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UBC Theses and Dissertations

The control of singing in varied thrushes Whitney, Carl Linn 1979

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THE CONTROL OF SINGING IN VARIED THRUSHES by CARL LINN WHITNEY B.S., Iowa State U n i v e r s i t y , 1970 M . S C i , The U n i v e r s i t y of B r i t i s h Columbia, 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1979 ... (c) C a r l L i n n Whitney, 1979 In presenting th i s thesis in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Univers ity of B r i t i s h Columbia, I agree that the Library shal l make i t f ree ly avai lable for reference and study. I further agree that permission for extensive copying of th i s thesis for scholar ly purposes may be granted by. the Head of my Department or by his representatives. It i s understood that copying or publ icat ion of th is thesis for f inanc ia l gain shal l not be allowed without my written permission. n 4 . * Zoology Department of _ The Univers ity of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 22 A p r i l 1 9 7 9 i i ABSTRACT Male v a r i e d t h r u s h e s (Zoothera n a e v i u s ) have r e p e r t o i r e s of t h r e e t o seven d i f f e r e n t songs. A t y p i c a l song i s a pure t o n e , about two seconds l o n g and modulated i n a m p l i t u d e ; i t can be d e s c r i b e d q u a n t i t a t i v e l y by i t s dominant f r e q u e n c y (Hz) and p e r i o d o f modulation;. . Beduced to t h e s e two d i m e n s i o n s , the songs w i t h i n r e p e r t o i r e s a r e more d i s s i m i l a r t o each o t h e r t h a n would be e x p e c t ed i f t h e y were drawn a t random from a l l t h e songs i n t h e p o p u l a t i o n . T h i s p a t t e r n i s c o n s i s t e n t w i t h a t h e o r y t h a t e x p l a i n s the a d a p t i v e s i g n i f i c a n c e of song and song r e p e r t o i r e s . A c c o r d i n g t o t h e t h e o r y , song h e l p s t e r r i t o r i a l males t o r e p e l o t h e r males and t o a t t r a c t and s t i m u l a t e f e m a l e s , and r e p e r t o i r e s enhance t h e s e e f f e c t s by r e d u c i n g t h e r a t e at which o t h e r b i r d s h a b i t u a t e . T h i s b e i n g so, i t i s to t h e advantage of males t o have r e p e r t o i r e s of d i s s i m i l a r songs, s i n c e o t h e r b i r d s w i l l g e n e r a l i z e l e s s between the songs and w i l l t h e r e f o r e h a b i t u a t e a t a s l o w e r r a t e . The r e l a t i o n s h i p between g e n e r a l i z a t i o n and s i m i l a r i t y o f songs forms the b a s i s of a t h e o r y of how s i n g i n g i s c o n t r o l l e d by t h e b r a i n o f a v a r i e d t h r u s h . The t h e o r y assumes t h a t a c o n t r o l c e n t e r f e e d s m o t i v a t i o n a l i m p u l s e s t o u n i t s i n c h a rge o f t h e d i f f e r e n t s o n g s i When the m o t i v a t i o n a l s t a t e o f a u n i t r e a c h e s a t h r e s h o l d , the u n i t produces a song and then i n h i b i t s i i t s e l f , d r o p p i n g a b r u p t l y t o a l o w e r m o t i v a t i o n a l s t a t e . At t h e same t i m e , i t i n h i b i t s each o f t h e o t h e r u n i t s an amount t h a t i s d i r e c t l y p r o p o r t i o n a l t o the s i m i l a r i t y o f the songs produced by the two u n i t s . Some random v a r i a b i l i t y i n i n h i b i t i o n i s not e x p l a i n e d by t h i s r e l a t i o n s h i p . k s i m u l a t i o n model based on t h i s t h e o r y a c c o u n t s f o r t h e f o l l o w i n g p a t t e r n s found i n sequences o f songs produced by v a r i e d t h r u s h e s : 1 ) Once a song has been sung, i t i s n o t u s u a l l y r e p e a t e d i m m e d i a t e l y i 2) Songs a r e n o t sung w i t h e q u a l f r e q u e n c y . 3) The most common song i n a r e p e r t o i r e i s more l i k e l y t han any o t h e r t o be r e p e a t e d i m m e d i a t e l y . 4) There i s a n e g a t i v e c o r r e l a t i o n between the l e n g t h s of s u c c e s s i v e r e c u r r e n c e i n t e r v a l s (where a r e c u r r e n c e i n t e r v a l i s d e f i n e d as one p l u s the number of o t h e r songs between s u c c e s s i v e r e p e t i t i o n s o f a g i v e n song) . 5) The two most d i s s i m i l a r songs i n a r e p e r t o i r e are more l i k e l y t o be sung i n s u c c e s s i o n t h a n would be expected i f they o c c u r r e d i n d e p e n d e n t l y of each o t h e r . 6) When s e v e r a l of the songs i n a r e p e r t o i r e a r e dropped and r e p l a c e d by o t h e r s , the ones t h a t remain occur i n d i f f e r e n t r e l a t i v e f r e q u e n c i e s ; , 7 ) Two ve r y s i m i l a r songs i n a r e p e r t o i r e can o c c u r w i t h q u i t e d i f f e r e n t f r e q u e n c i e s . 8) I n t e r v a l s between s u c c e s s i v e songs a r e v a r i a b l e but are u s u a l l y a t l e a s t s e v e r a l seconds l o n g . 9) The average l e n g t h s of the i n t e r v a l s f o l l o w i n g d i f f e r e n t songs a r e p o s i t i v e l y c o r r e l a t e d w i t h the r e l a t i v e f r e q u e n c i e s of the songs;. 1 0 ) The a b s o l u t e v a r i a b i l i t y o f t h e i n t e r v a l s between s u c c e s s i v e songs i s p o s i t i v e l y c o r r e l a t e d w i t h t h e r a t e o f s i n g i n g i The t h e o r y can be e l a b o r a t e d t o account f o r p a t t e r n s o f s i n g i n g i n o t h e r s p e c i e s . i v TABLE OF CONTENTS Page ABSTRACT .. ................. i i TABLE OF CONTENTS ................ .............. ^ .. . i v LIST OF TABLES ................. . : ........ 1 ..... : .. . v i LIST OF FIGURES ........................... '. ........ v i i i ACKNOWLEDGEMENTS ... .......... x i i I . . INTRODUCTION .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 THE BIRDS AND THEIR SONGS 3 ADAPTIVE SIGNIFICANCE OF SONG AND SONG REPERTOIRES 13 GENERAL METHODS ............................... 17, I I . SIMILARITY OF SONGS IN INDIVIDUAL REPERTOIRES 18 METHODS ,.. - i - . ' - i 24 RESULTS i U . 30 DISCUSSION 30 I I I . . CONTROL OF SINGING ................... i . . . ,i i . . . ^  . . . 52 INTERNAL AND EXTERNAL CONTROL . . . i i i . . . . . . . . . . . 53 A FIRST MODEL . . . . i 58 INHIBITION BETWEEN SONG UNITS . . . . . . . . . . . . . . . . i 69 FREQUENCIES OF DIFFERENT SONG TYPES ........... 86 INTERVALS BETWEEN SONGS , 97 SYNTHESIS .............. I . . . . . 1 117 IV. GENERAL DISCUSSION . . . . i l . . . . . . . . . . . . . i . 122 EVOLUTIONARY SIGNIFICANCE OF TEMPORAL PATTERNS OF SINGING ........ . . . . . . . . . . . . . i 122 V CONTROL OF SINGING ,* . . . 125 S i m i l a r i t y o f Songs i n I n d i v i d u a l R e p e r t o i r e s '.. 125 C o n t r o l o f S i n g i n g i n S e l e c t e d S p e c i e s ... 128 v i LIST OF TABLES T a b l e Page I ; F r e q u e n c i e s (and p r o p o r t i o n s ) o f d i f f e r e n t songs sung by the t h r u s h named Mick i n response t o p l a y b a c k and i n c o n t r o l sequence.......... 56 I I . F r e q u e n c i e s (and p r o p o r t i o n s ) of d i f f e r e n t songs sung by t h e t h r u s h named Bingo i n response t o p l a y b a c k and i n c o n t r o l sequence..,, 57 I I I . Sequences o f songs r e c o r d e d from the t h r u s h e s named Buddy, Chuck, and Bod......,....;..,.:..:.... 59 IV. M a t r i x o f i n h i b i t i o n v a l u e s ... . ,. ... 66 V i . P a r t of a sequence s i m u l a t e d u s i n g ' t h e v a l u e s of i n h i b i t i o n from T a b l e IV 67 VI. B e l a t i v e f r e q u e n c i e s o f songs r e p e a t e d i m m e d i a t e l y . , 68 V I I . Spearman rank c o r r e l a t i o n between s u c c e s s i v e 1 r e c u r r e n c e i n t e r v a l s . . 72 V i l l i T r a n s i t i o n f r e q u e n c i e s ! b e t w e e n t h e songs of t h e t h r u s h named Otis*•;•.„>.,:...•...•.. i . . . . . . . . . . . 74 IX.. T r a n s i t i o n f r e q u e n c i e s between t h e songs of the t h r u s h named John........ i . . . . . . . . 75 X. T r a n s i t i o n f r e q u e n c i e s between songs f o r two ' s i m u l a t i o n runs o f 500 songs e a c h . . . . . . . . i . . . . i . . . 76 X I . M a t r i x o f i n h i b i t i o n v a l u e s f o r r e p e r t o i r e shown i n F i g u r e 19............................... '.. 83 X l l i Observed/expected p r o b a b i l i t y o f the two most d i s s i m i l a r songs i n a r e p e r t o i r e b e i n g sung c o n s e c u t i v e l y 87 v i i X I I I . F r e q u e n c i e s (and p r o p o r t i o n s ) o f songs t h a t were i n t h e r e p e r t o i r e of the t h r u s h named F a t s both b e f o r e and a f t e r he a l t e r e d the r e p e r t o i r e . . . . . . . . 98 XIV.. F r e q u e n c i e s (and p r o p o r t i o n s ) of songs t h a t were i n t h e r e p e r t o i r e of the t h r u s h named Simon both b e f o r e and a f t e r he a l t e r e d the r e p e r t o i r e . . . . . . . . 99 XV. Spearman rank c o r r e l a t i o n between the r e l a t i v e f r e q u e n c i e s o f d i f f e r e n t songs w i t h i n i n d i v i d u a l r e p e r t o i r e s and t h e l e n q t h s of t h e i n t e r v a l s f o l l o w i n g them.......................... . . . . . . . . . . 107 XVI. Product-moment c o r r e l a t i o n s between 1) mean l e n g t h s o f i n t e r v a l s between s u c c e s s i v e songs and s t a n d a r d d e v i a t i o n s o f i n t e r v a l l e n g t h s , and 2) mean l e n g t h s of i n t e r v a l s between s u c c e s s i v e songs and c o e f f i c i e n t s of v a r i a t i o n o f i n t e r v a l lengths;,,. . j , . . . . . . . . . . . . . . . . . . . . . . . . .... 114 X V I I . M a t r i x of i n h i b i t i o n v a l u e s t h a t w i l l produce a f i x e d sequence: ABCDE ABCDE 12 9 v i i i LIST OF FIGURES F i g u r e Page 1.. Sonagram of a t y p i c a l v a r i e d t h r u s h song;.;....:.. 4 2: Sonograms o f t y p i c a l v a r i e d t h r u s h songs, showing d i f f e r e n c e s i n f r e q u e n c y (Hz) and p e r i o d of m o d u l a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3,i Sonagrams o f v a r i e d t h r u s h songs more complex t h a n those shown i n F i g u r e s 1 and 2i ,. 9 4.. Sonagrams o f a b e r r a n t songs sung by a male v a r i e d t h r u s h . . . ...... 11 5- Dominant f r e q u e n c i e s and p e r i o d s of m o d u l a t i o n of songs sung by the t h r u s h named Chubby............. 19 6. Dominant f r e q u e n c i e s and p e r i o d s of m o d u l a t i o n o f a l l 150 songs sung by the 30 male t h r u s h e s . . . . . . . . 21 7,. D i s t r i b u t i o n of v a r i e d t h r u s h songs a l o n g t h e dimension o f dominant f r e q u e n c y : observed d i s t r i b u t i o n ( s o l i d l i n e s ) and t h e o r e t i c a l d i s t r i b u t i o n o f best f i t (dashed l i n e s ) . . . . . . . . . . . 26 8. D i s t r i b u t i o n of v a r i e d t h r u s h songs a l o n g the d i m e n s i o n of p e r i o d o f m o d u l a t i o n : o b s e r v e d d i s t r i b u t i o n ( s o l i d l i n e s ) and t h e o r e t i c a l d i s t r i b u t i o n of b e s t f i t (dashed l i n e s ) . . . . . . . . . . . 28 9. Observed (shaded) and expected d i s t r i b u t i o n s o f s i m i l a r i t y t o n e a r e s t song a l o n g the d i m e n s i o n of dominant f r e q u e n c y i n v a r i e d t h r u s h songs 31 10. Observed (shaded) and e x p e c t e d d i s t r i b u t i o n s of s i m i l a r i t y t o n e a r e s t song a l o n g the d i m e n s i o n of p e r i o d o f m o d u l a t i o n i n v a r i e d t h r u s h songs....... 33 i x 11. a. Sonagram of a r u f o u s - s i d e d townee song, showing i n t r o d u c t o r y phrase and t r i l l of s i n g l e , r e p e a t e d , s y l l a b l e . , b*, Sonagrams of. r u f p u s - s i d e d towhee songs, showing v a r i a t i o n s i n s y l l a b l e l e n g t h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 12. Observed and expected d i s t r i b u t i o n s of s i m i l a r i t y t o n e a r e s t song a l o n g the dimension o f s y l l a b l e l e n g t h i n r u f o u s - s i d e d towhee songs....... ... 3 8 13. H a b i t u a t i o n of male r u f o u s - s i d e d towhees'to 1 p l a y b a c k o f songs on t h e i r t e r r i t o r i e s ; , ; . . , : . . . . . . . 40 14. Response o f r u f o u s - s i d e d towhees t o p l a y b a c k of song A a f t e r h a v i n g h a b i t u a t e d t o one of f o u r songs. A, B, C, o r D (see F i g u r e 11b).,..,.....,.... 43 15. R e s p o n s i v e n e s s and r a t e o f h a b i t u a t i o n , of r u f o u s ^ s i d e d towhees t o songs A, B, C, and D............. 45 16,i A f i r s t model o f the c o n t r o l o f s i n g i n g i n v a r i e d t h r u s h e s . . . . . . . . . . ................................ . 61 17. B e h a v i o r o f f i r s t model of c o n t r o l . . . . . . . . . . . . . . . . 63 18i E e l a t i o n s h i p between t h e l e n g t h s o f s u c c e s s i v e r e c u r r e n c e i n t e r v a l s f o r t h e most common song i n a seguence produced by the f i r s t model... 70 19. Banks of dominant f r e q u e n c y and p e r i o d o f mo d u l a t i o n o f songs sung by the t h r u s h named F a t s * 78 20. B e h a v i o r o f model i n which song u n i t s not o n l y i n h i b i t t h e m s e l v e s but a l s o i n h i b i t each o t h e r . . . . 80 21. R e l a t i o n s h i p of the ob s e r v e d / e x p e c t e d p r o b a b i l i t y o f two songs o c c u r i n g i n s u c c e s s i o n and t h e s i m i l a r i t y of the songs i n a s i m u l a t e d sequence based on t h e r e p e r t o i r e shown, i n F i g u r e 19,......... . 84 X 2 2 . R e l a t i o n s h i p of t h e r e l a t i v e f r e q u e n c y o f a song and t h e t o t a l amount of i n h i b i t i o n i t s c o n t r o l l i n g u n i t r e c e i v e s from o t h e r u n i t s i n a s i m u l a t e d sequence based on the r e p e r t o i r e shown i n F i g u r e 1 9 . . . . . . . 8 9 2 3 . Changes i n t h e r e p e r t o i r e o f t h e t h r u s h named F a t s * , . . . i i ; . . 91 2 4 . Changes i n t h e r e p e r t o i r e o f the t h r u s h named Simons..... .. A . A 9 3 2 5 . R e l a t i o n s h i p of t h e r e l a t i v e f r e q u e n c y of a song and t h e amount i t s c o n t r o l l i n g u n i t i n h i b i t s i t s e l f i n a s i m u l a t e d sequence based on t h e m a t r i x o f i n h i b i t i o n v a l u e s shown i n i n T a b l e IV.. 9 5 2 6 . L e n g t h s of i n t e r v a l s between s u c c e s s i v e songs i n sequence sung by the t h r u s h named Simon;...... A . . . 1 0 0 2 7 . L e n g t h s o f i n t e r v a l s between s u c c e s s i v e songs i n a s i m u l a t e d sequence based on t h e r e p e r t o i r e shown i n F i g u r e 1 9 , . . . . . . . . . . . . . . . . . . . 1 0 2 2 8 . I n t e r v a l s between s u c c e s s i v e songs i n a sequence, sung by t h e t h r u s h named John. ; . 1 0 5 2 9 . Asymmetry o f i n h i b i t i o n between song u n i t s . i n models h a v i n g a s t o c h a s t i c r e l a t i o n s h i p between t h e s i m i l a r i t y o f songs and t h e amount songs i n h i b i t each o t h e r . . . . . 1 0 9 3 0 . Sonagrams of two very s i m i l a r songs i n t h e r e p e r t o i r e o f t h e t h r u s h named F a t s . 1 11 3 1 . R e l a t i o n s h i p of the c o e f f i c i e n t o f v a r i a t i o n o f i n t e r v a l l e n g t h t o t h e average i n t e r v a l l e n g t h i n s i m u l a t e d sequences*.... 1 1 5 3 2 . F i n a l model o f the c o n t r o l . o f s i n g i n g i n v a r i e d t h r u s h e s . . 1 1 8 x i 33* E l a b o r a t i o n s of t h e v a r i e d t h r u s h model t o account f o r t h e c o n t r o l o f s i n g i n g i n o t h e r s p e c i e s . 133 34. I n t e r v a l s between s u c c e s s i v e songs i n a sequence sung by a r u f o u s - s i d e d towhee.................;... 136 x i i ACKNOWLEDGEMENTS I am g r a t e f u l t o my s u p e r v i s o r , Jamie. S m i t h , f o r h i s sup p o r t and encouragement d u r i n g t h e f i v e y e a r s o f t h i s s t u d y , and most e s p e c i a l l y f o r h i s p a t i e n c e d u r i n g t h e f i n a l months, when I was i n s h o r t s u p p l y o f t h i s v i r t u e m y s e l f i The members o f my r e s e a r c h committee, Lee Gass, C h a r l e y K r e b s , Robin L i l e y , Judy Myers, and Jamie S m i t h , h e l p f u l l y c r i t i c i z e d e a r l i e r d r a f t s o f t h i s t h e s i s . Susan H a r r i s o n and B i l l Webb backed me i n my b a t t l e s w i t h t h e computer* Mary P e r k i n s , a welcome companion on r e c o r d i n g t r i p s , h e l p e d me t o l u g equipment over th e s t e e p t e r r a i n o f Seymour and H o l l y b u r n Mountains. John Spence was a k i n d r e d s p i r i t t h r o u g h o u t my c a r e e r as a Ph.D. s t u d e n t ; whatever u n d e r s t a n d i n g I have o f t h e p h i l o s o p h i c a l i s s u e s t h a t u n d e r l i e t h i s r e s e a r c h can be t r a c e d t o d i s c u s s i o n s we have had d u r i n g t h e p a s t f i v e y e a r s . T o g e t h e r , John and Debbie Spence c o n t r i b u t e d t o my mental h e a l t h w h i l e I was w r i t i n g t h i s t h e s i s by i n d u l g i n g w i t h Joan M i l l e r and me i n weekly bouts o f beer d r i n k i n g . Joan M i l l e r h e l p e d by drawing t h e f i g u r e s and r e a d i n g a number o f d i f f e r e n t v e r s i o n s of t h e t h e s i s * More i m p o r t a n t l y , she e n t h u s i a s t i c a l l y d i s c u s s e d my r e s e a r c h with me and c o n t r i b u t e d much t o t h e development and c l a r i f i c a t i o n o f my i d e a s about how s i n g i n g i s c o n t r o l l e d i n b i r d s * 1 CHAPTEB I INTRODUCTION One of t h e t h i n g s t h a t b e h a v i o r a l b i o l o g i s t s attempt t o e x p l a i n about b e h a v i o r i s how i t i s c o n t r o l l e d by the nervous system; O b s e r v i n g t h a t b e h a v i o r i s o r g a n i z e d , t h a t i t f o l l o w s d e f i n i t e p a t t e r n s , we ask: What a r e t h e mechanisms u n d e r l y i n g t h e s e p a t t e r n s ? Dawkins (1976) has r e c e n t l y p o i n t e d out t h a t , c o n t r a r y t o what many of us may t h i n k , a s a t i s f a c t o r y u n d e r s t a n d i n g of c o n t r o l mechanisms does not f o l l o w as a n a t u r a l consequence of d e t a i l e d n e u r o a n a t o m i c a l and n e u r o p h y s i o l o g i c a l d e s c r i p t i o n s . T h i s i s e s p e c i a l l y t r u e f o r b e h a v i o r a l p a t t e r n s c o n t r o l l e d by complex nervous systems. What i s n e c e s s a r y , Dawkins a r g u e s , i s not a complete (and h o p e l e s s l y d e t a i l e d ) a c c o u n t of the a c t i v i t i e s o f neurons and synapses d u r i n g b e h a v i o r but i n s t e a d a " d i s t i l l a t i o n o f g e n e r a l p r i n c i p l e s at a h i g h e r l e v e l * " I n o t h e r words, t o summarize an analogy he makes between nervous systems and computers, a s o f t w a r e t h e o r y based on g e n e r a l p r i n c i p l e s i s a more a p p r o p r i a t e g o a l t h a n i s a complete d e s c r i p t i o n o f hardware: a s e t of programming i n s t r u c t i o n s i s a b e t t e r e x p l a n a t i o n of c o n t r o l than i s a w i r i n g diagram. Dawkins (1976) b e l i e v e s t h a t s o f t w a r e p r i n c i p l e s can be i n f e r r e d from b e h a v i o r a l e v i d e n c e a l o n e * T h i s i s not an e n t i r e l y new i d e a . The e t h o l o g i c a l l i t e r a t u r e abounds w i t h s t u d i e s o f the t e m p o r a l p a t t e r n i n g of b e h a v i o r , many o f them done under t h e assu m p t i o n t h a t the r e s u l t s would e l u c i d a t e t h e 2 u n d e r l y i n g c o n t r o l mechanisms ( S l a t e r 1973). An i m p o r t a n t p a r t of Dawkins' c o n t r i b u t i o n , however, has been t o emphasize something t h a t has n o t always been a p p r e c i a t e d i n t h e p a s t : a d e t a i l e d d e s c r i p t i o n o f b e h a v i o r may be no more e n l i g h t e n i n g t h a n a d e t a i l e d d e s c r i p t i o n of hardware u n l e s s i t i s done i n a t h e o r e t i c a l c o n t e x t * I n t h i s t h e s i s , I d e v e l o p a s o f t w a r e t h e o r y which t i e s t o g e t h e r a number of p a t t e r n s observed i n the s i n g i n g b e h a v i o r o f i n d i v i d u a l v a r i e d t h r u s h e s (Zoothera - n a e y i u s ) . The remainder of t h i s c h a p t e r and the whole o f C hapter I I b u i l d up t o t h e t h e o r y , which i s developed i n Chapter I I I . I n C hapter I , a f t e r i n t r o d u c i n g the b i r d s and t h e i r songs, I d e v e l o p an e v o l u t i o n a r y argument t h a t t h e songs w i t h i n the r e p e r t o i r e s o f i n d i v i d u a l s s h o u l d be more d i s s i m i l a r to each o t h e r t h a n i f they were drawn at random from a l l the songs i n t h e p o p u l a t i o n . T h i s p r e d i c t i o n i s c o n f i r m e d i n Chapter I I - I n Chapter I I I , I b e g i n w i t h a very s i m p l e t h e o r y of c o n t r o l and e l a b o r a t e i t , s t e p by s t e p , u n t i l i t a c c o u n t s not o n l y f o r the d i s s i m i l a r i t y of songs w i t h i n i n d i v i d u a l r e p e r t o i r e s but a l s o f o r p a t t e r n s based on 1) t h e r e l a t i v e f r e q u e n c i e s w i t h which d i f f e r e n t songs ar e sung, 2) the o r d e r i n which songs are sung, and 3) t h e l e n g t h s o f i n t e r v a l s between s u c c e s s i v e songs*. F i n a l l y , i n Chapter IV, I d i s c u s s t h e p o s s i b l e e v o l u t i o n a r y s i g n i f i c a n c e o f s e v e r a l p a t t e r n s d e s c r i b e d i n C hapter I I I , and I c o n s i d e r t h e c o n t r o l o f s i n g i n g i n o t h e r s p e c i e s . 3 THE BIRDS AND THEIR SONGS l a some p a r t s of t h e i r r a n g e , v a r i e d t h r u s h e s are known as " t i n w h i s t l e b i r d s . " The name r e f e r s t o t h e i r song, which L o u i s A g a s s i z F u e r t e s , the w i l d l i f e a r t i s t ; d e s c r i b e d as "a s i n g l e long-drawn n o t e , u t t e r e d i n s e v e r a l d i f f e r e n t k e y s , some o f t h e h i g h - p i t c h e d ones w i t h a s t r o n g v i b r a n t t r i l l . . Each note grows out of n o t h i n g , s w e l l s t o a f u l l t o n e , and then f a d e s away t o n o t h i n g u n t i l one i s c a r r i e d away w i t h t h e m y s t e r i o u s song" (quoted by B a i l e y 1924). More p r o s a i c but j u s t as f i t t i n g i s t h e d e s c r i p t i o n i n Nature West Coast by Smith e t a l . (1973): "a s i n g l e monotone note t h a t v a r i e s each t i m e , sounding l i k e someone w h i s t l i n g and humming at t h e same ti m e . " A sonagram of a t y p i c a l song i s shown i n F i g u r e 1. The song i s a pure t o n e , about two seconds l o n g and modulated i n a m p l i t u d e . I t can be d e s c r i b e d q u a n t i t a t i v e l y by i t s dominant f r e q u e n c y (Hz) and p e r i o d of m o d u l a t i o n , both measured from t h e sonagram. The songs o f v a r i e d t h r u s h e s a r e f a m i l i a r t o h i k e r s i n t h e Coa s t Mountains around Vancouver, but t h e b i r d s t h e m s e l v e s a r e o f t e n o v e r l o o k e d . S l i g h t l y s m a l l e r than r o b i n s , which t h e y resemble i n g e n e r a l appearance, they have orange wingbars and an orange eye s t r i p e * Males have a b l a c k b r e a s t band, f e m a l e s a gray b r e a s t band and d u l l e r orange f e a t h e r s . Only the males s i n g . I s t u d i e d the s i n g i n g b e h a v i o r of v a r i e d t h r u s h e s d u r i n g t h r e e summers, 1974-76, a t two a r e a s about 1000 m i n e l e v a t i o n 4 F i g u r e 1 . Sonagram o f a t y p i c a l v a r i e d t h r u s h song* L e f t s e t o f axes shows a t i m e v e r s u s f r e q u e n c y (Hz) p l o t o f the b e g i n n i n g of the song; t h e remainder of t h e song i s s i m i l a r . P e r i o d o f m o d u l a t i o n i s d e f i n e d as the i n t e r v a l (measured i n seconds) between s u c c e s s i v e p u l s e s . R i g h t s e t o f axes shows a p l o t o f r e l a t i v e i n t e n s i t y v e r s u s f r e q u e n c y t a k e n a t a s i n g l e p o i n t o f time d u r i n g the song: Dominant f r e q u e n c y i s d e f i n e d as t h e f r e q u e n c y (Hz) a t which i n t e n s i t y i s g r e a t e s t . .3 .4 increasing intensity 6 on Seymour and H o l l y b u r n Mountains near Vancouver. Thrushes breed h e r e i n the s u b a l p i n e f o r e s t . They a r r i v e i n A p r i l when snow s t i l l c o v e r s t h e ground, and males e s t a b l i s h t e r r i t o r i e s on which they can be heard s i n g i n g u n t i l l a t e • J u l y . P e r c h e d t y p i c a l l y i n the t o p s of c o n i f e r t r e e s , t h e y s i n g one song a f t e r a n o t h e r , t h e songs s e p a r a t e d by a few seconds o f s i l e n c e . The songs are u s u a l l y l o u d , and can be heard as f a r away as 1000 m. Each male has a r e p e r t o i r e of t h r e e t o seven songs, which d i f f e r i n dominant f r e q u e n c y and p e r i o d of m o d u l a t i o n ( F i g . 2) . Having sung a p a r t i c u l a r song, a male does not u s u a l l y r e t u r n t o i t u n t i l he has sung one or more o t h e r songs* One i n d i v i d u a l , f o r example, which had a r e p e r t o i r e o f f i v e songs (A,B,C,D,E), sang the f o l l o w i n g sequence: BACDBCDCBDACD BCEDBCADCBEDBCAC. Some songs are s l i g h t l y more complex than t h o s e shown i n F i g u r e 2, b u t even these ( F i g . 3) can be reduced t o t h e two d i m e n s i o n s , dominant frequency and p e r i o d of m o d u l a t i o n * With o n l y a s i n g l e e x c e p t i o n , the 31 males I s t u d i e d had songs t h a t c o u l d be d e s c r i b e d a l o n g t h e s e two d i m e n s i o n s . To a p p r e c i a t e t h e s u i t a b i l i t y o f v a r i e d t h r u s h songs f o r such p r e c i s e d e s c r i p t i o n , one has o n l y t o compare them t o t h e much more complex songs of most b i r d s ( s e e , f o r example, t h e sonagrams i n B i r d s o f N o r t h America by Bobbins e t a l . 1966),* The songs of the a b e r r a n t male ( F i g . : 4) resemble songs n o r m a l l y sung under q u i t e d i f f e r e n t c i r c u m s t a n c e s from t h o s e d e s c r i b e d above. I have observed o t h e r males s i n g i n g songs l i k e t h e ones shown i n F i g u r e 4 w h i l e hopping about on the ground f e e d i n g , w h i l e "warming up" t o t y p i c a l l o u d s i n g i n g , 7 F i g u r e 2* Sonagrams of t y p i c a l v a r i e d t h r u s h song, showing d i f f e r e n c e s i n frequency (Hz) and p e r i o d o f m o d u l a t i o n . Note t h a t the f i r s t song i s not modulated a t a l l . As i n F i g u r e 1, o n l y p a r t o f each song i s shown. 9 F i g u r e 3 . Sonagrams o f v a r i e d t h r u s h songs more complex than t h o s e shown i n F i g u r e s 1 and 2, 11 F i g u r e 4. Sonagrams of a b e r r a n t songs sung by a male v a r i e d t h r u s h . 1 2 (ziw) Aoiianbay 13 d u r i n g d i s p u t e s w i t h o t h e r males, and i n response t o p l a y b a c k . I n a l l t h e s e s i t u a t i o n s , however, the songs a r e u t t e r e d q u i e t l y and almost w i t h o u t pause. T h i s t h e s i s i s concerned o n l y w i t h the l o u d s i n g i n g t h a t males do from .high p e r c h e s . I t d e s c r i b e s the b e h a v i o r of the 30 males t h a t sang normal l o u d songs. The i d e a s i n t h i s t h e s i s grew out of an o b s e r v a t i o n I made in.. t h e r s p r i n g o f 1974 w h i l e h i k i n g through, a s u b a l p i n e f o r e s t where s e v e r a l v a r i e d t h r u s h e s were s i n g i n g . The o b s e r v a t i o n was s i m p l y , t h a t ..the songs i n the r e p e r t o i r e s of i n d i v i d u a l b i r d s were q u i t e d i s t i n c t , t h e b i r d s seeming t o obey a r u l e t h a t s a y s : s i n g o n l y songs t h a t a r e not v e r y s i m i l a r t o each o t h e r . T h i s r u l e , which i s t h e b a s i s o f the t h e o r y developed l a t e r i n t h i s t h e s i s , a l s o f i t s n e a t l y i n t o a t h e o r e t i c a l framework t h a t e x p l a i n s t h e a d a p t i v e s i g n i f i c a n c e of song and song r e p e r t o i r e s . ADAPTIVE SIGNIFICANCE OF SONG AND SONG EEPEBTOIRES L i k e many o t h e r s o n g b i r d s , v a r i e d t h r u s h e s defend t e r r i t o r i e s d u r i n g t h e b r e e d i n g season and s i n g w i t h i n t h e b o u n d a r i e s o f t h e i r t e r r i t o r i e s . T e r r i t o r i a l song i s commonly thou g h t t o have two g e n e r a l f u n c t i o n s : d e f e n s e . o f the t e r r i t o r y a g a i n s t o t h e r males, and a t t r a c t i o n and s t i m u l a t i o n of f e m a l e s . More than 100 y e a r s ago, Be r n a r d Altum (1868) suggested t h a t song i s a lo n g - r a n g e warning t o c o n s p e c i f i c males, r e p e l l i n g them from a d i s t a n c e . There have been t h r e e r e c e n t a t t e m p t s to t e s t t h i s h y p o t h e s i s . I n s u p p o r t of i t ; Peek 14 (1972) found t h a t s u r g i c a l l y muted red-winged b l a c k b i r d s ( A g e l a i u s phoeniceus) s u f f e r e d more t r e s p a s s i n g by o t h e r males and were more l i k e l y t o l o s e t h e i r t e r r i t o r i e s ; But, i n a s i m i l a r experiment w i t h t h e same s p e c i e s * Smith (1976) found no d i f f e r e n c e i n the a b i l i t y of muted and c o n t r o l males t o h o l d t h e i r t e r r i t o r i e s , and i t i s n o t c l e a r whether t h e s e c o n t r a d i c t o r y r e s u l t s r e f l e c t d i f f e r e n c e s i n e x p e r i m e n t a l procedure or d i f f e r e n c e s i n the r o l e o f song between t h e two p o p u l a t i o n s o f red-wings (Smith 1976). Krebs e t a l . (1978), t a k i n g a d i f f e r e n t approach i n a s t u d y of g r e a t t i t s (Parus m a j o r ) , found t h a t a f t e r the removal o f t e r r i t o r i a l p a i r s , new b i r d s c o l o n i z e d empty c o n t r o l a r e a s more q u i c k l y than t h e y d i d ar e a s o c c u p i e d o n l y by l o u d s p e a k e r s p l a y i n g back song. T h e r e f o r e , i n g r e a t t i t s song does r e p e l o t h e r males. There i s a l s o e v i d e n c e , though e q u a l l y s c a n t y , t h a t song a t t r a c t s and s t i m u l a t e s f e m a l e s . Although i t i s not known whether f e m a l e s l o c a t e mates by t h e i r song, fe m a l e c h a f f i n c h e s ( F r i n g i l l a c o e l e b s ) a r e a t t r a c t e d t o s o u r c e s of r e c o r d e d song ( M a r l e r 1956). V o c a l i z a t i o n s o f male b u d g e r i g a r s ( M e l o p s i t t a c u s undulatus) and r i n g doves ( S t r e p t o p e l i a r i s o r i a ) s t i m u l a t e g o n a d a l a c t i v i t y i n females (e.g. Brockway 1969; L o t t and Brody 1966). T h i s may a l s o be t r u e f o r c a n a r i e s ( S e r i n u s c a n a r i u s ) ; f e males b u i l d n e s t s more a c t i v e l y when exposed t o song (Hinde and S t e e l 1976), a p a t t e r n t h a t presumably has a hormonal b a s i s * V a r i e d t h r u s h e s are a l s o s i m i l a r t o many o t h e r song b i r d s i n h a v i n g r e p e r t o i r e s o f s e v e r a l d i f f e r e n t song t y p e s . I n a few s p e c i e s , males s i n g d i f f e r e n t songs i n d i f f e r e n t c o n t e x t s . 15 C h e s t n u t - s i d e d w a r b l e r s ( D e n d r o j c a g e n s y l v a n i e a ) , f o r example, s i n g c e r t a i n songs i n the c e n t e r s o f t h e i r t e r r i t o r i e s when no o t h e r males a r e nearby and o t h e r songs near t h e b o u n d a r i e s d u r i n g e n c o u n t e r s w i t h o t h e r males ( L e i n 1978). I n most s p e c i e s , however, males seem t o s i n g a l l t h e i r songs i n the same c o n t e x t s (Krebs 1977b).. There i s perhaps no b e t t e r example o f t h i s than v a r i e d t h r u s h e s , which s i n g one song t y p e a f t e r a n o ther ACDBCEDBCAD from t h e same p e r c h . But why do b i r d s s i n g more than one type of song i f t h e songs a l l o c c u r i n the same c o n t e x t s and presumably t h e r e f o r e c a r r y t h e same message? A l i k e l y answer i s t h a t r e p e r t o i r e s enhance one or b o t h o f the g e n e r a l e f f e c t s of song d i s c u s s e d above* Thus, a male t h a t has a r e p e r t o i r e might be b e t t e r a b l e t o m a i n t a i n h i s t e r r i t o r y a g a i n s t i n t r u d e r s . T h i s seems t o be t h e case f o r g r e a t t i t s . I n t r u d e r s ( b i r d s t h a t have e i t h e r no t e r r i t o r i e s o r i n f e r i o r ones) a r e q u i c k e r t o t a k e over empty t e r r i t o r i e s when r e s i d e n t males have been r e p l a c e d by r e c o r d i n g s o f s i n g l e song t y p e s than when t h e y have been r e p l a c e d by r e p e r t o i r e s of s e v e r a l song t y p e s (Krebs e t a l . 1978). The reason f o r t h i s d i f f e r e n c e i s not known, but one p o s s i b i l i t y i s t h a t p o t e n t i a l i n t r u d e r s h a b i t u a t e — - t h a t i s , c e ase t o be r e p e l l e d s o o n e r t o p l a y b a c k o f s i n g l e songs (Krebs 1977b). T h i s d i f f e r e n c e i n t h e r a t e o f h a b i t u a t i o n t o s i n g l e songs and r e p e r t o i r e s does, i n f a c t , o c c u r i n a n o t h e r c o n t e x t , t h a t of r e s i d e n t males r e s p o n d i n g a g g r e s s i v e l y t o pl a y b a c k of song on t h e i r t e r r i t o r i e s (Krebs e t a l . 1978). A male t h a t has a r e p e r t o i r e might a l s o be more a t t r a c t i v e 16 and s t i m u l a t i n g t o females,. I n c a n a r i e s , which s i n g complex songs of 30 t o 40 d i f f e r e n t s y l l a b l e s , f e m a l e s exposed t o normal songs b u i l d n e s t s sooner and l a y more eggs t h a n do t h o s e exposed t o songs h a v i n g an a r t i f i c i a l l y s m a l l number of s y l l a b l e s (Kroodsma 1976). These d i f f e r e n c e s might a l s o be due to d i f f e r e n c e s i n r a t e of h a b i t u a t i o n (Kroodsma 1976). Whether i t i s i m p o r t a n t f o r male s o n g b i r d s t o p r e v e n t f emales and p o t e n t i a l male i n t r u d e r s from h a b i t u a t i n g t o t h e i r song i s s t i l l an open q u e s t i o n . S t i l l , a p r e d i c t a b l e r e s u l t i n s t u d i e s o f h a b i t u a t i o n i s t h a t a f t e r a response has waned, i t can be r e s t o r e d by a d i f f e r e n t s t i m u l u s (Hinde 1970b; Thompson e t a l . 1973). I t would n o t be s u r p r i s i n g t h e r e f o r e i f r e p e r t o i r e s d i d reduce h a b i t u a t i o n * The amount t h a t h a b i t u a t i o n i s reduced s h o u l d depend on t h e s i m i l a r i t y o f t h e songs i n the r e p e r t o i r e . A p a t t e r n o f t e n observed i n s t u d i e s of h a b i t u a t i o n i s t h a t the r e s ponse t o one s t i m u l u s i s s m a l l e r a f t e r t h e a n i m a l has h a b i t u a t e d t o a n o t h e r s t i m u l u s . T h i s i s c a l l e d " s t i m u l u s g e n e r a l i z a t i o n " , a term a l s o a p p l i e d t o the s i t u a t i o n when a c o n d i t i o n e d response i s made t o a s t i m u l u s t h a t has not been a s s o c i a t e d w i t h r e i n f o r c e m e n t * I n e i t h e r c a s e , th e amount of g e n e r a l i z a t i o n n o r m a l l y i n c r e a s e s w i t h i n c r e a s i n g s i m i l a r i t y o f the s t i m u l i (Thompson e t a l . 1973; M a c K i n t o s h 1974). Thus, i f v a r i e d t h r u s h e s b e n e f i t by s i n g i n g i n ways t h a t reduce th e h a b i t u a t i o n of o t h e r b i r d s , t h e y might be expected t o have r e p e r t o i r e s o f d i s s i m i l a r songs. T h i s p r e d i c t i o n i s put t o a t e s t i n t h e n e x t c h a p t e r . 17 GENERAL METHODS Three g e n e r a l methods were used i n t h i s s t u d y : r e c o r d i n g of songs onto magnetic t a p e , a n a l y s i s of r e c o r d e d songs, by a sound s p e c t r o g r a p h , and p l a y i n g back of songs t o b i r d s t h r o u g h a l o u d s p e a k e r * Songs were r e c o r d e d with a S t e l l a y o x P r o f e s s i o n a l R e c o r d e r Type Sp7 and a S e n n h e i s e r MKH 815 condenser microphone. The r e c o r d i n g speed was 19 cm/s f o r songs t o be a n a l y z e d on t h e sound s p e c t r o g r a p h and 9.5 cm/s f o r l o n g sequences o f songs. D u r i n g the t h r e e y e a r s o f the s t u d y (1974-76), I r e c o r d e d b i r d s o n l y between 0500 and 1100 hours PDT from 20 May t o 27 J u l y . Not h a v i n g banded t h e b i r d s , I c o u l d n o t i d e n t i f y i n d i v i d u a l s w i t h c e r t a i n t y . But i f two r e c o r d i n g s made i n t h e same a r e a at d i f f e r e n t t i m e s c o n t a i n e d i d e n t i c a l songs, I assumed t h a t t h e y were o f t h e same i n d i v i d u a l . The sound s p e c t r o g r a p h was a Kay E l e c t r i c M i s s i l i z e r 675. The F l a t Shape and Wide Band s e t t i n g s were used f o r a l l sonagrams and t h e F l a t Shape and Narrow Band s e t t i n g s f o r p l o t s of f r e q u e n c y (Hz) v e r s u s i n t e n s i t y . For p l a y b a c k e x p e r i m e n t s , tape l o o p s 57 cm l o n g were p l a y e d a t 9.5 cm/s on a Oher 4000 R e p o r t - L tape r e c o r d e r * The playback r a t e of 10 songs/min was w i t h i n the normal range o f s i n g i n g r a t e s f o r v a r i e d t h r u s h e s * The songs were p l a y e d t h r o u g h a Nagra DH p o r t a b l e a m p l i f i e r / l o u d s p e a k e r , p l a c e d near t h e middle of a male's t e r r i t o r y . J u s t b e f o r e playback I made a rough map o f the t e r r i t o r y by o b s e r v i n g t h e male's movements f o r 30 t o 60 minutes* 18 CHAPTEE I I SIMILARITY OF SONGS IN INDIVIDUAL REPERTOIRES Eeduced to two d i m e n s i o n s , as shown i n F i g u r e 1, v a r i e d t h r u s h songs can be p l o t t e d as p o i n t s on a graph t h a t has dominant f r e q u e n c y r e p r e s e n t e d on one a x i s and p e r i o d o f m o d u l a t i o n on a n o t h e r . The r e p e r t o i r e of one b i r d i s graphed t h i s way i n F i g u r e 5, and a l l 150 songs r e c o r d e d from the 30 males a r e shown i n F i g u r e 6. The g e n e r a l q u e s t i o n asked i n t h i s c h a p t e r i s whether t h e songs i n i n d i v i d u a l r e p e r t o i r e s , f o r example t h e one shown i n F i g u r e 5, are more d i s s i m i l a r t o each o t h e r t h a n would be expected i f t h e y were drawn a t random from a l l t h e songs i n the p o p u l a t i o n , of which those i n F i g u r e 6 a r e assumed t o be a random sample. I n o t h e r words, the q u e s t i o n i s whether i n d i v i d u a l r e p e r t o i r e s a r e o r g a n i z e d t o r e d u c e . g e n e r a l i z a t i o n i n b i r d s l i s t e n i n g t o the songs. I n t h e a n a l y s i s , I assume t h a t b i r d s g e n e r a l i z e a l o n g t h e d i m e n s i o n s o f dominant frequency and p e r i o d o f m o d u l a t i o n ^ and t h a t t h e y g e n e r a l i z e a c c o r d i n g t o a r u l e which I gleaned from p u b l i s h e d s t u d i e s o f g e n e r a l i z a t i o n . The most d e t a i l e d o f t h e s e d e s c r i b e g e n e r a l i z a t i o n o f c o n d i t i o n e d r e s p o n s e s (e.g. M o s t o f s k y 1965; MacKintosh 1974}, not g e n e r a l i z a t i o n o f h a b i t u a t i o n , but I assume t h a t t h e r u l e a p p l i e s i n b oth s i t u a t i o n s . A c c o r d i n g t o t h i s l i t e r a t u r e , b i r d s s h o u l d g e n e r a l i z e a l o n g each d i m e n s i o n , not a c c o r d i n g t o the a b s o l u t e d i f f e r e n c e between two v a l u e s , but a c c o r d i n g t o t h e i r r a t i o . 19 F i g u r e 5 . Dominant f r e q u e n c i e s and p e r i o d s o f m o d u l a t i o n o f songs sung by t h e t h r u s h named Chubby,. 40 co E 30 A c o is D "D O 2 0 i g 10-0 2.5 3.0 3.5 4.0 4.5 Dominant Frequency ( k h z ) 21 F i g u r e 6. Dominant f r e q u e n c i e s and p e r i o d s o f mod u l a t i o n o f a l l 150 songs sung by t h e 30 male thrushes,. Numbers c o r r e s p o n d t o t h e number o f songs. E x p l a n a t i o n f o r t h e l o g a r i t h m i c s c a l i n g of t h e axes i s g i v e n i n the t e x t . 22 70.7 58.9-1 49.0-J 40.8 33.9-| ST" , 23.5i 1 16.3. 1 » H 1 1 1 11.3 9.4-1 •o 6.5-1 5.4-1 1 1 1 2 1 2 2 1 1 2 1 2 1 1 1 2 2 1 2 1 2 3 :3 4 2 3 2 1 2 2 5: 7 1 1 1 1 1 3 3 2 2 4 1 1 2 5 1 4 4 2 1 2 3 2 1 2 3 1 1 3 2 1 1 T I I I I i I 2810 3146. .3522 2656 2973 3522 3943 1 ' 3329 Dominant. Frequency (Hz) 1 1  4414 4942 ' 5533 3727 4172 4671 5229 23 For example, i f a t h r u s h g e n e r a l i z e s a c e r t a i n amount from a song h a v i n g a p e r i o d o f m o d u l a t i o n o f 6 ms t o one h a v i n g a p e r i o d o f 12 ms, i t sh o u l d g e n e r a l i z e an e q u a l amount from t h e l a t t e r song t o one h a v i n g a p e r i o d o f 24 ms. I was aisle t o f i n d o n l y one s t u d y o f g e n e r a l i z a t i o n a l o n g a d i m e n s i o n (the i n t e r v a l between p a i r s of c l i c k s ) t h a t c o r r e s p o n d s t o p e r i o d of m o d u l a t i o n , and i n t h a t s t u d y ( W i l k i n s o n and Howse .1975) a b u l l f i n c h ( P v r r h u l a p u r r h u l a ) behaved a c c o r d i n g t o t h e r u l e o f e q u a l g e n e r a l i z a t i o n t o e q u a l r a t i o s * Many s t u d i e s ( s e v e r a l o f which a r e . summarized i n Mos t o f s k y 1965 and Mac K i n t o s h 1974)- show t h a t the same r u l e a p p l i e s t o g e n e r a l i z a t i o n a l o n g the dimension of f r e q u e n c y (Hz) i I f t h e axes o f F i g u r e 5 a r e t r a n s f o r m e d l o g a r i t h m i c a l l y (as i n F i g u r e 6 ) , a g i v e n change a l o n g one a x i s from any p o i n t on the a x i s r e p r e s e n t s a c o n s t a n t amount o f g e n e r a l i z a t i o n . There i s s t i l l t h e problem, however, t h a t two songs may d i f f e r i n both dominant f r e q u e n c y and p e r i o d o f m o d u l a t i o n , i n which case t h e o v e r a l l amount of g e n e r a l i z a t i o n i s some f u n c t i o n o f t h e g e n e r a l i z a t i o n a l o n g each a x i s . Although s e v e r a l " c o m b i n a t i o n " r u l e s have been c o n s i d e r e d i n s t u d i e s o f g e n e r a l i z a t i o n a l o n g two d i m e n s i o n s , no s i n g l e r u l e seems t o a p p l y g e n e r a l l y . About a l l we know a t p r e s e n t i s t h a t i f a c e r t a i n amount of g e n e r a l i z a t i o n o c c u r s between two s t i m u l i t h a t d i f f e r a l o n g one d i m e n s i o n , l e s s w i l l o c c u r i f they d i f f e r a l o n g two (Blough 1972),. T h e r e f o r e , i n t h i s a n a l y s i s , no c o m b i n a t i o n r u l e was assumed, and i n s t e a d a c o n s e r v a t i v e approach was adopted i n which each d i m e n s i o n was c o n s i d e r e d 24 i n d e p e n d e n t l y . The g e n e r a l procedure was t o c a l c u l a t e an i n d e x of g e n e r a l i z a t i o n by measuring t h e d i s t a n c e (on a graph such as F i g u r e 6) from each song t o the n e a r e s t song i n t h e same r e p e r t o i r e ; . The d a t a f o r a l l 150 songs were combined i n t o an observed d i s t r i b u t i o n of " s i m i l a r i t y t o n e a r e s t song", which was compared t o t h e d i s t r i b u t i o n e x p e c t e d i f i n d i v i d u a l b i r d s a c q u i r e d t h e i r songs a t random from t h o s e i n t h e . p o p u l a t i o n . METHODS To s i m p l i f y t h e a n a l y s i s , each song was reduced from a dominant f r e q u e n c y and a p e r i o d of m o d u l a t i o n t o a rank based on each o f t h e s e dimensions* A l t h o u g h t h e e x a c t procedure used t o e s t a b l i s h t h e s e r a n k s d i f f e r e d s l i g h t l y between t h e two d i m e n s i o n s , t h e g o a l i n each case was t o put the data i n a form where a g i v e n d i f f e r e n c e ( i n number of ranks) i m p l i e s a c o n s t a n t amount o f g e n e r a l i z a t i o n . Thus i f a t h r u s h g e n e r a l i z e s a c e r t a i n amount' from a song i n rank 2 of dominant f r e g u e n c y t o a song i n rank 5, i t s h o u l d g e n e r a l i z e an e q u a l amount from t h e l a t t e r song t o one i n rank 8. F i f t e e n r a n k s , based on t h e l o g a r i t h m s o f t h e o r i g i n a l d a t a , were e s t a b l i s h e d a l o n g each d i m e n s i o n . For dominant f r e q u e n c y , t h e minimum v a l u e o f the f i r s t rank was the l o g a r i t h m of the l o w e s t dominant f r e q u e n c y (2510 Hz) o b s e r v e d f o r a l l 150 songs, and t h e maximum v a l u e of the 15th rank was t h e l o g a r i t h m o f t h e h i g h e s t f r e q u e n c y (5854 Hz). The procedure was not q u i t e so s i m p l e f o r p e r i o d of m o d u l a t i o n , s i n c e a few songs (3% i n my 25 sample) are not modulated a t a l l . These were p l a c e d a r b i t r a r i l y i n t h e 15th rank , and t h e o t h e r 14 r a n k s were e s t a b l i s h e d as d e s c r i b e d f o r dominant frequency* The s i m i l a r i t y t o n e a r e s t song, a l o n g each d i m e n s i o n , was c a l c u l a t e d f o r each song i n each o f t h e 30 r e p e r t o i r e s . The procedure used t o c a l c u l a t e t h e expected d i s t r i b u t i o n s of s i m i l a r i t y t o n e a r e s t song was the same f o r both d i m e n s i o n s : A l l 150 songs were p l a c e d i n t o a f r e q u e n c y d i s t r i b u t i o n h a v i n g 15 r a n k s e s t a b l i s h e d as above, a t h e o r e t i c a l d i s t r i b u t i o n was f i t t e d t o t h i s f r e q u e n c y d i s t r i b u t i o n (the o b j e c t b e i n g t o smooth the o r i g i n a l d i s t r i b u t i o n ) , each of the 30 r e p e r t o i r e s was s i m u l a t e d 30 t i m e s by a random s e l e c t i o n o f songs from t h e t h e o r e t i c a l d i s t r i b u t i o n , and the s i m i l a r i t y t o n e a r e s t song was c a l c u l a t e d f o r each song. The t h e o r e t i c a l d i s t r i b u t i o n s were c a l c u l a t e d u s i n g t h e program FBEQ i n t h e U n i v e r s i t y of B. C: Computing C e n t r e L i b r a r y ; T h i s program c a l c u l a t e s goodness o f f i t t o seven t h e o r e t i c a l d i s t r i b u t i o n s : n o r m a l , P o i s s o n , b i n o m i a l , n e g a t i v e b i n o m i a l , gamma, l o g n o r m a l , and e x p o n e n t i a l * The program was a p p l i e d not only t o the o r i g i n a l f r e q u e n c y d i s t r i b u t i o n s but a l s o t o two t r a n s f o r m a t i o n s o f t h e s e , square r o o t and l o g a r i t h m i c . Thus, a t o t a l of 21 goodness o f f i t t e s t s was done f o r each f r e q u e n c y d i s t r i b u t i o n . The t h e o r e t i c a l d i s t r i b u t i o n s used i n t h e f i n a l a n a l y s i s were tho s e t h a t gave the b e s t f i t a c c o r d i n g t o a c h i - s q u a r e c r i t e r i o n . The o r i g i n a l d i s t r i b u t i o n s and t h e t h e o r e t i c a l d i s t r i b u t i o n s t hey were r e p l a c e d by a r e shown i n F i g u r e s 7 and 8,: 26 F i g u r e 7. D i s t r i b u t i o n of v a r i e d t h r u s h songs a l o n g t h e d i m e n s i o n of dominant f r e q u e n c y : observed d i s t r i b u t i o n ( s o l i d l i n e s ) and t h e o r e t i c a l d i s t r i b u t i o n of b e s t f i t (dashed l i n e s ) . T h e o r e t i c a l d i s t r i b u t i o n i s the normal d i s t r i b u t i o n f i t t e d t o sguare r o o t s of o r i g i n a l d a t a * 27 28 F i g u r e 8. D i s t r i b u t i o n of v a r i e d t h r u s h songs a l o n g t h e d i m e n s i o n of p e r i o d of m o d u l a t i o n : observed d i s t r i b u t i o n ( s o l i d l i n e s ) and t h e o r e t i c a l d i s t r i b u t i o n of b e s t f i t (dashed l i n e s ) . ~ T h e o r e t i c a l d i s t r i b u t i o n i s t h e n e g a t i v e b i n o m i a l f i t t e d t o square r o o t s o f o r i g i n a l data.: 29 at LO CO C\J CO CO CD in CO CNJ T v. M UU I) co C\J oil c\J r - T ^ T - ^ CNJ C\J C\J CO AouanbaJj < DC 30 RESULTS For dominant f r e q u e n c y , fewer songs t h a n e x p e c t e d a r e i n the same rank ( F i g * 9 ) , and a c c o r d i n g t o a Kolmogorov-Smirnov one-sample t e s t ( S i e g e l 1956) the d i f f e r e n c e between t h e ob s e r v e d and expected d i s t r i b u t i o n s i s s i g n i f i c a n t (p<0.01). F o r p e r i o d o f m o d u l a t i o n , almost as many songs as e x p e c t e d a r e i n the same r a n k , but f a r fewer t h a n expected d i f f e r by j u s t one rank ( F i g * 1 0 ) , and a g a i n t h e d i f f e r e n c e between t h e d i s t r i b u t i o n s i s s i g n i f i c a n t (p<0.01). DISCUSSION The a n a l y s i s upholds the h y p o t h e s i s t h a t r e p e r t o i r e s a r e o r g a n i z e d t o reduce g e n e r a l i z a t i o n i n b i r d s l i s t e n i n g t o t h e songs. The n e x t s t e p s h o u l d be t o t e s t the assumptions t h a t b i r d s g e n e r a l i z e a l o n g the di m e n s i o n s of dominant f r e q u e n c y and p e r i o d o f m o d u l a t i o n , and t h a t t h e y obey t h e r u l e o f e q u a l g e n e r a l i z a t i o n t o e q u a l r a t i o s . I attempted t o t e s t the f i r s t a ssumption i n a f i e l d e x p e r i m e n t . The g e n e r a l p r o c e d u r e was t o p l a y back songs on t h e t e r r i t o r i e s of i n d i v i d u a l males, h a b i t u a t i n g them t o one song, t h e n measuring t h e i r response t o a n o t h e r song. The p r e d i c t i o n was t h a t the more s i m i l a r t h e two songs, the s m a l l e r would be t h e response t o t h e . second song. T h i s k i n d o f experiment r e q u i r e s t h a t one q u a n t i f y t h e r e s p o n s e s of b i r d s . U n f o r t u n a t e l y , v a r i e d t h r u s h e s were not c o o p e r a t i v e * A l t h o u g h sometimes t h e y responded by s i n g i n g . t y p i c a l l o u d songs and by f l y i n g back and f o r t h o v er t h e l o u d s p e a k e r , two b e h a v i o r a l 31 F i g u r e 9. Observed (shaded) and expected d i s t r i b u t i o n s o f s i m i l a r i t y t o n e a r e s t song a l o n g t h e d i m e n s i o n of dominant f r e q u e n c y i n v a r i e d t h r u s h songs; The d i f f e r e n c e i s s i g n i f i c a n t , p<0.01 (Kolmogorov-Smirnov one-sample t e s t : D-Max=0.175; N=150). 8CH 7CH 60-50-40-32 30-20-104 • J A M n lid 0 1 2 3 4 6 8 Distance to Nearest Song (number of ranks) 33 F i g u r e 10. Observed (shaded) and expected d i s t r i b u t i o n s o f s i m i l a r i t y to n e a r e s t song a l o n g t h e di m e n s i o n of p e r i o d o f m o d u l a t i o n i n v a r i e d t h r u s h songs. The d i f f e r e n c e i s s i g n i f i c a n t , p<0.01 (Kolmogorov-Smirnov one-sample t e s t : D-Max=0.167; N=150). 60H 50H cn c o CO 40 o 30M CD .Q £ = 3 20 10-0 1 8 9-15 Distance to Nearest Song (number of ranks) 35 pattern's t h a t can be measured, more o f t e n they perched near t h e l o u d s p e a k e r , s i n g i n g q u i e t l y and c o n t i n u o u s l y , o r s i m p l y f l e w away from the v i c i n i t y o f the speaker and d i d not r e t u r n . At the same t i m e I was s t u d y i n g v a r i e d t h r u s h e s , I a l s o s t u d i e d r u f ous-'sided towhees ( P i p i l o e r y t h r g p h t h a l m u s ) , and t h e y proved more s u i t a b l e f o r p l a y b a c k e x p e r i m e n t s . I n s o u t h w e s t e r n B r i t i s h C o l u m b i a , towhees have very s i m p l e songs. Except f o r a b r i e f i n t r o d u c t o r y p h r a s e , which does not always o c c u r , t h e songs a r e monotonous t r i l l s , composed of a s i n g l e , r e p e a t e d s y l l a b l e ( F i g . 11a). A l t h o u g h the d e t a i l e d c o n f i g u r a t i o n of s y l l a b l e s v a r i e s between songs, the songs can be reduced t o a s i n g l e d i m e n s i o n , s y l l a b l e l e n g t h ( F i g * 11b), which ranged from y 6 t o 253 ms i n the songs I r e c o r d e d . The r e s u l t s of an a n a l y s i s s i m i l a r t o t h e one done on v a r i e d t h r u s h e s s u g g e s t s t h a t r u f o u s - s i d e d towhees a l s o have r e p e r t o i r e s made up o f songs l e s s s i m i l a r t o each o t h e r t h a n i f t h e y had been t a k e n a t randpm from the songs i n t h e p o p u l a t i o n ( F i g . 12). A l t h o u g h the r e s u l t s a r e not s t a t i s t i c a l l y s i g n i f i c a n t (0. 20>p>0. 15) , t h e y a r e c l o s e enough t o w a r r a n t r e p e a t i n g the a n a l y s i s w i t h a l a r g e r sample s i z e . I d i d a f i e l d e x p e r i m e n t , s i m i l a r i n d e s i g n t o t h e o n e ^ attempted w i t h v a r i e d t h r u s h e s , t o determine whether towhees g e n e r a l i z e a l o n g t h e dimension of s y l l a b l e l e n g t h . Males p r e d i c t a b l y sang and f l e w o ver t h e l o u d s p e a k e r and u s u a l l y h a b i t u a t e d t o the f i r s t song d u r i n g 25 minutes of p l a y b a c k ( F i g ; 13). Even so, t h e e x p e r i m e n t — ^ - a n e x c e e d i n g l y t i m e -consuming one-'—was i n t h e end a f a i l u r e , f o r r e a s o n s w o r t h w h i l e t o r e l a t e * 36 F i g u r e 11 . a. Sonagram o f a r u f o u s - s i d e d towhee song, showing i n t r o d u c t o r y phrase and t r i l l o f s i n g l e , r e p e a t e d s y l l a b l e . S y l l a b l e l e n g t h e q u a l s the i n t e r v a l (measured i n seconds) from t h e b e g i n n i n g o f one s y l l a b l e t o the b e g i n n i n g o f t h e next;* Only t h e f i r s t p a r t o f t h e song i s shown, but remainder i s j u s t a c o n t i n u a t i o n o f t h e t r i l l , b. Sonagrams of r u f o u s - s i d e d towhee songs, showing v a r i a t i o n s i n s y l l a b l e l e n g t h . From l e f t t o r i g h t : songs A, B, C, and D. 38 F i g u r e 12. Observed (shaded) and expected d i s t r i b u t i o n s o f s i m i l a r i t y to n e a r e s t song a l o n g the di m e n s i o n o f s y l l a b l e l e n g t h i n r u f o u s - s i d e d towhee songs. The d i f f e r e n c e i s not s i g n i f i c a n t , 0.20>p>0.15 (Kolmogorov-Smirnov one-sample t e s t : D-Max=0.153; N=53). 39 CO CD c o CO CD _Q E 20-18-16-14-12-10 8-6-4-2-0 1 8 Distance to Nearest Song (number of ranks) 40 F i g u r e 13. H a b i t u a t i o n o f male r u f o u s - s i d e d towhees t o pla y b a c k of songs on t h e i r t e r r i t o r i e s . P l a y b a c k was done f o r 25 minutes a c c o r d i n g t o t h e methods d e s c r i b e d on p. 17. R e s u l t s shown a r e averages f o r 48 i n d i v i d u a l s ; 0 1 8 CO CO c o CO 6 4-2-1 • « e « ° * « © c © e © © © 13 Minute 25 24 D C CO -t—' CE 14 © © e©°© © © © © © o "l3 25 Minute 42 The experiment was designed so t h a t the b i r d s h a b i t u a t e d t o a d i f f e r e n t song i n each of t h e f o u r t r e a t m e n t s (the songs a r e shown i n F i g u r e 11b), then responded f o r 5 minutes t o the same song (song A i n F i g u r e 11b) i n a l l t r e a t m e n t s . Thus, i n one t r e a t m e n t , b i r d s h a b i t u a t e d t o the same song (song A) t h e y l a t e r responded t o , i n the second t r e a t m e n t a d i f f e r e n t song but w i t h t h e same s y l l a b l e l e n g t h (song B ) , i n t h e t h i r d a song w i t h a d i f f e r e n t s y l l a b l e l e n g t h (song C ) , and i n t h e f o u r t h a song w i t h an even more d i f f e r e n t s y l l a b l e l e n g t h (song D). S u p e r f i c i a l l y , t h e r e s u l t s appear t o be as p r e d i c t e d ( F i g * 1 4 ) , but a c c o r d i n g t o an a n a l y s i s of v a r i a n c e t h e r e were no s i g n i f i c a n t d i f f e r e n c e s among t h e t r e a t m e n t s (p>0.50). A l s o , an a n a l y s i s o f c o v a r i a n c e showed t h a t t h e b i r d s d i d 1 n o t respond e q u a l l y o r h a b i t u a t e a t the same r a t e t o t h e f o u r songs ( F i g . 1 5). T h i s c o u l d e x p l a i n some of the d i f f e r e n c e s i n F i g u r e 14, s i n c e i t i s known from o t h e r s t u d i e s (see Hinde 1970b) t h a t an a n i m a l ' s r e s p o n s i v e n e s s t o one s t i m u l u s can a f f e c t i t s subsequent r e s p o n s i v e n e s s t o a n o t h e r s t i m u l u s . T h e r e f o r e , i n t h i s experiment t h e response t o the second song was not n e c e s s a r i l y j u s t a f u n c t i o n o f t h e amount of g e n e r a l i z a t i o n between t h e two songs. T h i s s o u r c e o f a m b i g u i t y c o u l d be e l i m i n a t e d by a more e l a b o r a t e e x p e r i m e n t a l d e s i g n . I f t h e f i r s t song was the same f o r a l l t r e a t m e n t s , and o n l y t h e second song v a r i e d , the response t o t h e second song would be a f u n c t i o n of t h e amount of g e n e r a l i z a t i o n between the two songs and o f t h e re s p o n s e t h e second song would have e l i c i t e d i f t h e b i r d had not a l r e a d y h a b i t u a t e d t o another song. Any 43 F i g u r e 14- Response of r u f o u s - s i d e d towhees t o p l a y b a c k o f song A a f t e r h a v i n g h a b i t u a t e d t o one of f o u r songs. A, B, C, or D (see F i g . 11b). L e t t e r s on t h e graphs r e f e r to the f o u r songs* R e s u l t s shown are averages f o r 12 i n d i v i d u a l s i n each t r e a t m e n t . A n a l y s e s of v a r i a n c e based on t h e o v e r a l l means and v a r i a n c e s f o r t h e 5 minute p e r i o d showed no s i g n i f i c a n t d i f f e r e n c e s among the t r e a t m e n t s f o r e i t h e r measure o f response (p>0.50 i n b o t h c a s e s ) * Minute 45 F i g u r e 15,. R e s p o n s i v e n e s s and r a t e o f h a b i t u a t i o n o f r u f o u s -s i d e d towhees t o songs A, B, C, and D. The r e g r e s s i o n l i n e s were c a l c u l a t e d f o r t h e p e r i o d s d u r i n g which the o v e r a l l r a t e s of response were d e c l i n i n g (see F i g . 13). I n t h e upper graph, t h e s l o p e s o f the l i n e s a r e n o t s i g n i f i c a n t l y d i f f e r e n t (p>0.20) but the Y - i n t e r c e p t s a r e s i g n i f i c a n t l y d i f f e r e n t (p<0.0001); i n t h e lower g r a p h , the s l o p e s are s i g n i f i c a n t l y d i f f e r e n t (p<0.02); 9H Minute 47 d i f f e r e n c e s i n t h e l a t t e r c o u l d be measured i n s e p a r a t e e x p e r i m e n t s and t h e r e s u l t s used as a c o n t r o l . These e x p e r i m e n t s would have t o be r e p e a t e d s e v e r a l t i m e s , w i t h d i f f e r e n t songs each time (to c o n t r o l f o r o t h e r , unmeasured d i f f e r e n c e s among s o n g s ) , j u s t t o e s t a b l i s h t h a t b i r d s g e n e r a l i z e a l o n g a p a r t i c u l a r d i m e n s i o n , a t a s k t h a t c o u l d t a k e s e v e r a l y e a r s , s i n c e b i r d s are n o r m a l l y r e s p o n s i v e t o p l a y b a c k o n l y f o r a l i m i t e d time d u r i n g t h e b r e e d i n g season ( f o r a d e t a i l e d s t u d y see P e t r i n o v i c h e t a l . 1976). I t might t a k e much l o n g e r t o d e t e r m i n e t h e r u l e s g o v e r n i n g g e n e r a l i z a t i o n a l o n g t h a t dimension* One's t i m e would s u r e l y be b e t t e r s p e n t working w i t h b i r d s i n the l a b o r a t o r y where g e n e r a l i z a t i o n c o u l d be s t u d i e d i n d e t a i l u s i n g p r o c e d u r e s s t a n d a r d i n e x p e r i m e n t a l p s y c h o l o g y (see e.g. MacKintosh 1974, chap. 10). There i s s t i l l a n o t h e r assumption c r i t i c a l t o t h e i n t e r p r e t a t i o n o f t h e r e s u l t s i n t h i s c h a p t e r . T h i s a s s u m p t i o n f o l l o w s from the f a c t t h a t song has a d e v e l o p m e n t a l h i s t o r y i n i n d i v i d u a l b i r d s . A l t h o u g h i t i s not known how v a r i e d t h r u s h e s a c q u i r e t h e i r songs, i f they are a n y t h i n g l i k e c h a f f i n c h e s and white-crowned sparrows ( Z o n o t r i c h i a leucophry_s) , t h e two s p e c i e s i n which the development o f song has been s t u d i e d most i n t e n s i v e l y (see e*g. Thorpe 1958; M a r l e r 1970), a d u l t b i r d s s i n g songs t h a t t h e y l e a r n e d as j u v e n i l e s . The g e n e r a l q u e s t i o n asked a t the b e g i n n i n g o f t h i s c h a p t e r can now be r e p h r a s e d : Are t h e songs i n i n d i v i d u a l r e p e r t o i r e s more d i s s i m i l a r t o each o t h e r than would be expected i f they were a c q u i r e d at random from a l l t h e songs t h a t were i n t h e 48 p o p u l a t i o n when the b i r d s a c t u a l l y l e a r n e d the songs? The a n a l y s i s assumes t h a t t h e d i s t r i b u t i o n of songs a l o n g t h e two d i m e n s i o n s was the same when the b i r d s l e a r n e d t h e i r songs as when I r e c o r d e d my sample. Of c o u r s e , t h e r e i s no way o f t e s t i n g t h i s assumption f o r t h e p r e s e n t a n a l y s i s , but i t s h o u l d not be i g n o r e d i n f u t u r e s t u d i e s * T u r n i n g now t o q u e s t i o n s . o f a d a p t i v e s i g n i f i c a n c e , i f the t h e o r y summarized i n C hapter I i s c o r r e c t , males t h a t have r e p e r t o i r e s o f d i s s i m i l a r songs may be more, e f f e c t i v e a t r e p e l l i n g p o t e n t i a l i n t r u d e r s o r a t a t t r a c t i n g and s t i m u l a t i n g f e m a l e s . These i d e a s c o u l d be t e s t e d e x p e r i m e n t a l l y . The i d e a t h a t r e p e r t o i r e s a i d males i n r e p e l l i n g p o t e n t i a l i n t r u d e r s has been developed most f u l l y by Krebs (1977b) i n h i s s o - c a l l e d "Beau Ges t e " h y p o t h e s i s . G i v e n t h a t t h e s u i t a b i l i t y o f an a r e a f o r b r e e d i n g d e c l i n e s as t h e d e n s i t y o f r e s i d e n t s i n c r e a s e s , i t i s t o t h e advantage of s e t t l e r s to e s t i m a t e the number of b i r d s a l r e a d y p r e s e n t . A c c o r d i n g t o t h e h y p o t h e s i s t h e y do t h i s by l i s t e n i n g t o t h e song of t e r r i t o r i a l males* By t h e same argument, i t i s t o the advantage o f r e s i d e n t s t o appear more common tha n t h e y a c t u a l l y a r e , so t h a t s e t t l e r s w i l l move on. Hence the e v o l u t i o n " o f r e p e r t o i r e s : each male, by s i n g i n g s e v e r a l d i f f e r e n t song t y p e s , c r e a t e s t h e i m p r e s s i o n t h a t he i s s e v e r a l d i f f e r e n t i n d i v i d u a l s . The v a l u e o f t h i s h y p o t h e s i s i s t h a t i t p r e d i c t s s e v e r a l b e h a v i o r a l p a t t e r n s t h a t might be a s s o c i a t e d w i t h r e p e r t o i r e s * One i s t h a t " w i t h i n r e p e r t o i r e v a r i a b i l i t y i n song t y p e s s h o u l d be a t l e a s t as g r e a t as between r e p e r t o i r e v a r i a b i l i t y " (Krebs 1977b). These two s o u r c e s of v a r i a b i l i t y w i l l be e q u a l when i n d i v i d u a l s s i n g a random sample of songs from t h e p o p u l a t i o n . I f r e s i d e n t b i r d s and i n t r u d e r s a r e p l a y i n g a d e c e p t i o n and d e t e c t i o n game, t h i s would seem a good s t r a t e g y f o r r e s i d e n t s ; t h e r e would be no f i x e d p a t t e r n of songs w i t h i n r e p e r t o i r e s and t h e r e f o r e no code f o r i n t r u d e r s t o break. But Krebs (1977b) has gone on t o s u g g e s t t h a t h a b i t u a t i o n i s the p r o c e s s by which i n t r u d e r s " c o u n t " t h e number of r e s i d e n t s , and i f t h i s i s so a g r e a t e r v a r i a b i l i t y o f song t y p e s w i t h i n r e p e r t o i r e s than between r e p e r t o i r e s might be a b e t t e r s t r a t e g y , . T h i s i s j u s t a n o t h e r way o f s a y i n g t h a t b i r d s might be e x p e c t e d t o have r e p e r t o i r e s o r g a n i z e d so as t o reduce g e n e r a l i z a t i o n by i n t r u d e r s ; T h e r e f o r e , the p r e d i c t e d v a r i a b i l i t y of songs w i t h i n r e p e r t o i r e s compared to t h e i r v a r i a b i l i t y between r e p e r t o i r e s depends on how the h y p o t h e s i s i s s t a t e d , i n p a r t i c u l a r on whether h a b i t u a t i o n i s c o n s i d e r e d t h e p r o c e s s by which i n t r u d e r s e s t i m a t e th e d e n s i t y o f s i n g i n g males. Krebs (1977a) has found no c l e a r p a t t e r n i n h i s work w i t h g r e a t t i t s . I f t h e songs, which a r e r e l a t i v e l y s i m p l e , c o n s i s t i n g o f a s i n g l e phrase r e p e a t e d s e v e r a l t i m e s , are d e s c r i b e d by the l e n g t h of t h e p h r a s e , th e v a r i a b i l i t y of songs w i t h i n r e p e r t o i r e s i s g r e a t e r than t h a t between r e p e r t o i r e s . But i f t h e songs are c l a s s i f i e d by the form of t h e n o t e s w i t h i n the p h r a s e s , the v a r i a b i l i t y w i t h i n r e p e r t o i r e s i s no d i f f e r e n t from t h a t between r e p e r t o i r e s . Thus, no c o n c l u s i o n can be drawn about whether the r e p e r t o i r e s o f g r e a t t i t s a re o r g a n i z e d so as t o reduce g e n e r a l i z a t i o n . I f r e p e r t o i r e s have e v o l v e d t o d e c e i v e i n t r u d e r s , males s h o u l d behave i n ways t h a t uphold t h e d e c e p t i o n . G r e a t t i t s 50 seem t o , f o r example, by s i n g i n g t h e i r songs i n bouts o f a s i n g l e t y p e (AAA.*.EBB--.) and by c h a n g i n g p e r c h e s a t the same t i m e t h e y change songs (Krebs e t a l . 1978)* V a r i e d t h r u s h e s , on the o t h e r hand, s i n g one song type a f t e r a n o t h e r from prominent p e r c h e s , a p r a c t i c e t h a t would be u n l i k e l y t o f o o l even the l e a s t p e r c e p t i v e i n t r u d e r . I t would be i n t e r e s t i n g t o f i n d o ut whether r e p e r t o i r e s a i d male v a r i e d t h r u s h e s i n r e p e l l i n g i n t r u d e r s ; i f so, t h e e x p l a n a t i o n would s u r e l y n o t be a Beau Geste d e c e p t i o n * One f i n a l t h o u g h t about our e f f o r t s t o u n d e r s t a n d t h e a d a p t i v e s i g n i f i c a n c e of r e p e r t o i r e s and t h e b e h a v i o r a l p a t t e r n s a s s o c i a t e d w i t h them. We s h o u l d t a k e Bertram's (1976) a d v i c e , p r o f f e r e d e s p e c i a l l y , f o r the s t u d y o f b e h a v i o r a l p a t t e r n s used i n s o c i a l i n t e r a c t i o n s , and attempt t o i d e n t i f y and measure a l l t h e s e l e c t i v e p r e s s u r e s o p e r a t i n g on t h e s e t r a i t s , not j u s t the p o s i t i v e ones. One example w i l l i l l u s t r a t e t h e p o i n t . I n g r e a t t i t s , males are known t o b e n e f i t because t h e i r r e p e r t o i r e s aire more e f f e c t i v e than s i n g l e songs a t r e p e l l i n g p o t e n t i a l i n t r u d e r s (Krebs e t a l . 1978). I f Krebs (1977b) i s c o r r e c t t h a t r e p e r t o i r e s r e d uce th e r a t e at which p o t e n t i a l i n t r u d e r s h a b i t u a t e t o song, then th e same s h o u l d be t r u e f o r t e r r i t o r i a l n e i g h b o r s . I f s o , t h e a g g r e s s i v e i n t e r a c t i o n s by which a d j a c e n t males e s t a b l i s h t h e i r t e r r i t o r i e s c o u l d be p r o l o n g e d beyond the t i m e ' t h a t would o t h e r w i s e be n e c e s s a r y f o r t e r r i t o r i a l e s t a b l i s h m e n t . T h i s would have o b v i o u s c o s t s i n t i m e , energy, and r i s k of i n j u r y (and might a l s o have some s u b t l e b e n e f i t s ) . I t would be f a t a l t o t h e Beau Geste h y p o t h e s i s i f t h e s e c o s t s outweighed t h e 5 1 b e n e f i t s r e s u l t i n g i n t r u d e r s * from the more e f f e c t i v e r e p u l s i o n of 52 CHAPTER I I I CONTROL OF SINGING Samuel F. Rathbun, who i n t r o d u c e d A r t h u r C l e v e l a n d B e n t , t h e g r e a t monographer of North American b i r d s , t o v a r i e d t h r u s h e s (Bent 1949), s t u d i e d t h e i r s i n g i n g b e h a v i o r e a r l y i n t h i s c e n t u r y . U s i n g o n l y h i s e a r s and a p e n c i l and paper, he r e c o r d e d sequences such as the f o l l o w i n g , i n which each song i s d e s c r i b e d by i t s p i t c h : "High l o w — medium l o w — - v e r y l o w , t h i s f o l l o w e d by a h a r s h n o t e ; h i g h very h i g h l o w — -medium h i g h l ow, then a pause as i f t h e b i r d was r e f l e c t i n g on i t s performance; h i g h — - m e d i u m medium—-low..." Rathbun shared h i s n o t e s w i t h Bent* who ob s e r v e d t h a t the. songs w i t h i n t h e s e sequences were not d e l i v e r e d i n a f i x e d o r d e r , and a l s o t h a t some songs were used more commonly than o t h e r s . F o r example, i n one sequence of 56 songs, t h e most common song was sung 21 t i m e s , the r a r e s t o n l y once (Bent 1949). T h i s d e s c r i p t i o n , a c c u r a t e i n s o f a r as i t goes, i s the most d e t a i l e d account I have been a b l e t o f i n d o f t h e s i n g i n g b e h a v i o r of v a r i e d t h r u s h e s * I n t h i s c h a p t e r , I e l a b o r a t e t h i s d e s c r i p t i o n , t h a n k s i n l a r g e p a r t t o t e c h n o l o g y and a n a l y t i c t e c h n i q u e s developed l o n g a f t e r Rathbun made h i s o b s e r v a t i o n s . I r e c o r d e d sequences of up t o 110 sonqs from each o f t h e 30 males and determined the dominant f r e q u e n c y and p e r i o d o f m o d u l a t i o n o f each song t y p e , t h e o r d e r i n which the songs were sung, and t h e l e n g t h s o f t h e pauses between songs. These were t h e b a s i c d a t a used f o r 53 f u r t h e r a n a l y s i s . My d e s c r i p t i o n . o f s i n g i n g b e h a v i o r i s - d e v e l o p e d - t h r o u g h an i n t e r p l a y w i t h t h e o r y . A c c o r d i n g t o Weizenbaum (1976), a t h e o r y i s " l i k e a map *.. pf a p a r t i a l l y e x p l o r e d t e r r i t o r y * I t s f u n c t i o n i s o f t e n h e u r i s t i c , t h a t i s , t o .guide t h e e x p l o r e r i n f u r t h e r d i s c o v e r y * " I n t h i s c h a p t e r , I b e g i n w i t h a ve r y s k e t c h y map and e l a b o r a t e i t t h r o u g h a s e r i e s of e x p l o r a t i o n s * The f i n a l v e r s i o n i s f a r from complete, but i t p o i n t s o u t s e v e r a l p r o m i s i n g a r e a s f o r f u r t h e r i n v e s t i g a t i o n . The p r o c e s s by which I e l a b o r a t e the map, or. t h e o r y , i n v o l v e s b u i l d i n g models t h a t s a t i s f y i t . The b e h a v i o r o f t h e models c o n s t i t u t e s p r e d i c t i o n s , w h i c h - a r e then checked a g a i n s t t h e b e h a v i o r of r e a l b i r d s . I f t h e two do not match, t h e t h e o r y i s a l t e r e d a c c o r d i n g l y , new models b u i l t ; and f u r t h e r p r e d i c t i o n s made. I t was c o n v e n i e n t t o w r i t e the models i n FORTRAN and r u n them on a computer, but they a r e r e p r e s e n t e d p i c t o r i a l l y i n t h i s c h a p t e r , w i t h l i t t l e l o s s of i n f o r m a t i o n . INTERNAL AND EXTERNAL CONTROL A c c o r d i n g t o Hinde (1970a) , t h e r e a r e t h r e e p o s s i b l e s o u r c e s o f c o n t r o l over the p a t t e r n i n g of complex muscular movements s u c h as t h e ones r e s p o n s i b l e f o r song: " e n v i r o n m e n t a l s t i m u l i p r o p r i o c e p t i v e s t i m u l i , and c o o r d i n a t i n g mechanisms w i t h i n the c e n t r a l nervous system i t s e l f . " Suppose an i s o l a t e d v a r i e d t h r u s h i s s i n g i n g i n a room t h a t o t h e r w i s e has a c o n s t a n t environment; The p a t t e r n i n g o f t h e b i r d ' s songs may be c o n t r o l l e d by i t s c e n t r a l nervous system, by 54 p r o p r i o c e p t i v e f e e d b a c k , and perhaps a l s o by an e n v i r o n m e n t a l s t i m u l u s , t h e sound o f i t s songs. I have not attempted i n t h i s s t u d y t o s e p a r a t e t h e s e p o s s i b l e s o u r c e s o f c o n t r o l but have i n s t e a d combined them i n t o a s i n g l e c a t e g o r y of " i n t e r n a l " c o n t r o l . I n n a t u r e , a p o t e n t i a l l y i m p o r t a n t s o u r c e of e x t e r n a l c o n t r o l i s the sound of o t h e r b i r d s s i n g i n g . The d e n s i t y o f v a r i e d t h r u s h e s on my study a r e a s was l o w , s i n g i n g males u s u a l l y 300 m o r more a p a r t , which seems t o be t y p i c a l f o r t h e s p e c i e s (Bent 1949), but i n some p l a c e s I c o u l d s t i l l h e a r as many as f o u r o r f i v e i n d i v i d u a l s s i n g i n g at the same t i m e . Presumably the b i r d s c o u l d a l s o hear each o t h e r . I n s e v e r a l o t h e r s p e c i e s , males a r e known t o p r a c t i c e "matched c o u n t e r s i n g i n g " (Krebs 1977b), t h a t i s , t o s i n g s i m i l a r song t y p e s back and f o r t h a t each other,. L o n g - b i l l e d marsh wrens (Telmatodytes p a l u s t r i s ) , f o r example, have very l a r g e r e p e r t o i r e s of up t o 114 d i f f e r e n t song t y p e s , which t h e y s i n g as v a r i e d t h r u s h e s do t h e i r s m a l l e r r e p e r t o i r e s , r a r e l y r e p e a t i n g a song type i m m e d i a t e l y (Verner 1975). By r e c o r d i n g b i r d s s i m u l t a n e o u s l y on the same tap e r e c o r d e r , Verner (1975) found t h a t t e r r i t o r i a l n e i g h b o r s match songs about 30% o f t h e time,. I was not a b l e t o use t h i s t e c h n i q u e t o s t u d y i n t e r a c t i o n s between male v a r i e d t h r u s h e s , however, because they a r e t o o w i d e l y spaced. Matched c o u n t e r s i n g i n g has a l s o been s t u d i e d e x p e r i m e n t a l l y . Male c a r d i n a l s (Richmondena c a r d i n a l i s ) (Lemon 1968), c h a f f i n c h e s (Hinde 1958), and g r e a t t i t s (Krebs 1977a) respond t o r e p e a t e d playback of one o f t h e i r own songs by 55 matching i t , and t h i s has t h e e f f e c t o f them s i n g i n g r e l a t i v e l y more of t h i s song and r e l a t i v e l y l e s s of t h e o t h e r songs i n t h e i r r e p e r t o i r e s . The same i s t r u e o f European b l a c k b i r d s (Turdus m e r u l a ) , whose i n d i v i d u a l songs, as d e f i n e d by Todt (1975), a r e o r g a n i z e d i n a manner r a t h e r s i m i l a r t o whole sequences of songs d e l i v e r e d by v a r i e d t h r u s h e s . They c o n s i s t o f a number o f d i f f e r e n t "song p h r a s e s " , each about t h e same l e n g t h as a v a r i e d t h r u s h song. N o r m a l l y a b l a c k b i r d s i n g s one phrase, pauses f o r s e v e r a l seconds, t h e n s i n g s a d i f f e r e n t p h r a se. But t h i s p a t t e r n can be d i s r u p t e d by p l a y b a c k * I f , f o r example, a b i r d i s p l a y e d t h e most common phrase i n i t s r e p e r t o i r e , i t responds not o n l y by s i n g i n g t h e phrase more o f t e n but a l s o by s i n g i n g i t more o f t e n t w i c e i n s u c c e s s i o n . Todt (1975) has developed a t h e o r y o f c o n t r o l t h a t a c c o u n t s f o r t h e s e r e s u l t s . My a t t e m p t s t o do t h i s k i n d o f experiment i n t h e f i e l d w i t h v a r i e d t h r u s h e s were met w i t h l i t t l e s u c c e s s , because males d i d not u s u a l l y respond t o p l a y b a c k o f songs on t h e i r t e r r i t o r i e s by s i n g i n g l o u d songs (see p,» 3 0 ) , I was not a b l e t o r e c o r d any sequences of l o u d songs g i v e n by males i n response t o p l a y b a c k of one of t h e i r own songs and o n l y two sequences g i v e n i n response t o a n e i g h b o r ' s song. I n t h e s e two c a s e s , t h e r e l a t i v e f r e q u e n c i e s o f the songs were no d i f f e r e n t from t h o s e i n c o n t r o l sequences (Table I and I I ) ; l i k e w i s e , t h e o r d e r i n which t h e songs were sung was not d i f f e r e n t i n any o b v i o u s way. Thus, v a r i e d t h r u s h e s do not seem t o change t h e o r g a n i z a t i o n of t h e i r l o u d s i n g i n g i n response t o t h e s t i m u l u s o f a s i n g l e song t y p e b r o a d c a s t r e p e a t e d l y * I t r i e d s e v e r a l 56 Table I . F r e q u e n c i e s (and p r o p o r t i o n s ) o f d i f f e r e n t songs sung by the t h r u s h named M i c k i n r e s p o n s e t o p l a y b a c k arid i n c o n t r o l sequence. C o n t r o l sequence r e c o r d e d j u s t b e f o r e p l a y b a c k sequence. Frequency ( P r o p o r t i o n )  Song A Song B Song.C Song D Song E Song F T o t a l C o n t r o l 13(.23) 6 ( . l l ) 17(.30) Playback 6(.17) 4 ( . l l ) 9(.26) 0(.00) 2 (.06), 6 (.11) 15 (.26) 57 5(.14) 9(.26) 35 57 Table II. Frequencies (and proportions) of d i f f e r e n t songs sung by the thrush named Ringo i n response to playback and i n control sequence. Control sequence recorded just before playback sequence. Frequency (Proportion)  Song A Song B Song C Song D Song E Song F Total Control 10 (.17) 12 (. 20) 7 (.12) 12 (.20) 9 (.15) 9 (.15) 59 Playback 8(.17) 10(.21) 7(.15) 9(.19) 6(.13) 8(.17) 48 5 8 t i m e s t o r e p e a t t h e experiment w i t h a more r e a l i s t i c s timulus., a seguence r e c o r d e d from a b i r d , but no males responded by s i n g i n g l o u d songs. Although I admit the p o s s i b i l i t y t h a t f u r t h e r e x p e r i m e n t s w i l l show t h a t v a r i e d t h r u s h e s a f f e c t each o t h e r ' s s i n g i n g b e h a v i o r , i n the absense of such e v i d e n c e I s h a l l assume t h a t e x t e r n a l c o n t r o l i s u n i m p o r t a n t . The t h e o r y I d e v e l o p i n . t h i s c h a p t e r i s an e x p l a n a t i o n o n l y o f the i n t e r n a l c o n t r o l o f s i n g i n g * A FIRST MODEL I f we b e g i n by c o n s i d e r i n g o n l y t h e o r d e r i n which songs are sung (Table I I I ) , d i s r e g a r d i n g f o r t h e moment t h e i n t e r v a l s between songs and t h e c h a r a c t e r i s t i c s of the d i f f e r e n t song t y p e s , we can see t h r e e o b v i o u s p a t t e r n s : 1) Once a song type has been sung, i t i s not u s u a l l y r e p e a t e d i m m e d i a t e l y , 2 ) t h e number o f o t h e r songs sung b e f o r e i t i s r e p e a t e d i s not always t h e same, and 3) some song t y p e s a r e more common than o t h e r s . These p a t t e r n s can be produced by a v e r y s i m p l e model. Imagine t h a t i n the b i r d ' s b r a i n t h e r e i s a c o n t r o l c e n t e r i n charge o f si n g i n g . . I t f e e d s m o t i v a t i o n a l i m p u l s e s t o u n i t s i n charge of the d i f f e r e n t song t y p e s . A song can be sung o n l y when t h e m o t i v a t i o n a l s t a t e of i t s c o n t r o l l i n g u n i t r e a c h e s a t h r e s h o l d , and once the song has been sung, t h e u n i t i n h i b i t s i t s e l f , d r o p p i n g a b r u p t l y t o a l o w e r m o t i v a t i o n a l s t a t e , and then g r a d u a l l y r e c o v e r s , under t h e i n f l u e n c e o f t h e c o n t r o l c e n t e r , toward i t s t h r e s h o l d . The system i t s e l f i s shown i n Table I I I . Sequences of songs rec o r d e d from the thrushes named Buddy, Chuck, and Rod. Use of the same l e t t e r s to represent songs i n the r e p e r t o i r e s o f d i f f e r e n t i n d i v i d u a l s does not imply t h a t the songs are s i m i l a r . Buddy, sequence 4: BABCBABCABCBABCABBCBACBABCBABCABCABBCABBACBCBCBA Chuck, sequence 3: EABDECBDFDCBDEBDACFEDEDBADCBDEABDCBDFDCBFDABDCB Rod, sequence 1: CBCBACBACBADBCABCBCADBACBCBADCBCACBACBACBCA 60 F i g u r e 16, and i t s b e h a v i o r i s diagrammed i n F i g u r e 17.. I t makes no d i f f e r e n c e t o t h e b e h a v i o r o f the system whether a l l song u n i t s have t h e same t h r e s h o l d and d i f f e r o n l y i n t h e amount t h e y i n h i b i t themselves ( F i g . 17a) o r whether t h e y i n h i b i t t h emselves the same amount and d i f f e r o n l y i n t h e i r t h r e s h o l d s ( F i g * 17b),; But s i n c e t h e former i s e a s i e r to r e p r e s e n t g r a p h i c a l l y , a l l subsequent f i g u r e s w i l l show t h e d i f f e r e n t song u n i t s h aving t h e same thre s h o l d , ; The computer models a r e o n l y s l i g h t l y more complex than F i g u r e 17a. A f t e r a song has been produced and t h e a p p r o p r i a t e song u n i t i n h i b i t e d , t h e computer d e c i d e s which song w i l l be sung n e x t . I t does t h i s i n a way analogous t o drawing t h e d i a g o n a l r e c o v e r y l i n e s on a p i c t o r i a l model such as F i g u r e 17a, e x c e p t t h a t i t i n t r o d u c e s a s m a l l amount of random v a r i a b i l i t y i n t h e r a t e o f r e c o v e r y . I t c a l c u l a t e s how l o n g ( i n a r b i t r a r y u n i t s ) each song u n i t w i l l t a k e t o r e c o v e r t o t h e t h r e s h o l d , t h e n adds a n o r m a l l y d i s t r i b u t e d random v a r i a b l e w i t h a mean e q u a l t o z e r o and a s t a n d a r d d e v i a t i o n e q u a l t o 1% of the o r i g i n a l r e c o v e r y t i m e . The song u n i t t h a t ends up h a v i n g t h e s h o r t e s t r e c o v e r y t i m e produces the n e x t song* The purpose of the random v a r i a b l e i s t o reduce the chance o f two o r more song u n i t s r e a c h i n g the t h r e s h o l d at t h e same t i m e , but i f t h i s s h o u l d happen anyway, the computer i s i n s t r u c t e d t o choose one o f t h e songs at random. A s i m u l a t i o n b e g i n s w i t h a l l t he song u n i t s a t t h r e s h o l d . The computer chooses from them a t random and then goes on t o produce a sequence o f 510 songs. Only the f i n a l 500 songs a r e used f o r a n a l y s i s * The magnitudes of i n h i b i t i o n (measured i n a r b i t r a r y u n i t s ) 61 F i g u r e 16. A f i r s t model of t h e c o n t r o l of s i n g i n g i n v a r i e d t h r u s h e s . Dashed arrows r e p r e s e n t f a c i l i t a t i o n ; s o l i d l i n e s e n d i n g i n open c i r c l e s r e p r e s e n t i n h i b i t i o n . See t e x t f o r d e t a i l s . 63 F i g u r e 17 . B e h a v i o r of f i r s t model o f c o n t r o l . M o t i v a t i o n a l s t a t e i s r e p r e s e n t e d on v e r t i c a l a x i s , i n c r e a s i n g toward t h e t h r e s h o l d ; sequence o f songs i s shown a l o n g h o r i z o n t a l a x i s . When the m o t i v a t i o n a l s t a t e of a song u n i t r e a c h e s t h e t h r e s h o l d , the u n i t produces a song, then i n h i b i t s i t s e l f , d r o p p i n g i m m e d i a t e l y t o a l o w e r m o t i v a t i o n a l s t a t e * a. A l l song u n i t s have t h e same t h r e s h o l d but i n h i b i t t h e m s e l v e s t o d i f f e r e n t l e v e l s o f m o t i v a t i o n . b. Song u n i t s have d i f f e r e n t t h r e s h o l d s but i n h i b i t t h e m s e l v e s t o the same l e v e l of m o t i v a t i o n * The b e h a v i o r o f t h i s system i s i d e n t i c a l t o t h a t o f t h e one shown i n p a r t a. A THRESHOLDS A B C 7!7p B A A B A7\ \/ y /; • i Q A B A A S\ y' V y j | j y y S y y / ! / v B T H R E S H O L D C T H R E S H O L D B |a^L__ T H R E S H O L D A '''y y Y/A A A / / i / /y y // \/\/ \/ $A A \?/ ' \\// / 65 a r e c a l l e d " i n h i b i t i o n v a l u e s ; " They can be ar r a n g e d i n a m a t r i x (Table IV)-.- Note t h a t , s i n c e song u n i t s i n h i b i t o n l y t h e m s e l v e s i n t h i s f i r s t model, a l l the v a l u e s - a re e q u a l t o zero except t h o s e i n t h e d e s c e n d i n g d i a g o n a l ( I n h i b i t i o n } * , , where i r e f e r s t o t h e song type.).. Only t h e r e l a t i v e s i z e o f t h e s e numbers makes any d i f f e r e n c e t o t h e outcome.of t h e s i m u l a t i o n . T a b l e V shows p a r t of a sequence s i m u l a t e d u s i n g t h e v a l u e s o f i n h i b i t i o n from T a b l e IV. T h i s model c l e a r l y behaves l i k e a r e a l b i r d i n the ways l i s t e d a t the b e g i n n i n g of t h i s s e c t i o n , and the same i s t r u e f o r s i m i l a r models " B a s e d on d i f f e r e n t v a l u e s o f i n h i b i t i o n . Now some p r e d i c t i o n s . I n a d e t e r m i n i s t i c model l i k e F i g u r e 17a, o n l y t h e most common song t y p e — t h e one produced by the u n i t t h a t i n h i b i t s i t s e l f l e a s t can o c c u r t w i c e i n s u c c e s s i o n ; i n t h e s t o c h a s t i c computer model, t h e r e i s a l s o a s m a l l chance t h a t o t h e r songs can oc c u r t w i c e i n s u c c e s s i o n , but t h i s never happened i n any o f my s i m u l a t i o n s * - Of t h e t h r u s h e s t h a t r e p e a t e d song t y p e s i m m e d i a t e l y , most d i d so o n l y w i t h t h e song they used most f r e q u e n t l y , j u s t as p r e d i c t e d , but s e v e r a l r e p e a t e d l e s s common songs i m m e d i a t e l y ( T a b l e V I ) . The b e h a v i o r of t h e l a t t e r b i r d s can be s i m u l a t e d . b y a model t h a t has more random v a r i a b i l i t y i n t h e r a t e o f r e c o v e r y t o w a r d t h e t h r e s h o l d than does t h e model d e s c r i b e d above,. I f t h e songs i n a sequence a r e numbered i n order., t h e " r e c u r r e n c e i n t e r v a l s " between s u c c e s s i v e r e p e t i t i o n s o f each song t y p e can be c a l c u l a t e d by s u b t r a c t i n g t h e number of t h e i - t h r e p e t i t i o n from t h a t o f t h e i - t h + 1 r e p e t i t i o n . 66 Table IV. M a t r i x o f i n h i b i t i o n v a l u e s . I n t h i s m a t r i x , song u n i t s i n h i b i t o n l y t h e m s e l v e s . I n h i b i t i o n on  U n i t A U n i t B U n i t C U n i t D U n i t E U n i t A 5.0 0.0 0.0 0.0 0.0 U n i t B 0. 0 7. 0 0. 0 0. 0 0. 0 I n h i b i t i o n . f r o m - U n i t v_ 0. 0 0. 0 10. 0 0. 0 0. 0 U n i t D o. 0 0. 0 , 0. o . 12. 0 0. 0 U n i t E 0. 0 0. 0 0. 0 0. 0 15. 0 67 Table V. P a r t o f a sequence s i m u l a t e d u s i n g t h e v a l u e s o f i n h i b i t i o n from T a b l e IV. AEBDACABACEDBABACDAEBACBDAAEBCADBACBAEDACBABACEDBAACBDAEBAC t 6 8 Table V I . R e l a t i v e - f r e q u e n c i e s o f songs r e p e a t e d i m m e d i a t e l y . Most common song i n a. r e p e r t o i r e e q u a l s 1. Use o f the s a m e ' l e t t e r s t o r e p r e s e n t songs i n the r e p e r t o i r e s o f d i f f e r e n t i n d i v i d u a l s ..does n o t i m p l y t h a t t h e songs a r e s i m i l a r . Name o f B i r d Song R e l a t i v e Frequency .Buddy B 1 Chuck D 1 Chuckr :B 2. Chuck A 3 Chubby B 1 F a t s E 1 Simon D. 1 G a r f u n k e l A 1 Mick F 2 69 For example, i n t h e sequence, ABCBDACBBACB, t h e r e c u r r e n c e i n t e r v a l s f o r song B are 2, 4, 1, and 3. I n s i m u l a t e d sequences, t h e r e i s a n e g a t i v e c o r r e l a t i o n between t h e l e n g t h s o f s u c c e s s i v e r e c u r r e n c e i n t e r v a l s ( F i g . 1 8 ) , and the same i s t r u e i n seguences r e c o r d e d from b i r d s (Table V I I ) . Thus t h e s i m p l e model d e s c r i b e d i n t h i s s e c t i o n not o n l y mimics g e n e r a l p a t t e r n s o f s i n g i n g but a l s o c o r r e c t l y p r e d i c t s more s p e c i f i c p a t t e r n s * INHIBITION BETWEEN SONG ONITS In the model* song u n i t s o p e r a t e i n d e p e n d e n t l y o f each o t h e r * T h i s may n o t be t h e case i n birds,,.-however, and my o b j e c t i v e i n t h i s s e c t i o n i s t o i n v e s t i g a t e t h e p o s s i b i l i t y 1 " of i n h i b i t o r y c o n n e c t i o n s between song u n i t s . Sequences of b e h a v i o r a l e v e n t s a r e o f t e n d e s c r i b e d u s i n g Markov a n a l y s i s , which measures t h e e x t e n t t o which an event can be p r e d i c t e d from one or more p r e c e d i n g e v e n t s ( C h a t f i e l d and Lemon 197 0,),. The f i n d i n g t h a t one event f o l l o w s a n o t h e r e i t h e r more o r l e s s o f t e n than would be ex p e c t e d i f -the two o c c u r r e d i n d e p e n d e n t l y o f each o t h e r i s sometimes used t o make i n f e r e n c e s about t h e u n d e r l y i n g c o n t r o l of the sequence (see e..g*-.. Lemon and C h a t f i e l d 1973) . We have a l r e a d y seen t h a t * v a r i e d t h r u s h e s do not s i n g t h e i r songs i n random o r d e r . No d e t a i l e d q u a n t i t a t i v e a n a l y s i s i s n e c e s s a r y t o show t h a t t h e y a v o i d r e p e a t i n g song t y p e s i m m e d i a t e l y . . I t i s l e s s o b v i o u s , however, whether they obey some r u l e i n s w i t c h i n g between song t y p e s o r whether t h e y 70 F i g u r e 18* R e l a t i o n s h i p between t h e l e n g t h s o f s u c c e s s i v e r e c u r r e n c e i n t e r v a l s f o r t h e most common song i n a sequence produced by t h e f i r s t model. Sequence o f 500 songs was s i m u l a t e d u s i n g the v a l u e s o f i n h i b i t i o n from T a b l e I V . Length of Preceeding Recurrence Interval (arbitrary units) 72 Table V I I . Spearman ;r.ank^correlation between s u c c e s s i v e recurrence i n t e r v a l s . C a l c u l a t e d f o r most common song i n sequences i n which t h i s song o c c u r r e d 10 or more times. Use of the same l e t t e r s t o r e p r e s e n t songs i n the r e p e r t o i r e s o f d i f f e r e n t i n d i v i d u a l s does not imply t h a t the songs a r e . s i m i l a r . Name of B i r d Song C o r r e l a t i o n (r„) £ Buddy B -0.51 <0.025 Chuck D -0.48 N.S. Chubby B -0.01 N.S. Fats E -0.11 N.S. E l v i s A -0.18 :N.. S,. J e r r y Lee C -0.33 ' N.S,.. Bobbie 'B' -0..11 N..S. O t i s ,B +0.09 :N,..S.. . Simon C -0.67 Garfunkel C -0.47 \ 0 . 0 1 Freddie A -0.26 -N.S. Mick C -0.34 N.S. Eddie, sequence.! A -0.51 <0.05 Eddie, sequence 2 A -0.03 N.S. J i m i C -0.49 <0.05 Rod C -0.39 N.S. E l t o n A -0.19 N.S. Paul A -0.60 <0.05 John, sequence 1 A -0.01 N.S. John, sequence 3 A -0.15 N.S. George C -0.23 N.S. Ringo ..D -0.27 N.S. On e - t a i l e d b i n o m i a l t e s t : 21 c o r r e l a t i o n s < 0 1 c o r r e l a t i o n > 0 P < 0.0001 73 s i m p l y choose songs a t random* I f t h e l a t t e r > t h e . p r o b a b i l i t y o f a song b e i n g chosen e q u a l s i t s own fr e q u e n c y d i v i d e d by the sum of t h e f r e q u e n c i e s of the. songs t h a t c o u l d be chosen. F o r example, a b i r d t h a t has a r e p e r t o i r e of t h r e e songs. A, B, C, which o c c u r i n t h e f r e q u e n c i e s , 0.5, 0,*3.,.< and 0.2, r e s p e c t i v e l y , would s i n g song B 60% (0.3/{0.3+0.2j) o f t h e t i m e and song C 40% of the time a f t e r song A* Markov a n a l y s i s i s one way t o d i s t i n g u i s h between t h e s e a l t e r n a t i v e s * A n a l y s i s of the sequences r e c o r d e d from b i r d s r e v e a l e d some s i g n i f i c a n t d e v i a t i o n s from t h e random e x p e c t a t i o n ( T a b l e s V I I I and IX)...- I might have been more i n c l i n e d t o _ i n t e r p r e t t h e s e as a m a n i f e s t a t i o n of c o n n e c t i o n s between song u n i t s had I not got s i m i l a r r e s u l t s i n a Markov a n a l y s i s of sequences s i m u l a t e d u s i n g the computer model (Table X)>. The d e v i a t i o n s . , which d i f f e r w i d e l y from one sequence o f 500 songs t o an o t h e r (Table X ) , seem t o depend on t h e s t a r t i n g c o n d i t i o n s of the s i m u l a t i o n ; i f a sequence b e g i n s w i t h song B, f o r example, the d e v i a t i o n s a r e not t h e same as i f t h e sequence b e g i n s w i t h song E. When t h e d a t a from a number o f sequences are combined, the e f f e c t s of s t a r t i n g c o n d i t i o n s d i s a p p e a r , and s w i t c h e s appear t o be made a t random., which i s what would be expected from a model i n which song u n i t s o p e r a t e i n d e p e n d e n t l y of each o t h e r . I s u s p e c t the same would be t r u e f o r a s i n g l e , l o n g s i m u l a t i o n o f , say, 5,000 songs o r more. S t i l l , f o r the s h o r t e r sequences o f 500 songs, which a re l o n g e r t h a n any I r e c o r d e d from v a r i e d t h r u s h e s , Markov a n a l y s i s wrongly suggests t h a t song u n i t s i n f l u e n c e each o t h e r . Can i t be t r u s t e d t h e n t o r e v e a l a n y t h i n g about the c o n t r o l of song i n b i r d s ? Table V I I I . T r a n s i t i o n f r e q u e n c i e s between the songs o f the t h r u s h named O t i s . Data f o r a l l sequences combined. Songs Observed Frequency E x p e c t e d F r e q u e n c y AB 24 21.9 AC 6 14.0 AD 24 18.1 BA 23 23.8 BC 29 20.2 BD 18 26.0 CA 12 13.2 CB 17 17.4 CD 16 14.4 DA 18 18.0 DB 29 23. 8 DC 10 15.3 Chi-s q u a r e goodness o f f i t t e s t : Chi-square=21.17 d.f.=11 p < 0.05 75 Table IX. T r a n s i t i o n f r e q u e n c i e s between t h e songs o f th e t h r u s h named John. .Data f o r a l l sequences combined. Songs Observed Frequency E x p e c t e d Frequency AB 2 3 1 9 . 4 AC 2 1 2 6 . 2 AD 1 3 1 0 . 5 AE 8 8 . 9 BA 1 8 1 6 . 0 B.C . 1 2 1 2 . 1 BD" . 3 4 . 8 BE 4 4 . 1 CA 38 2 3 . 6 CB 4 1 3 . 2 CD 3 7 . 1 CE 5 6 . 1 DA 6 7 . 8 DB 6 4 . 4 DC 8 5 . 9 DE 0 2 . 0 EA 4 6 . 9 EB 4 3 . 9 EC 9 5 . 2 ED 1 2 . 1 Chi-square goodness o f f i t t e s t : C hi-square=32.6 d . f . = 1 9 p < 0 . 0 5 76 Table X. T r a n s i t i o n f r e q u e n c i e s between songs f o r two s i m u l a t i o n runs o f 500 songs each. S t a r t i n g c o n d i t i o n s were d i f f e r e n t f o r the two r u n s . Observed F r e q u e n c i e s -i Average Songs Run 1 Run 2 E x p e c t e d F r e q u e n c i e s AB 89 42 56. 6 AC 15 54 39. 6 AD 41 20 33.1 AE 11 40 26. 7 BA 42 83 51. 3 BC 3,4 7 27. 6. -BD 17 22 23.0 BE 28 8 18.6 CA 64 21 32. 6 CB 8 35 25.0 CD 4 20 14. 6 CE 8 8 11. 8 DA DB DC DE 20 21 19 10 41 25 4 0 26.2 20.2 14.1 9.5 EA EB EC ED 30 2 16 8 11 18 19 8 20.3 15.6 10.9 0.1 1 Expected v a l u e s d i f f e r e d o n l y s l i g h t l y between t h e two r u n s . 77 I t c a n , I t h i n k , i f used i n t h e c o n t e x t o f a t h e o r y . The t h e o r y developed i n t h i s . s e c t i o n was i n s p i r e d by the f i n d i n g , d e s c r i b e d e a r l i e r , t h a t v a r i e d t h r u s h e s have r e p e r t o i r e s o f songs l e s s s i m i l a r t o each o t h e r t h a n would be e x p e c t e d i f t h e y were t a k e n a t random from t h o s e i n t h e p o p u l a t i o n . I t appears t h a t i n h i b i t o r y p r o c e s s e s o p e r a t e d u r i n g t h e development o f song i n i n d i v i d u a l b i r d s t o p r e v e n t them from a c q u i r i n g s i m i l a r song t y p e s , and o n l y a s h o r t f l i g h t o f i m a g i n a t i o n i s n e c e s s a r y t o suppose t h a t s i m i l a r i n h i b i t o r y p r o c e s s e s o p e r a t e d u r i n g s i n g i n g , w i t h song u n i t s i n h i b i t i n g each o t h e r an amount d i r e c t l y r e l a t e d t o the s i m i l a r i t y o f t h e songs t h e y c o n t r o l . I t i s h e l p f u l t o p i c t u r e song u n i t s as d i s t r i b u t e d i n a two d i m e n s i o n a l s p a c e , s i m i l a r t o F i g u r e 5-, w i t h dominant f r e q u e n c y a l o n g one d i m e n s i o n and p e r i o d of M o d u l a t i o n a l o n g t h e o t h e r . F i g u r e .19 shows t h e r e p e r t o i r e o f an i n d i v i d u a l b i r d p l o t t e d a c c o r d i n g to t h e l o g a r i t h m i c r a n k s developed i n t h e l a s t c h a p t e r . A c c o r d i n g t o t h e t h e o r y , i n h i b i t i o n , l i k e g e n e r a l i z a t i o n , i s a d e c r e a s i n g f u n c t i o n of the d i s t a n c e between songs (or song u n i t s ) on t h i s graph. The b e h a v i o r o f such a system i s shown p i c t o r i a l l y i n F i g u r e 20. Note t h a t the- magnitude o f i n h i b i t i o n on a song u n i t i s assumed t o be independent o f t h e u n i t ' s c u r r e n t m o t i v a t i o n a l s t a t e . A r e a s o n a b l e a l t e r n a t i v e assumption i s t h a t the m o t i v a t i o n a l s t a t e o f a u n i t can not drop below some minimum v a l u e , and I have a l s o s t u d i e d t h e b e h a v i o r of models i n which t h a t v a l u e i s s e t e q u a l t o t h e t h r e s h o l d minus the i n t e n s i t y of s e l f i n h i b i t i o n ( T h r e s h o l d - I n h i b i t i o n . . . ) . 78 F i g u r e 19- Ranks o f dominant f r e q u e n c y and p e r i o d o f m o d u l a t i o n of songs sung by t h e t h r u s h named F a t s -Period of Modulation (rank) o j . D o 3 CD n c CD •D O cn-J ® © ® 0) o r 80 F i g u r e 20. B e h a v i o r o f model i n which song u n i t s not o n l y i n h i b i t t h e m s e l v e s but a l s o i n h i b i t each o t h e r . Note t h a t s e l f i n h i b i t i o n i s always g r e a t e r than i n h i b i t i o n o f o t h e r u n i t s . THRESHOLD B A 7T / / A / i A A CO H. 8 2 I n h i b i t i o n i s independent o f m o t i v a t i o n a l s t a t e e x c e p t when the m o t i v a t i o n a l s t a t e would o t h e r w i s e drop below t h i s v a l u e . T h i s change i n t h e model, however, does not a l t e r any of t h e p r e d i c t i o n s I make i n t h e remainder o f t h i s c h a p t e r * The computer s i m u l a t i o n s were based on r e a l song r e p e r t o i r e s , e s p e c i a l l y t h e one shown i n F i g u r e 19. T h i s was s i m p l y a c o n v e n i e n t way t o i n s u r e t h a t the p a t t e r n s o f i n h i b i t i o n used i n the s i m u l a t i o n s were t o p o l o g i c a l l y p o s s i b l e i n two d i m e n s i o n a l space. The i n h i b i t i o n between song u n i t s was c a l c u l a t e d u s i n g the f o l l o w i n g a r b i t r a r y f o r m u l a : I n h i b i t i o n , - ; = C/\/(D+1 j ,. where C = a c o n s t a n t , and D = t h e E u c l i d e a n d i s t a n c e between two song u n i t s . A c c o r d i n g t o the f o r m u l a , the i n h i b i t i o n between song u n i t s i s always s y m m e t r i c a l ( I n h i b i t i o n - = I n h i b i t i o n ^ ) and has a maximum e q u a l t o the c o n s t a n t ( f o r u n i t s t h a t have i d e n t i c a l r a n k s ) . The s e l f i n h i b i t i o n o f song u n i t s was made s l i g h t l y l a r g e r * t h a n t h e c o n s t a n t , s i n c e i t i s assumed t h a t a song u n i t i n h i b i t s i t s e l f more than i t does any o t h e r u n i t , even one t h a t produces a v e r y s i m i l a r song. The v a l u e s of i n h i b i t i o n f o r t h e r e p e r t o i r e shown i n F i g u r e 19 are g i v e n i n Table XI* Not s u r p r i s i n g l y , Markov a n a l y s i s of s i m u l a t e d sequences r e v e a l s a n e g a t i v e c o r r e l a t i o n between the s i m i l a r i t y o f two song t y p e s and the observed p r o b a b i l i t y of them b e i n g produced c o n s e c u t i v e l y d i v i d e d by the p r o b a b i l i t y expected i f they o c c u r i n d e p e n d e n t l y of each o t h e r ( F i g . 2 1 ) . There i s a problem, however, i n a p p l y i n g t h i s a n a l y s i s t o sequences r e c o r d e d from b i r d s . The amount o f i n h i b i t i o n between songs i s assumed to be Table X I . M a t r i x o f i n h i b i t i o n v a l u e s f o r r e p e r t o i r e shown i n F i g u r e 19 . U n i t A U n i t B 6. 65 I n h i b i t i o n from U n i t C 7. 35 U n i t D 6. 65 U n i t E 7.70 I n h i b i t i o n on U n i t B U n i t C U n i t D U n i t E 6.65 7.35 6. 65 7. 70 20.0 6.48 6.97 6. 28 6.48 20.0 5.41 12. 73 6.97 5.41 20.0 5. 43 6.28 12.73 5. 43 20 .0 84 F i g u r e 2 1 . R e l a t i o n s h i p of the o b s e r v e d / e x p e c t e d p r o b a b i l i t y o f two songs o c c u r r i n g i n s u c c e s s i o n and the s i m i l a r i t y o f the songs i n a s i m u l a t e d sequence based on t h e r e p e r t o i r e shown i n F i g u r e 19. S 1.25-1.0-.75-£ .504 .254 T 2 4 "5 Number of Ranks Apart 86 governed by t h e same r u l e s t h a t a p p l y t o g e n e r a l i z a t i o n , and as I mentioned i n t h e l a s t c h a p t e r t h e s e r u l e s a r e known o n l y f o r d i f f e r e n c e s a l o n g one d i m e n s i o n . Thus, p r e c i s e e s t i m a t e s o f i n h i b i t i o n can not be c a l c u l a t e d f o r songs t h a t d i f f e r a l o n g b o t h d i m e n s i o n s . S t i l l , t h e E u c l i d e a n d i s t a n c e between songs s h o u l d , i f the t h e o r y i s c o r r e c t , be a t l e a s t a rough i n d e x of i n h i b i t i o n ; A c c o r d i n g t o t h i s l i n e of r e a s o n i n g , t h e song u n i t s c o n t r o l l i n g the two most d i s s i m i l a r songs i n a r e p e r t o i r e (C and D i n F i g u r e 19) s h o u l d i n h i b i t each o t h e r r e l a t i v e l y l i t t l e , a n d t h e r e f o r e t h e s e songs s h o u l d be sung c o n s e c u t i v e l y more o f t e n t h a n expected by chance* Seguences l o n g enough f o r t h i s a n a l y s i s were r e c o r d e d from 22 b i r d s , and t h e r e s u l t s uphold t h e p r e d i c t i o n (Table X I I ) . T h i s i s a t l e a s t t e n t a t i v e s u p p o r t f o r t h e i d e a t h a t song u n i t s i n h i b i t each o t h e r i n r e l a t i o n t o the s i m i l a r i t y o f t h e songs t h e y c o n t r o l * Assuming t h a t i n h i b i t i o n i s governed by t h e same r u l e s as g e n e r a l i z a t i o n , a more r i g o r o u s t e s t can be made a f t e r t h e s e r u l e s have been d e s c r i b e d . FREQUENCIES OF DIFFERENT SONG TYPES I n d i v i d u a l t h r u s h e s s i n g some o f t h e i r song t y p e s more o f t e n t h a n others;; I n t h e second sequence shown i n T a b l e I I I , f o r example, song D o c c u r s 14 t i m e s , song E o c c u r s 6 t i m e s , and F o n l y 3 t i m e s . We can now c o n s i d e r an e x p l a n a t i o n f o r t h e s e d i f f e r e n c e s . I n t h e model d e s c r i b e d i n the l a s t s e c t i o n , a l l song u n i t s i n h i b i t t h e m s e l v e s the same amount, and the f r e q u e n c y o f each Table X I I . Observed/Expected p r o b a b i l i t y o f the two most d i s s i m i l a r songs i n a r e p e r t o i r e b e i n g sung consecu-t i v e l y . Name of B i r d Songs Ob s e r v e d / E x p e c t e d Buddy A,B 1 . 1 2 Chubby A,B 1 . 2 7 F a t s C,D 1 . 0 7 E l v i s A,E 1 . 4 5 J e r r y Lee A,B 0 . 8 2 F r a n k i e D,E 1 . 4 9 L i t t l e R i c h a r d C,D 0 . 7 9 Bobbie C,(F,B, A t i e d ) 0 . 8 5 Dwayne B,C 0 . 9 6 O t i s A,C 0 . 6 6 Simon D,E 1 . 3 9 G a r f u n k e l D,H 1 . 2 3 F r e d d i e A,B 1 . 0 1 Mick A,C 0 . 79 E d d i e B, E 1 . 0 4 J i m i B,D 1 . 1 1 Rod B,C 1 . 2 4 E l t o n A,B 1 . 1 3 P a u l B,C 1 . 0 1 John B,E 1 . 0 1 George A, B 1 . 3 1 Ringo B,F 1 . 1 0 O n e - t a i l e d b i n o m i a l t e s t : 1 6 v a l u e s > 1 . 0 0 6 v a l u e s < 1 . 0 0 p = 0 . 0 3 6 88 song t y p e i s n e g a t i v e l y c o r r e l a t e d w i t h the amount o f i n h i b i t i o n i t s c o n t r o l l i n g u n i t r e c e i v e s from o t h e r song u n i t s ( F i g * 2 2 ) . Once the r u l e s t h a t govern i n h i b i t i o n between song u n i t s have been d e s c r i b e d , i t w i l l be p o s s i b l e t o p r e d i c t t h e r e l a t i v e f r e q u e n c i e s of t h e d i f f e r e n t song types,; I n t h e meantime, however, t h a n k s t o a n a t u r a l experiment done by two male t h r u s h e s , t h e model can a t l e a s t be put t o a weak t e s t . The two b i r d s a l t e r e d t h e i r r e p e r t o i r e s by c o m p l e t e l y d r o p p i n g s e v e r a l songs and r e p l a c i n g them w i t h o t h e r s . A lthough I can not say t h i s w i t h c e r t a i n t y , s i n c e the b i r d s were not marked f o r i n d i v i d u a l i d e n t i f i c a t i o n , the e v i d e n c e seems good* An i n d i v i d u a l r e c o r d e d i n 1976 had t h r e e songs, i n a r e p e r t o i r e o f f i v e , i d e n t i c a l (as judged from sonagrams) t o t h o s e o f a b i r d r e c o r d e d i n e x a c t l y t h e same a r e a i n 1975; t h e o t h e r c a s e was s i m i l a r , e x c e p t the r e c o r d i n g s were made o n l y a few weeks a p a r t i n 1976. The r e p e r t o i r e s of the two b i r d s , both b e f o r e and a f t e r t h e changes, a r e shown i n F i g u r e s 23 and 24. Now r e c a l l t h e f i r s t model, i n which each song u n i t i n h i b i t s o n l y i t s e l f , and the f r e q u e n c y o f the sonq i t produces i s n e g a t i v e l y c o r r e l a t e d w i t h the magnitude o f s e l f i n h i b i t i o n ( F i g * 2 5 ) . Suppose t h i s model, i s c o r r e c t . The a b s o l u t e f r e q u e n c i e s o f t h e songs t h a t remain i n a r e p e r t o i r e a f t e r i t i s a l t e r e d (e.g. A, B, and C i n F i g u r e 23) may change, but t h e r e l a t i v e f r e q u e n c i e s w i l l n o t . But i f t h e second model a p p l i e s t h e r e l a t i v e f r e q u e n c i e s w i l l a l s o change* and t h i s i s because t h e r e l a t i v e amounts o f i n h i b i t i o n imposed by o t h e r song u n i t s change* 89 F i g u r e 22. R e l a t i o n s h i p of the r e l a t i v e f r e q u e n c y of a song and t h e t o t a l amount o f i n h i b i t i o n i t s c o n t r o l l i n g u n i t r e c e i v e s from o t h e r u n i t s i n a s i m u l a t e d sequence based on the r e p e r t o i r e shown i n F i g u r e 19. 90 24 26 28 30 32 Total Inhibition from Other Song Units gure 23. Changes i n the r e p e r o i r e o f t h e t h r u s h named F a t s . ^ 1975 ^ 1976 ©both years D 15 Dominant Frequency (rank) 93 F i g u r e 24,. Changes i n t h e r e p e r t o i r e o f t h e t h r u s h named Simon. 94 ® © ® B H 1st repertoire 2nd repertoire O both 10 15 20 Dominant Frequency (rank) 95 F i g u r e 25. R e l a t i o n s h i p o f t h e r e l a t i v e f r e q u e n c y of a song and the amount i t s c o n t r o l l i n g u n i t i n h i b i t s i t s e l f i n a s i m u l a t e d sequence based on the m a t r i x o f i n h i b i t i o n v a l u e s shown i n Table I V . 96 200H Self Inhibition (arbitrary units) 97 The r e s u l t s weakly s u p p o r t t h e second model ( T a b l e s X I I I and X I V ) . The changes i n the r e l a t i v e f r e q u e n c i e s of t h e songs a r e not s t a t i s t i c a l l y s i g n i f i c a n t , but t h e y are f a r g r e a t e r i n magnitude t h a n any I have observed i n seguences r e c o r d e d from i n d i v i d u a l s when t h e i r r e p e r t o i r e s remained t h e same (see e.g. T a b l e s I and I I ) * A l s o , t h e changes appear t o be i n t h e d i r e c t i o n s p r e d i c t e d by t h e model. I n t h e f i r s t r e p e r t o i r e ( F i g . 2 3 ) , t h e r e l a t i v e f r e q u e n c y of song A changes t h e most, d e c r e a s i n g from 0.32 t o 0.18. T h i s o c c u r s when t h e b i r d a c q u i r e s song G, which i s much more s i m i l a r t o A than a r e e i t h e r of t h e songs i t d r o p s (B and C ) . I n t h e second r e p e r t o i r e ( F i g . 2 4 ) , song A i n c r e a s e s by 0.14 when the b i r d r e p l a c e s two songs v e r y s i m i l a r t o i t (B and E) w i t h j u s t one r a t h e r s i m i l a r song ( F ) ; song D d e c r e a s e s by t h e same amount when t h e b i r d adds two songs more s i m i l a r t o i t (G and H) than e i t h e r o f t h e two i t drops. INTERVALS BETWEEN SONGS F i n a l l y , we can c o n s i d e r t h e i n t e r v a l s between songs. There are two o b v i o u s p a t t e r n s : . 1) i n t e r v a l s a re q u i t e v a r i a b l e , and 2) t h e y a r e u s u a l l y a t l e a s t s e v e r a l seconds l o n g ( F i g . 2 6 ) . I n a l l the sequences I r e c o r d e d , t h e i n t e r v a l s were never s h o r t e r than 3.2 s, e x c e p t a few t i m e s when a b i r d would s t a r t t o s i n g one song, then s t o p s h o r t and s w i t c h i m m e d i a t e l y t o a n o t h e r song. These p a t t e r n s a l s o o c c u r i n sequences produced by t h e model ( F i g s . 20 and 27) . We can see i n t u i t i v e l y t h a t t h e 98 Table X I I I . F r e q u e n c i e s (and p r o p o r t i o n s ) o f songs t h a t were i n the r e p e r t o i r e o f t h e t h r u s h named F a t s b o t h b e f o r e and a f t e r he a l t e r e d the r e p e r t o i r e . Frequency ( P r o p o r t i o n ) Song B e f o r e Change A f t e r Change A 20 (. 32) 14 (.18) D 18 (.29) 31 (.40) E 25 (.40) 33 (.42) Chi-square two sample t e s t : Chi-square=4.06 d.f.=2 0.20>p>0.10 99 Table XIV. F r e q u e n c i e s (and p r o p o r t i o n s ) o f songs t h a t were i n the r e p e r t o i r e o f the t h r u s h named Simon b o t h b e f o r e and a f t e r he a l t e r e d the r e p e r t o i r e . F requency ( P r o p o r t i o n ) Song B e f o r e Change A f t e r Change A 14 (.21) 23 (.35) C 27 (.41) 26 (. 39) D 25 (.38) ,16 (.24) C h i - s q u a r e two sample t e s t : Chi-square=4.18 d.f.=2 0. 20 > p > 0.10 100 F i g u r e 26. L e n g t h s o f i n t e r v a l s between s u c c e s s i v e songs i n sequence sung by the t h r u s h named Simon. 30-20-1 10 6 7 8 9 10 11 12 13 14 15 16 17 18 Length of Interval (sec) 102 F i g u r e 27- L e n g t h s o f i n t e r v a l s between s u c c e s s i v e songs i n s i m u l a t e d sequence based on the r e p e r t o i r e shown i n F i g u r e 19. A r b i t r a r y u n i t s a r e s c a l e d t o c o r r e s p o n d t o seconds. 30 20 o c CD cr 104 f" ? ! i 4 5 6 7 8 9 10 11 12 13 14 15 16 Length of Interval (arbitrary units) 104 s h o r t e s t i n t e r v a l s must be between songs produced by the u n i t s t h a t i n h i b i t each o t h e r l e a s t . T h i s l e a d s t o t h e p r e d i c t i o n t h a t , i n seguences r e c o r d e d from b i r d s , t h e i n t e r v a l s between t h e two most d i s s i m i l a r songs i n a r e p e r t o i r e s h o u l d be s h o r t e r t han t h e av e r a g e . U n f o r t u n a t e l y , t h i s p r e d i c t i o n i s i m p r a c t i c a b l e t o t e s t , because the average l e n g t h of i n t e r v a l s o f t e n changes d u r i n g a sequence ( F i g . 2 8 ) . There i s a c o n s i s t e n t r e l a t i o n s h i p i n s i m u l a t e d sequences between t h e f r e q u e n c y of a song t y p e and the average l e n g t h o f the i n t e r v a l s f o l l o w i n g i t . I n a sample of 10 seguences based on t h e r e p e r t o i r e shown i n F i g u r e 1 9 , t h e Spearman rank c o r r e l a t i o n between t h e s e two v a r i a b l e s was always n e g a t i v e ( p = 0 i 0 0 2 ; two t a i l e d b i n o m i a l t e s t ) , r a n g i n g from - 0 . 3 0 t o - 1 . 0 0 . The r e s u l t s were s i m i l a r f o r sequences based on o t h e r r e p e r t o i r e s * An i n t u i t i v e e x p l a n a t i o n f o r t h i s r e l a t i o n s h i p i s t h a t t h e most common song i n a r e p e r t o i r e i s produced by t h e song u n i t i n h i b i t e d l e a s t by o t h e r u n i t s , and s i n c e i n h i b i t i o n i s s y m m e t r i c a l , t h i s u n i t i n h i b i t s o t h e r s r e l a t i v e l y l i t t l e hence o t h e r songs can be produced sooner. S u r p r i s i n g l y , a s i m i l a r a n a l y s i s o f sequences r e c o r d e d from b i r d s r e v e a l e d e x a c t l y t h e o p p o s i t e r e l a t i o n s h i p : t h e most common songs are f o l l o w e d by l o n g e r , not s h o r t e r , i n t e r v a l s (Table XV). The model i s wrong. But i t needs o n l y a s i n g l e m o d i f i c a t i o n t o behave a p p r o p r i a t e l y . The r e l a t i o n s h i p o f i n h i b i t i o n t o the d i s t a n c e between song u n i t s must be made s t o c h a s t i c * I d i d t h i s by s e t t i n g each I n h i b i t i o n . - ; e q u a l t o the o r i g i n a l d e t e r m i n i s t i c v a l u e p l u s a n o r m a l l y d i s t r i b u t e d random v a r i a b l e w i t h a mean e q u a l t o z e r o and a s t a n d a r d 105 F i g u r e 28. I n t e r v a l s between s u c c e s s i v e songs i n a sequence sung by the th r u s h named John. I n t e r v a l s are shown i n sequence from l e f t to r i g h t . Length of Interval (sec) 13 CD CT CD 901 107 Table XV. Spearman rank c o r r e l a t i o n between t h e r e l a t i v e f r e q u e n c i e s o f d i f f e r e n t songs w i t h i n i n d i v i d u a l r e p e r t o i r e s and t h e l e n g t h s o f t h e i n t e r v a l s f o l l o w i n g them. Data from a l l sequences l o n g e r t h a n 40 songs. Name o f B i r d C o r r e l a t i c Buddy +0.50 Chuck +0.89 Chubby +0.95 F a t s +0.10 E l v i s +0.77 J e r r y Lee -0.50 Bobbie -0.15 O t i s , sequence 2 -0.20 O t i s , sequence 3 -1.00 O t i s , sequence 4 + 0. 45 Simon , +0.30 G a r f u n k e l +0.49 Mic k +0.52 E d d i e , sequence 1 +0.74 E d d i e , sequence 2 -0.12 J i m i +0.33 Rod +0.80 E l t o n -1.00 P a u l +0.17 John, sequence 1 +0.10 John, sequence 3 +0.70 George +0.17 Ringo + 0.77 F r e d d i e + 0. 40 T w o - t a i l e d b i n o m i a l t e s t : 18 c o r r e l a t i o n s > 0 6 c o r r e l a t i o n s < 0 p=0.03 108 d e v i a t i o n e q u a l t o 25% of the o r i g i n a l v a l u e * I n a sample o f 10 sequences produced by t h i s model (the s t o c h a s t i c v a l u e s o f I n h i b i t i o n c a l c u l a t e d s e p a r a t e l y f o r each s e q u e n c e ) , t h e c o r r e l a t i o n between the f r e q u e n c y o f each sonq t y p e and t h e average l e n g t h o f t h e i n t e r v a l s f o l l o w i n g i t was p o s i t i v e i n a l l but one case (p=0.021; two t a i l e d b i n o m i a l t e s t ) . T h i s r e l a t i o n s h i p i s more d i f f i c u l t t o und e r s t a n d i n t u i t i v e l y , but i t seems r e l a t e d t o t h e f a c t t h a t i n h i b i t i o n between song u n i t s i s no l o n g e r s y m m e t r i c a l ( F i g . 29) . Now a p a r t i c u l a r song i s common p a r t l y because the u n i t t h a t c o n t r o l s i t has a p o w e r f u l i n h i b i t o r y e f f e c t on o t h e r u n i t s , and t h i s means th e y can n o t r e c o v e r t o t h e t h r e s h o l d as q u i c k l y . I have e v i d e n c e t h a t t h i s i s a r e a l i s t i c m o d i f i c a t i o n of t h e t h e o r y - For example, i n t h e r e p e r t o i r e shown i n F i q u r e 19, songs C and E l i e o n l y one rank a p a r t on t h e dim e n s i o n o f dominant f r e q u e n c y and do not d i f f e r a t a l l i n p e r i o d of mo d u l a t i o n * T h e i r sonagrams a r e very s i m i l a r ( F i g . 3 0 ) . I f i n h i b i t i o n i s s y m m e t r i c a l , t h e f r e q u e n c i e s of t h e s e two songs s h o u l d be about the same, s i n c e t h e y would be i n h i b i t e d e q u a l l y by o t h e r song u n i t s * But i n r e a l i t y E i s the most common song i n t h e r e p e r t o i r e , o c c u r r i n g 27% o f t h e t i m e , and C t h e r a r e s t song, o c c u r r i n g o n l y 11% o f the time- One i n t e r p r e t a t i o n of t h i s d i f f e r e n c e i s t h a t i n h i b i t i o n i s a s y m m e t r i c a l between t h e two song u n i t s , w i t h E d o m i n a t i n g . As I mentioned e a r l i e r , the r a t e of s i n g i n g o f t e n changes d u r i n g a sequence. A l t h o u g h I have not attempted t o e x p l a i n t h e s e changes, i t i s s t i l l i n t e r e s t i n g t o c o n s i d e r how t h e model can be made t o a l t e r i t s r a t e o f s i n g i n g . Two s i m p l e 109 F i g u r e 29. Asymmetry o f i n h i b i t i o n between song u n i t s i n models h a v i n g a s t o c h a s t i c r e l a t i o n s h i p between t h e s i m i l a r i t y of songs and the amount songs i n h i b i t each o t h e r . F o r d e t a i l s , see t e x t . Asymmetry i s d e f i n e d here as I n h i b i t i o n ^ - / I n h i b i t i o n ^ , where I n h i b i t i o n Lj i s al w a y s e q u a l t o or g r e a t e r t h a n I n h i b i t i o n ^ . 18 16-14-12->» I CT 0 o Lt 8 " 6-4-24 1.0 1.5 2.0 q — - a 2.5 3.0 Asymmetry (inhibition y / inhibitionjj) 1 1 1 F i g u r e 30* Sonagrams o f two very s i m i l a r songs i n t h e r e p e r t o i r e of the t h r u s h named F a t s * Song E on l e f t , song C on r i g h t , . 112 CD ID • '"st CO CM (ZMH) Aouanbajj 113 ways a r e 1) t o change t h e r a t e a t which song u n i t s r e c o v e r toward t h e t h r e s h o l d , and 2) t o m u l t i p l y a l l v a l u e s o f i n h i b i t i o n by a c o n s t a n t * As f a r as t h e b e h a v i o r of the model goes, t h e s e a r e e g u i v a l e n t , s i n c e m u l t i p l y i n g t h e r a t e o f r e c o v e r y by a f a c t o r X has e x a c t l y the same e f f e c t as m u l t i p l y i n g t h e v a l u e s o f i n h i b i t i o n by 1/X. I t i s i n t u i t i v e l y o b v i o u s t h a t i n the model t h e a b s o l u t e v a r i a b i l i t y o f t h e i n t e r v a l s between songs (as measured, f o r example, by the s t a n d a r d d e v i a t i o n ) i s p o s i t i v e l y c o r r e l a t e d w i t h t h e mean l e n g t h o f t h e i n t e r v a l s * T h i s i s a l s o t r u e f o r t h r u s h e s ( T a b l e X V I ) * A more p r e c i s e p r e d i c t i o n can be made u s i n g a measure o f r e l a t i v e v a r i a b i l i t y , the c o e f f i c i e n t of v a r i a t i o n (100% x S.D./Mean). I n the model, t h e c o e f f i c i e n t of v a r i a t i o n i s c o n s t a n t f o r a l l r a t e s o f s i n g i n g ( F i g . 31). I n most t h r u s h e s , however, t h e r e i s a p o s i t i v e c o r r e l a t i o n between t h e c o e f f i c i e n t of v a r i a t i o n and t h e r a t e o f s i n g i n g (Table X V I ) . The r e s u l t s a r e n o t s t a t i s t i c a l l y s i g n i f i c a n t (Table XVI) but are c l o s e enough t o warrant t h e c o l l e c t i o n of more d a t a . I f t h i s p a t t e r n i s c o r r o b o r a t e d , t h e model w i l l have t o be changed. Two f u r t h e r p r e d i c t i o n s , f r o m t h i s s t o c h a s t i c v e r s i o n of t h e model a r e t h a t n e i t h e r t h e r e l a t i v e f r e q u e n c i e s o f song t y p e s n o r the o r d e r i n which t h e y are sung w i l l change as t h e r a t e o f s i n g i n g changes. Both p r e d i c t i o n s appear t o be upheld i n the sequences I r e c o r d e d , but p r o p e r t e s t s w i l l r e q u i r e more da t a * Table XVI. Product-moment c o r r e l a t i o n s between 1) mean l e n g t h s o f i n t e r v a l s between s u c c e s s i v e songs and s t a n d a r d d e v i a t i o n s o f i n t e r v a l l e n g t h s , and 2) mean l e n g t h s o f i n t e r v a l s between s u c c e s s i v e songs and c o e f f i c i e n t s o f v a r i a t i o n o f i n t e r v a l l e n g t h s . C o r r e l a t i o n s b a sed on means, s t a n d a r d d e v i a t i o n s , and c o e f f i c i e n t s o f v a r i a t i o n c a l c u l a t e d f o r groups o f f i v e c o n s e c u t i v e i n t e r v a l s i n sequences l o n g e r t h a n 40 songs. C o r r e l a t i o n C o e f f i c i e n t (r) Name of B i r d 1 Mean v s . S.D. Mean v s . C.V. Buddy + 0 . 3 8 + 0 . 2 1 Chuck + 0 . 6 6 + 0 . 3 8 Chubby - 0 . 3 9 - 0 . 7 7 F a t s + 0 . 45 + 0 . 2 9 E l v i s + 0 . 1 9 - 0 . 4 2 J e r r y Lee - 0 . 0 4 - 0 . 1 5 Bobbie + 0 . 7 8 +0 . 46 O t i s , sequence 2 - 0 . 1 2 - 0 . 3 1 O t i s , sequence 3 + 0 . 0 4 - 0 . 1 6 O t i s , sequence 4 - 0 . 0 3 - 0 . 4 0 Simon +0 . 83 + 0 . 5 5 G a r f u n k e l + 0 . 3 8 + 0 . 1 9 Mick + 0 . 7 9 +0 . 49 E d d i e , sequence 1 +0 . 77 + 0 . 6 0 E d d i e , sequence 2 + 0 . 4 5 + 0 . 3 3 J i m i + 0 . 6 0 + 0 . 3 8 Rod + 0 . 3 5 + 0 . 0 3 E l t o n + 0 . 9 8 + 0 . 9 2 P a u l + 0 . 6 7 + 0 . 5 5 John, sequence 1 + 0 . 7 9 + 0 . 6 7 John, sequence 3 + 0 . 6 3 + 0 . 3 7 George + 0 . 6 1 + 0 . 4 6 Ringo + 0 . 6 7 + 0 . 49 F r e d d i e + 0 . 0 4 - 0 . 2 0 2 O n e - t a i l e d b i n o m i a l t e s t : 20 c o r r e l a t i o n s > 0 p = 0 . 0 0 0 8 4 c o r r e l a t i o n s < 0 T w o - t a i l e d b i n o m i a l t e s t : 17 c o r r e l a t i o n s > 0 p = o . 0 6 4 7 c o r r e l a t i o n s < 0 115 Figure 31. Relationship of the c o e f f i c i e n t of variation of i n t e r v a l length to the average i n t e r v a l length i n simulated sequences. 116 504 40H .1 30-254 c CD ' g i o O 2CH 10-J 0 10 20 Average Length of Intervals (arbitrary units) 117 SYNTHESIS The f i n a l v e r s i o n of the model i s summarized i n F i g u r e 32. A l l song u n i t s i n h i b i t t h e m s e l v e s a c o n s t a n t amount t h a t i s g r e a t e r than t h e maximum amount t h e y can i n h i b i t each o t h e r ; The magnitude of i n h i b i t i o n between song u n i t s i s ' p o s i t i v e l y c o r r e l a t e d w i t h t h e s i m i l a r i t y o f the songs produced by t h e u n i t s b u t i s not c o m p l e t e l y p r e d i c t a b l e by t h i s r e l a t i o n s h i p . Some random v a r i a b i l i t y r e m a i n s , and t h i s a l l o w s the i n h i b i t i o n between two u n i t s t o be a s y m m e t r i c a l , one i n h i b i t i n g t he o t h e r more than i t i s i n h i b i t e d i n r e t u r n . T h i s model behaves more l i k e a v a r i e d t h r u s h t h a n any o f t h e l e s s e l a b o r a t e models developed i n t h i s chapter,. I t can account f o r a l l of the f o l l o w i n g p a t t e r n s found i n sequences of songs produced by t h r u s h e s : 1) Once a song type has been sung, i t i s n o t u s u a l l y r e p e a t e d i m m e d i a t e l y . 2) Song t y p e s a r e n o t sung w i t h e q u a l f r e q u e n c y . 3) The most common song t y p e i s more l i k e l y than any o t h e r t o be r e p e a t e d i m m e d i a t e l y . 4) The l e n g t h s o f s u c c e s s i v e r e c u r r e n c e i n t e r v a l s a r e n e g a t i v e l y c o r r e l a t e d * 5) The two most d i s s i m i l a r songs i n a r e p e r t o i r e a r e more l i k e l y t o be sung i n s u c c e s s i o n than would be e x p e c t e d i f t h e y o c c u r r e d i n d e p e n d e n t l y o f each o t h e r ; 6) When s e v e r a l o f t h e song t y p e s i n a r e p e r t o i r e a r e dropped and r e p l a c e d by o t h e r s , t h e ones t h a t remain a r e sung i n d i f f e r e n t r e l a t i v e f r e q u e n c i e s . 7) Two very s i m i l a r song t y p e s i n a r e p e r t o i r e can o c c u r w i t h q u i t e d i f f e r e n t f r e q u e n c i e s . , 8) I n t e r v a l s between songs a re v a r i a b l e but are u s u a l l y a t l e a s t s e v e r a l 1 1 8 F i g u r e 32. F i n a l model of t h e c o n t r o l o f s i n g i n g i n v a r i e d t h r u s h e s . Dashed l i n e s ending i n arrows r e p r e s e n t f a c i l i t a t i o n ; s o l i d l i n e s e n d i n g i n open c i r c l e s r e p r e s e n t i n h i b i t i o n . Amount o f i n h i b i t i o n ( i n d i c a t e d by t h e t h i c k n e s s of the s o l i d l i n e s ) i s p o s i t i v e l y c o r r e l a t e d w i t h the s i m i l a r i t y o f t h e songs produced by song u n i t s , but can be a s y m m e t r i c a l , owing t o s t o c h a s t i c v a r i a t i o n i n t h i s r e l a t i o n s h i p . 119 120 seconds l o n g . 9) The average l e n g t h s o f t h e i n t e r v a l s f o l l o w i n g d i f f e r e n t song t y p e s i n a r e p e r t o i r e : a r e p o s i t i v e l y c o r r e l a t e d w i t h t h e f r e q u e n c i e s o f the songs t y p e s ; 10) The a b s o l u t e v a r i a b i l i t y of t h e i n t e r v a l s between songs i s p o s i t i v e l y c o r r e l a t e d w i t h t h e r a t e of s i n g i n g . The.model a l s o a ccounts f o r t h e f a c t t h a t t h r u s h e s have r e p e r t o i r e s o f d i s s i m i l a r songs. We have a l r e a d y seen t h a t when i n h i b i t i o n between song u n i t s i s s y m m e t r i c a l , the f r e q u e n c y of a song type i s r e l a t e d t o t h e t o t a l amount of i n h i b i t i o n i t s c o n t r o l l i n g u n i t r e c e i v e s from o t h e r song u n i t s (Fig,: 2 2 ) . I f t h e i n h i b i t i o n i s l a r g e enough, the song i s n e v e r produced,. The same i s t r u e o f the p r e s e n t model, a l t h o u g h now t h e f r e q u e n c y of a song i s r e l a t e d not only t o t h e i n h i b i t i o n r e c e i v e d from o t h e r song u n i t s but a l s o t o t h e i n h i b i t i o n i t s u n i t imposes on them. I n both models, s i n c e i n h i b i t i o n i s p o s i t i v e l y c o r r e l a t e d w i t h s i m i l a r i t y , a song i s most l i k e l y t o be t o t a l l y s u p p r e s s e d i f i t i s s i m i l a r to o t h e r songs. Thus the e x p r e s s e d r e p e r t o i r e i s a nonrandom s u b s e t o f d i s s i m i l a r songs* I f v a r i e d t h r u s h e s o p e r a t e i n t h e same way as t h i s model, they may use o n l y p a r t o f t h e i r r e p e r t o i r e s d u r i n g l o u d s i n g i n g . T h i s , i n f a c t , seems t o be t h e c a s e , s i n c e t h e y s i n g a v a r i e t y o f q u i e t songs, some o f which resemble t y p i c a l l o u d songs (the sonagrams of t h e q u i e t songs a r e s i m i l a r t o t h o s e i n F i g u r e s 1-3). I t i s i n t r i g u i n g t o wonder whether these songs ar e e v e r i n c o r p o r a t e d i n t o t h e r e p e r t o i r e s used d u r i n g l o u d s i n g i n g * Perhaps t h e y a r e , and perhaps t h i s i s how the two b i r d s mentioned e a r l i e r ( F i g s . 23 and 24) a l t e r e d t h e i r 1 2 1 r e p e r t o i r e s . T h i s i d e a c o u l d be t e s t e d by r e c o r d i n g i n d i v i d u a l l y marked b i r d s over a l o n g p e r i o d o f - t i m e . 122 CHAPTER IV GENERAL DISCUSSION EVOLUTIONARY SIGNIFICANCE OF TEMPORAL PATTERNS OF SINGING When a v a r i e d t h r u s h s i n g s , i t i s engaged i n an e x e r c i s e i n c o n t r a s t s . Drawing from a r e p e r t o i r e of d i s s i m i l a r song t y p e s , i t d e l i v e r s them w i t h what H a r t s h o r n e (1973) c a l l s "immediate v a r i e t y " , one song type a f t e r a n o t h e r , o n l y r a r e l y r e p e a t i n g a song t y p e i m m e d i a t e l y . I t adds t o t h e c o n t r a s t by t e n d i n g t o s w i t c h between song t y p e s t h a t a r e e s p e c i a l l y d i s s i m i l a r . A l s o , i f i t s h o u l d happen t o r e p e a t a song type i m m e d i a t e l y o r a f t e r a r e l a t i v e l y s h o r t r e c u r r e n c e i n t e r v a l , i t w i l l t h e n w a i t a l o n g e r t ime b e f o r e Using the song type a g a i n * I began t h i s t h e s i s by d e v e l o p i n g an e v o l u t i o n a r y argument t h a t e x p l a i n s why v a r i e d t h r u s h e s have r e p e r t o i r e s of d i s s i m i l a r songs, and i t i s t e m p t i n g t o extend th e argument to t h e s e p a t t e r n s of t e m p o r a l o r d e r — t h a t i s , t o c l a i m t h e y a r e a d a p t a t i o n s t o reduce h a b i t u a t i o n i n o t h e r b i r d s . But, r e a s o n a b l e as t h i s a s s e r t i o n may seem, i t has no a p r i o r i b a s i s i n t h e t h e o r y of h a b i t u a t i o n . D e s p i t e t h e enormous amount o f a t t e n t i o n t h a t p s y c h o l o g i s t s have p a i d t o h a b i t u a t i o n (see e.g. t h e two volumes e d i t e d by Peeke and Herz 1973), th e t h e o r y i s s t i l l f a r from complete* At p r e s e n t , i t does n o t even a l l o w one t o p r e d i c t , f o r example, t h a t v a r i e d t h r u s h e s w i l l h a b i t u a t e more s l o w l y t o song t y p e s p r e s e n t e d i n the o r d e r t h e y a r e n o r m a l l y sung than t o t h e same songs i n bouts o f each t y p e . T h i s i s s u e w i l l have t o be r e s o l v e d e x p e r i m e n t a l l y . 123 S t i l l , we can s p e c u l a t e on why v a r i e d t h r u s h e s have e v o l v e d t o s i n g w i t h immediate v a r i e t y . I have a l r e a d y mentioned one p o s s i b l e d i s a d v a n t a g e t o t h i s p a t t e r n of s i n g i n g , t h a t i n d i v i d u a l s would be u n l i k e l y t o be a b l e t o p e r p e t r a t e a Beau Geste d e c e p t i o n (Krebs 1977b) on p o t e n t i a l i n t r u d e r s . A p o s s i b l e advantage i s s u g g e s t e d by Kroodsma's (1977) s t u d y o f North American, wrens; S i n g i n g w i t h immediate v a r i e t y o c c u r s e s p e c i a l l y i n s p e c i e s t h a t have polygynous b r e e d i n g s y s t e m s , and t h e r e f o r e : seems t o be a s s o c i a t e d w i t h i n t e n s e s e x u a l s e l e c t i o n imposed by f e males (Kroodsma 1977). The i m p l i c a t i o n i s t h a t f e m a l e s p r e f e r t o mate w i t h males who can d i s p l a y l a r g e r e p e r t o i r e s i n a s h o r t t i m e * A l t h o u g h v a r i e d t h r u s h e s have much s m a l l e r r e p e r t o i r e s than t h e s e wrens and are not known t o be p o l y g y n o u s , mate s e l e c t i o n by f e males might s t i l l be r e s p o n s i b l e f o r the e v o l u t i o n of males who s w i t c h i m m e d i a t e l y between songs. I t i s c l e a r t h a t t h i s form o f s e x u a l s e l e c t i o n and s e l e c t i o n f o r Beau Geste d e c e p t i o n o f o t h e r males would have very d i f f e r e n t e f f e c t s on t h e o r d e r i n which males s i n g t h e i r songs and on o t h e r b e h a v i o r a l p a t t e r n s a s s o c i a t e d w i t h s i n g i n g . I f f e m a l e s spend o n l y a s h o r t amount of t i m e l i s t e n i n g t o each male t h e y e n c o u n t e r w h i l e s e a r c h i n g f o r mates, a male whose b e h a v i o r i s adapted t o i m p r e s s f e m a l e s s h o u l d i n s u r e not o n l y t h a t t h e y hear a l a r g e sample of h i s songs but a l s o t h a t t h e y a r e a b l e t o a s s o c i a t e him w i t h t h e s e songs. He s h o u l d s i n g each song o n l y once b e f o r e s w i t c h i n g , and he s h o u l d e i t h e r s i n g l o n g bouts, from a s i n g l e p e r c h or move c o n s p i c u o u s l y between perches* On t h e o t h e r hand, i f t h e o b j e c t i s t o d e c e i v e 124 p o t e n t i a l i n t r u d e r s , a male s h o u l d s i n g a bout o f one song t y p e from a h i d d e n p e r c h , t h e n move f u r t i v e l y t o a n o t h e r perch and s i n g a bout o f a n o t h e r song t y p e . Suppose a t e r r i t o r i a l male i s f a c e d w i t h t h e double c h a l l e n g e o f a t t r a c t i n g a mate and r e p e l l i n g p o t e n t i a l i n t r u d e r s . I do n o t know i f t h i s i s t r u e f o r v a r i e d t h r u s h e s , b u t i t p r o b a b l y i s f o r some s p e c i e s . I t i s d i f f i c u l t t o imagine a s i m p l e s o l u t i o n t h a t would, a l l o w him b o t h t o i m p r e s s f e m a l e s w i t h h i s l a r g e r e p e r t o i r e and a l s o t o d e c e i v e o t h e r males; t h r o u g h n a t u r a l s e l e c t i o n ^ he would p r o b a b l y o p t f o r one or t h e o t h e r of the s t r a t e g i e s . I t i s i n t e r e s t i n g , however, t h a t i n some s p e c i e s t e m p o r a l p a t t e r n s o f s i n g i n g change over t i m e * F o r example, as t h e b r e e d i n g season p r o g r e s s e s , Bewick's wrens (Thryomanes b e w i c k i i ) s i n g l o n g e r bouts of each song t y p e but a t s l o w e r r a t e s (Kroodsma 1977) . S i m i l a r l y , but from morning t o a f t e r n o o n , l o n g - b i l l e d marsh wrens change from s w i t c h i n g i m m e d i a t e l y between song t y p e s t o sometimes d e l i v e r i n g them s e v e r a l t i m e s i n s u c c e s s i o n , a change t h a t a g a i n c o i n c i d e s w i t h a r e d u c t i o n i n t h e r a t e o f s i n g i n g . F o r r e a s o n s unknown, t h i s o c c u r s i n I l l i n o i s p o p u l a t i o n s , where r e p e r t o i r e s a r e r e l a t i v e l y s m a l l , b u t not i n Washington p o p u l a t i o n s (Kroodsma 1977). I r e c o r d e d v a r i e d t h r u s h e s a t d i f f e r e n t t i m e s d u r i n g t h e day and the b r e e d i n g season but found no e v i d e n c e t h a t i n d i v i d u a l s a l t e r t h e i r s i n g i n g b e h a v i o r i n ways such as t h e s e * T h i s i s a s u b j e c t t h a t d e s e r v e s f u r t h e r a t t e n t i o n , however, s i n c e changes i n t e m p o r a l p a t t e r n s o f s i n g i n g may r e f l e c t changes i n t h e f u n c t i o n a l s i g n i f i c a n c e o f song* 125 CONTROL. OF SINGING The t h e o r y I have deve l o p e d i n t h i s t h e s i s i s r i c h i n i t s e x p l a n a t o r y power* I t t i e s t o g e t h e r p a t t e r n s based on 1) t h e s i m i l a r i t y of song t y p e s w i t h i n r e p e r t o i r e s , 2) t h e r e l a t i v e f r e q u e n c i e s o f d i f f e r e n t song t y p e s , 3) the o r d e r of songs i n sequences, and 4) t h e l e n g t h s o f pauses between songs. I t has been a u s e f u l t o o l f o r e x p l o r i n g t h e c o n t r o l of s i n g i n g i n v a r i e d t h r u s h e s , and we can now ask whether i t s u t i l i t y e x t e n d s to o t h e r s p e c i e s * an o b v i o u s problem i s t h a t t h e songs of most s p e c i e s a r e f a r more complex t h a n t h o s e o f v a r i e d t h r u s h e s . To reduce them t o a few s i m p l e d i m e n s i o n s from which t h e i r s i m i l a r i t y can be measured i s l i k e l y t o be an a r b i t r a r y e x e r c i s e , t h e r e s u l t s b e a r i n g l i t t l e r e l a t i o n to the way the b i r d s t h e m s e l v e s p e r c e i v e s i m i l a r i t y * A l t h o u g h t h e i m p o r t a n t d i m e n s i o n s c o u l d perhaps be i d e n t i f i e d e x p e r i m e n t a l l y , I t h i n k a more p r o d u c t i v e approach w i l l be t o c o n c e n t r a t e on s p e c i e s t h a t have r e l a t i v e l y s i m p l e songs. S i m i l a r i t y .of. Songs_in_ I n d i v i d u a l - R e p e r t o i r e s There a r e s e v e r a l examples i n t h e l i t e r a t u r e o f a t t e m p t s t o f i n d p a t t e r n s based on the s i m i l a r i t y of song t y p e s . I have a l r e a d y r e v i e w e d t h e e v i d e n c e t h a t g r e a t t i t s have r e p e r t o i r e s o f d i s s i m i l a r song t y p e s (Krebs 1977a). U n l i k e v a r i e d t h r u s h e s , g r e a t t i t s d e l i v e r t h e i r songs i n b o u t s of a s i n g l e t y p e , and i t would be i n t e r e s t i n g t o know whether, i n s p i t e of t h i s d i f f e r e n c e , t h e y a l s o t e n d t o s w i t c h between e s p e c i a l l y 126 d i s s i m i l a r songs* P r o b a b l y the most e x h a u s t i v e s e a r c h f o r t h i s p a t t e r n has been made by Verner (1975) i n h i s s t u d y of l o n g - b i l l e d marsh wrens. A t y p i c a l marsh wren song b e g i n s w i t h a s e r i e s o f i n t r o d u c t o r y n o t e s , has a "main p h r a s e " composed of a s e r i a l l y r e p e a t e d s y l l a b l e , and may end w i t h a n o t e . o r a buzz. Verner measured t h e s i m i l a r i t y o f the song t y p e s i n an i n d i v i d u a l ' s r e p e r t o i r e a c c o r d i n g t o each o f 15 s e p a r a t e v a r i a b l e s * E l e v e n of t h e s e are based on i n t e r v a l s c a l e s and i n c l u d e such t h i n g s as t h e l e n g t h o f the s y l l a b l e i n t h e main phrase and the dominant f r e q u e n c y o f n o t e s * and t h e o t h e r f o u r a re based on t h e p r e s e n c e o r absence o f such f e a t u r e s a s t e r m i n a l n o t e s . The r e s u l t s a r e not i n s t r o n g s u p p o r t of t h e i d e a t h a t marsh wrens s w i t c h between e s p e c i a l l y d i s s i m i l a r songs: s u c c e s s i v e song t y p e s a r e s i g n i f i c a n t l y more d i s s i m i l a r t h a n ones chosen at random from the sequence o n l y w i t h r e s p e c t t o two v a r i a b l e s , t h e p r e s e n c e o r absence o f t e r m i n a l buzzes and " m u s i c a l " (as judged by Verner) n o t e s . There i s b e t t e r e v i d e n c e f o r t h i s p a t t e r n i n r o c k wrens ( S a l p i h c t e s o b s o l e t u s ) . T h e i r songs a r e t r i l l s o f s e r i a l l y r e p e a t e d s y l l a b l e s , and i n d i v i d u a l s have r e p e r t o i r e s o f up t o 100 o r more d i f f e r e n t t y p e s , which t h e y d e l i v e r by s w i t c h i n g i m m e d i a t e l y from one t o a n o t h e r . They c y c l e t h r o u g h t h e i r r e p e r t o i r e s by s i n g i n g a few song t y p e s , each s e v e r a l t i m e s , g r a d u a l l y r e p l a c i n g them w i t h o t h e r s and e v e n t u a l l y r e t u r n i n g t o t h e o r i g i n a l ones: ABCADBA. . .IGIHJIK... PROEQPS.,. . BACABDA*, Eeduced t o e i t l i e r o f two d i m e n s i o n s , r a t e of s y l l a b l e r e p e t i t i o n and f r e q u e n c y range (maximum minus minimum 127 f r e q u e n c y ) * song t y p e s t h a t o c c u r c l o s e t o g e t h e r t e n d t o be more d i s s i m i l a r than ones drawn a t random from a sequence (Kroodsma 1975). A somewhat s i m i l a r p a t t e r n may o c c u r i n Swainson's t h r u s h e s ( H y l o c i c h l a u s t u l a t a ) . I n d i v i d u a l s have r e p e r t o i r e s o f t h r e e t o seven song t y p e s (Dobson and Lemon 1977), and they s i n g each song type o n l y once b e f o r e switching,* Nelson (1973) s t u d i e d a sequence r e c o r d e d from a male t h a t had a r e p e r t o i r e o f s i x song t y p e s * He r e p o r t e d t h a t each song t y p e , d e s p i t e b e i n g a r a t h e r complex " r o l l i n g s e r i e s o f r a p i d f l u t e - l i k e n o t e s , r i s i n g up t h e s c a l e " (Bobbins e t a l * 1966), seemed t o be sung t o a d i f f e r e n t key. D e f i n i n g a "keyn o t e " (the l o w e r f r e g u e h c y of the second s y l l a b l e o r "pr i m a r y p a t t e r n " ) f o r each song t y p e , and r a n k i n g t h e k e y n o t e s a c c o r d i n g t o t h e i r f r e q u e n c y , he found t h a t the b i r d u s u a l l y s w i t c h e d between songs t h a t had k e y n o t e s e i t h e r two o r t h r e e r a n k s a p a r t . Shown g r a p h i c a l l y , the seguence o f songs, as reduced t o these r a n k s , forms a " r h o m b o i d a l " p a t t e r n . Nelson a p p l i e d t h e same a n a l y s i s t o a sequence o f songs r e c o r d e d from a western meadowlark ( S t u r n e l l a n e g l e c t a ) , t h i s time r a n k i n g each song type by t h e f r e g u e n c y of i t s f i r s t l o n g n o t e , and a g a i n t h e s w i t c h e s between song t y p e s appeared t o form a rhomboidal p a t t e r n . O t h e r w o r k e r s , however, have o b t a i n e d r e s u l t s c o n t r a d i c t o r y t o t h e s e . I n a d e t a i l e d s t u d y o f Swainson's t h r u s h e s , Dobson and Lemon (1977) r e j e c t e d N e l s o n ' s c l a i m t h a t song t y p e s a r e sung t o d i f f e r e n t keys and t h u s , as a consequence, h i s concept of a keynote. F a l l s and Krebs (1975) were a b l e t o rank t h e songs o f western meadowlarks i n the same 128 way t h a t N e l s o n d i d but found no evidence f o r r h o m b o i d a l p a t t e r n i n g * N e v e r t h e l e s s , i t seems premature t o r e j e c t Nelson's i d e a s e n t i r e l y ; p a t t e r n s ( a l t h o u g h not n e c e s s a r i l y r h o m b o i d a l ones) may e x i s t b u t l i e u n d i s c o v e r e d f o r want of a p p r o p r i a t e d e s c r i p t i o n s o f songs. C o n t r o l o f S i n g i n g i n S e l e c t e d S p e c i e s Now c o n s i d e r i n more d e t a i l t h e c o n t r o l of s i n g i n g i n s e v e r a l o f t h e s e s p e c i e s . A l t h o u g h Swainson's t h r u s h e s a r e s i m i l a r t o a number of o t h e r s p e c i e s , i n c l u d i n g v a r i e d t h r u s h e s , i n t h a t t h e y s w i t c h i m m e d i a t e l y between d i f f e r e n t song t y p e s , t h e y have the u n u s u a l h a b i t o f s i n g i n g t h e i r songs i n a f i x e d o r r n e a r l y f i x e d o r d e r . For example, one male r e c o r d e d by Dobson and Lemon (1977) sang h i s r e p e r t o i r e o f f i v e song t y p e s i n the f o l l o w i n g o r d e r f o r t h e e n t i r e sample of 346 songs; ABCDEABCDE. Fox sparrows ( P a s s e r e l l a i l i a c a ) may behave i n a s i m i l a r manner ( M a r t i n 1977) . Suppose t h a t s i n g i n g i n t h e s e . s p e c i e s i s c o n t r o l l e d by a system s i m i l a r t o t h e one I have proposed f o r v a r i e d t h r u s h e s . What a r e t h e c h a r a c t e r i s t i c s o f a model t h a t w i l l produce a f i x e d sequence such as ABCDE? Although I have not e x p l o r e d t h i s g u e s t i o n i n d e t a i l , i t seems n e c e s s a r y f i r s t t o assume t h a t t h e m o t i v a t i o n a l s t a t e of each song u n i t can not drop below T h r e s h o l d minus I n h i b i t i o n - (see p,. 7 7 ) . Then a model based on a m a t r i x of i n h i b i t i o n v a l u e s such as the. one shown i n T a b l e XVII w i l l produce the f i x e d sequence (the u s u a l p i c t o r i a l models a r e o n l y c o n f u s i n g i n t h i s case because the system 129 Table X V I I . M a t r i x o f i n h i b i t i o n v a l u e s t h a t w i l l produce a f i x e d sequence: ABCDEABCDE. I n h i b i t i o n on U n i t A U n i t B U n i t _ C U n i t D U n i t E I n h i b i t i o n from U n i t A 25.0 5.0 10.0 15.0 20.0 U n i t B 20.0 25.0 5.0 10.0 15.0 U n i t C 15.0 20.0 25.0 5.0 10.0 U n i t D 10.0 15.0 20.0 25.0 5.0 U n i t E 5.0 10.0 15.0 20.0 25.0 130 i n c l u d e s so many song u n i t s ; t o c o n v i n c e y o u r s e l f o f t h e argument, do t h e c a l c u l a t i o n s by hand). Note i n t h e m a t r i x t h e p a t t e r n ,of a s y m m e t r i c a l i n h i b i t i o n between song u n i t s . T h i s p a t t e r n c o r r e s p o n d s t o the o r d e r i n which songs a r e produced. U n i t A, f o r example,, i n h i b i t s B l e s s t h a n i t does any o f t h e o t h e r u n i t s , and as a r e s u l t , song B always, f o l l o w s song A; b u t u n i t 3 i n h i b i t s A more than i t does any o t h e r u n i t e x c e p t i t s e l f , hence song A i s never sung u n t i l a f t e r songs C, D, and E. Assume f o r t h e sake of argument t h a t the o r d e r i n which songs a r e sung i s r e l a t e d t o t h e i r s i m i l a r i t y . I n the model developed f o r v a r i e d t h r u s h e s , t h e a s y m m e t r i c a l i n h i b i t i o n between song u n i t s i s s i m p l y a p r o d u c t o f random v a r i a t i o n i n th e r e l a t i o n s h i p between i n h i b i t i o n and s i m i l a r i t y , but i n t h i s model i t f o l l o w s a d e f i n i t e p a t t e r n , and t h i s p a t t e r n r e q u i r e s an e x p l a n a t i o n . I t w i l l be a c h a l l e n g e t o t r y t o d e v e l o p t h e t h e o r y a l o n g t h e s e l i n e s . A f t e r r e j e c t i n g N e l s o n * s (1973) t h e o r y of r h o m b o i d a l p a t t e r n i n g , Dobson and Lemon (1977) proposed an a l t e r n a t i v e , t h a t t h e f i x e d o r d e r i n which Swainson's t h r u s h e s s i n g t h e i r songs "may be c o n s i d e r e d a p e r f e c t f i r s t o r d e r Markov sequence." That i s t o say, each song t y p e i s a p e r f e c t p r e d i c t o r o f t h e song t y p e t o f o l l o w . Although i t may be t r u e t h a t Swainson's t h r u s h e s n o r m a l l y s i n g t h e i r songs i n a f i x e d o r d e r , t h i s i s not always the c a s e , and t h e e x c e p t i o n s a r e i n s t r u c t i v e . The b i r d s t u d i e d by N e l s o n , f o r example, sang t h e f o l l o w i n g seguence o f 52 songs (the symbols a r e mine): CEBDFCEDEBDFCEAFDFCDFCEBDFCEAFDEBDFCEBDFCEAFCEBDFCEB Note t h a t , a l t h o u g h s h o r t p a t t e r n s such a s BDFCE a r e r e p e a t e d 131 f r e q u e n t l y , t h e sequence does not a t a l l resemble a p e r f e c t f i r s t o r d e r Markov sequence. There i s no way the songs c o u l d be r e a r r a n g e d t o form such a sequence, because they do n o t o c c u r w i t h e q u a l f r e q u e n c y . Songs C, D, E, and F oc c u r 10 or 11 t i m e s each, song B o c c u r s 7 t i m e s , and A j u s t 3 t i m e s . A b i r d r e c o r d e d by Dobson and Lemon a l s o d e v i a t e d from the Markov model, b u t i n a r a t h e r d i f f e r e n t way. He n o r m a l l y sang h i s seven song t y p e s i n a f i x e d o r d e r , ABCDEFG, but o c c a s i o n a l l y o m i t t e d one o r two, s i n g i n g i n s t e a d , ABCEFG, ABCFG, o r ABCDEF. Dobson and Lemon e x p l a i n e d the second d e v i a n t p a t t e r n by p o i n t i n g out t h a t , s i n c e song t y p e C i s almost i d e n t i c a l t o E, t h i s may have " c o n f u s e d the b i r d i n t o assuming he had a l r e a d y sung the p a r t i c u l a r song type i n q u e s t i o n and he t h e r e f o r e s k i p p e d on t o a l a t e r p o i n t i n t h e s e r i e s . . . . " T h i s i s an i n t e r e s t i n g s u g g e s t i o n , but i t does not e x p l a i n t h e o t h e r two anomalous p a t t e r n s . I f we j u s t a c c e p t t h a t Swainson's t h r u s h e s o c c a s i o n a l l y s k i p o v e r a song t y p e , f o r some u n a c c o u n t a b l e r e a s o n , we can go on t o ask how t h e y d e c i d e which song t y p e t o s i n g n e x t . A f i r s t o r d e r Markov model w i t h t r a n s i t i o n p r o b a b i l i t i e s of 1.0 o f f e r s no e x p l a n a t i o n . Yet they do not seem t o choose at.random ( t h a t i s , . f r o m . a l l . s o n g t y p e s e x c e p t t h e l a s t one sung and t h e one om i t t e d ) ,. s i n c e t h e b i r d r e c o r d e d by Dobson and Lemon never o m i t t e d more t h a n two songs i n any of t h e e i g h t t i m e s he sang a b e r r a n t sequences. My own model i s more h e l p f u l * I f I f o r c e i t t o . s k i p a song t y p e , i t c a r r i e s on, j u s t l i k e t he b i r d , with the n e x t song or t h e one. a f t e r t h a t * A l t h o u g h t h i s i s an e n c o u r a g i n g r e s u l t , t h e r e a l . t e s t o f t h e model w i l l come when i t i s used t o p r e d i c t p a t t e r n s t h a t 132 have not a l r e a d y been observed* U n l i k e Swainson's t-hrushes o r , v a r i e d t h r u s h e s , w e s t e r n meadowlarks r e p e a t each song t y p e a number of t i m e s b e f o r e s w i t c h i n g . T h i s i s a common p a t t e r n o f s i n g i n g , and t o e x p l a i n i t o t h e r a u t h o r s have proposed a system t h a t i n c l u d e s not o n l y s e l f i n h i b i t i o n but a l s o s e l f f a c i l i t a t i o n of song u n i t s (Hinde 1958; Lemon and C h a t f i e l d 1971). S e l f f a c i l i t a t i o n i s assumed t o be r e s p o n s i b l e f o r t h e immediate r e p e t i t i o n of song t y p e s , s e l f i n h i b i t i o n f o r s w i t c h e s between song types;. T h i s t h e o r y has never been e x p r e s s e d as a model* but t h e r e i s no reason why t h i s c o u l d not be done, and t h e model used t o make p r e d i c t i o n s . Now c o n s i d e r an a l t e r n a t i v e t h e o r y which i s s i m i l a r t o t h e t h e o r y developed f o r v a r i e d t h r u s h e s i n t h a t song u n i t s i n h i b i t , but do not f a c i l i t a t e , t h e m s e l v e s and each o t h e r . I f t h e magnitude of s e l f i n h i b i t i o n i s made l e s s than the minimum amount o f i n h i b i t i o n between u n i t s , a model based on t h i s t h e o r y produces one song type r e p e a t e d l y ( F i g . 3 3 a ) . Thus s e l f f a c i l i t a t i o n i s not a n e c e s s a r y c o n d i t i o n f o r t h e immediate r e p e t i t i o n p f song t y p e s . S t i l l , one might d e v e l o p a roundabout argument i n f a v o r o f t h e f i r s t t h e o r y . R e c a l l t h e d i s c u s s i o n o f matched c o u n t e r s i n g i n g i n the l a s t c h a p t e r . I n matched c o u n t e r s i n g i n g , a song u n i t i n one i n d i v i d u a l i s f a c i l i t a t e d by the sound o f a n o t h e r i n d i v i d u a l ' s songs, and t h i s s u g g e s t s t h a t i t may a l s o be f a c i l i t a t e d by t h e sound of i t s own songs* T h i s argument does not a p p l y t o western meadowlarks, s i n c e t h e y do n o t p r a c t i c e matched c o u n t e r s i n g i n g ( F a l l s and Krebs 1975), but i t does a p p l y t o c a r d i n a l s (Lemon 1968), c h a f f i n c h e s (Hinde 1958), and g r e a t t i t s (Krebs 1977a). 133 F i g u r e 33. E l a b o r a t i o n s o f t h e v a r i e d t h r u s h model t o a c c o u n t f o r t h e c o n t r o l of s i n g i n g i n o t h e r s p e c i e s . a. Magnitude o f s e l f i n h i b i t i o n i s l e s s than minimum amount o f i n h i b i t i o n between u n i t s . b. Magnitude o f s e l f i n h i b i t i o n g r a d u a l l y i n c r e a s e s u n t i l i t i s l a r g e r t h a n i n h i b i t i o n o f another u n i t . c. I n h i b i t i o n of o t h e r u n i t s g r a d u a l l y d e c l i n e s . d* S i m i l a r t o p a r t c, except t h a t t h e r e i s some s t o c h a s t i c v a r i a t i o n i n s e l f i n h i b i t i o n . A THRESHOLD A A A A A A ,A A A / /A/A/A/A// B THRESHOLD A TT B B W\AA A$ \/ v "/ if \y \S D THRESHOLDJ- A A ; > J ? 7 / i A /A B / v / \ / \//\y/ / v v V. \y 135 C o n t r a r y t o the argument, however^ s u r g i c a l l y deafened c h a f f i n c h e s do not a l t e r t h e t e m p o r a l o r g a n i z a t i o n of t h e i r s i n g i n g ( K o n i s h i and Nottebohm 1969). C o n t i n u i n g w i t h the second t h e o r y , we must n e x t e x p l a i n t h e s w i t c h e s between song t y p e s . Western meadowlarks s i n g each o f t h e i r song t y p e s i n b o u t s t h a t average 3 t o 15 songs i n l e n g t h ( F a l l s and Krebs 1975); w i t h i n t h e s e b o u t s , t h e i n t e r v a l s between s u c c e s s i v e songs become i n c r e a s i n g l y l o n g e r ( F i s h e t a l . 1962),. I n terms o f t h e model, i t appears t h a t t h e s e l f i n h i b i t i o n of the u n i t t h a t i s p r o d u c i n g songs g r a d u a l l y i n c r e a s e s u n t i l t h e u n i t b e g i n s t o i n h i b i t i t s e l f more th a n i t does one of t h e o t h e r u n i t s . At t h i s p o i n t t h e o t h e r u n i t can r e a c h the t h r e s h o l d and i t s e l f produce a bout of songs ( F i g . 33b) . I t would be g r a t i f y i n g i f t h i s mechanism of s w i t c h i n g a p p l i e d t o a l l s p e c i e s i n which i n d i v i d u a l s r e p e a t song t y p e s a number o f t i m e s b e f o r e s w i t c h i n g , but I have e v i d e n c e t h a t t h i s i s not t h e c a s e ; R u f o u s - s i d e d towhees s i n g l o n g b o u t s o f each song t y p e , and a l t h o u g h t h e r a t e o f s i n g i n g can f l u c t u a t e c o n s i d e r a b l y , i t does not n e c e s s a r i l y d e c l i n e and r e a c h a minimum a t t h e end of b o u t s ( F i g . 34). I n towhees, t h e r e f o r e , s w i t c h i n g must depend.on something b e s i d e s a b u i l d - u p o f s e l f i n h i b i t i o n , a t . l e a s t as i n h i b i t i o n i s m a n i f e s t e d by an i n c r e a s e i n the i n t e r v a l s between songs. An a l t e r n a t i v e mechanism, which assumes t h a t t h e i n h i b i t i o n o f o t h e r song u n i t s d e c l i n e s d u r i n g b o u t s , i s shown i n F i g u r e 33c. D e s p i t e t h i s d i f f e r e n c e between western meadlowlarks and r u f o u s - s i d e d towhees, one p a t t e r n a s s o c i a t e d w i t h s w i t c h e s i s 136 F i g u r e 3 4 . I n t e r v a l s between s u c c e s s i v e songs i n a sequence sung by a r i i f o u s - s i d e d towhee. I n t e r v a l s are shown i n sequence from l e f t t o r i g h t . The b i r d s w i t c h e d t o a d i f f e r e n t song type a t t h e end o f t h i s sequence. a • 84 I 1 6+ • a a a c 3 a • a a n a > 10 20 30 40 50 Interval Number H U) 138 common t o both s p e c i e s : i n t e r v a l s between bouts t e n d t o be l o n g e r than i n t e r v a l s between s u c c e s s i v e songs w i t h i n b o u t s ( F i s h et a l . 1962; p e r s ; o b s e r v . ) . Reported a l s o i n c h a f f i n c h e s (Hinde 1958) and dark-eyed j u n c o s (Junco hyemajis) ( K o n i s h i 1964), t h i s p a t t e r n must be a ccounted f o r by a t h e o r y o f c o n t r o l ; There i s something wrong t h e r e f o r e w i t h the model shown i n F i g u r e 33c, s i n c e i t produces sequences i n which i n t e r v a l s between bouts are always th e same l e n g t h o r s h o r t e r than i n t e r v a l s w i t h i n b o u t s ; An i n t e r e s t i n g m o d i f i c a t i o n o f t h i s model i s shown i n F i g u r e 33d. The o n l y change has been t o add a random v a r i a b l e t o t h e r a t e a t which song u n i t s r e c o v e r toward t h e t h r e s h o l d ; As a r e s u l t , t h e i n t e r v a l s between s u c c e s s i v e r e p e t i t i o n s of a song t y p e are v a r i a b l e , j u s t as t h e y a r e i n seguences produced by b i r d s ( F i g * 3 4 ) ; I have n o t s t u d i e d a computer v e r s i o n o f t h i s model, but i n t u i t i v e l y i t seems t h a t i n t e r v a l s between b o u t s w i l l now t e n d t o be l o n g e r than i n t e r v a l s w i t h i n bouts. To see t h i s , i m agine t h a t u n i t A i s p r o d u c i n g a bout of songs, and t h a t t h e m o t i v a t i o n a l s t a t e o f u n i t B i s a p p r o a c h i n g t h a t o f A (because B i s i n h i b i t e d l e s s each t i m e song A i s sung) but i s s t i l l somewhat l o w e r . I f A, a f t e r p r o d u c i n g a song, happens by chance t o r e c o v e r a t a s l o w e r r a t e than u s u a l , B may reach t h e t h r e s h o l d f i r s t - - - b u t o n l y a f t e r a r e l a t i v e l y l o n g i n t e r v a l ( F i g . 33d). Undoubtedly t h e r e a r e o t h e r ways t o account f o r t h e p a t t e r n , but t h i s model has t h e v i r t u e of making a p r e d i c t i o n : T h e . l e n g t h s o f t h e i n t e r v a l s f o l l o w i n g bouts s h o u l d be i n v e r s e l y c o r r e l a t e d w i t h t h e l e n g t h s of the bouts. T h i s i s because t h e m o t i v a t i o n a l s t a t e s o f o t h e r u n i t s g r a d u a l l y i n c r e a s e d u r i n g a bout. 139 T h e r e f o r e t h e l o n g e r t h e bout t h e l e s s time t h e y w i l l t a k e to rea c h t h e t h r e s h o l d . Another p a t t e r n , known t o o c c u r i n western meadowlarks ( F a l l s and Krebs 1975) and g r e a t t i t s (Krebs 1977a) and p r o b a b l y widespread among b i r d s t h a t r e p e a t song t y p e s i n b o u t s , i s t h e tendency o f i n d i v i d u a l s t o c y c l e through t h e i r r e p e r t o i r e s , a v o i d i n g low r e c u r r e n c e i n t e r v a l s ( d e f i n e d .here as t h e number o f s w i t c h e s between s u c c e s s i v e b o u t s of the same song t y p e ) . R e l a t e d t o t h i s , Krebs (1977a) found t h a t i n p l a y b a c k e x p e r i m e n t s g r e a t t i t s a r e l e s s l i k e l y t o match one o f t h e i r own songs i f t h e y have j u s t f i n i s h e d s i n g i n g i t th a n i f they have a l r e a d y s w i t c h e d s e v e r a l t i m e s s i n c e s i n g i n g i t . I t w i l l be i n t e r e s t i n g t o c o n s i d e r t h e s e p a t t e r n s t h e o r e t i c a l l y . Assuming t h a t they r e f l e c t l o n g term e f f e c t s of s e l f i n h i b i t i o n , a model t h a t a c c o u n t s f o r them may p r e d i c t r e l a t e d p a t t e r n s . Two p o s s i b i l i t i e s a r e 1) a n e g a t i v e c o r r e l a t i o n between the l e n g t h s of s u c c e s s i v e r e c u r r e n c e i n t e r v a l s s i m i l a r t o t h e one found i n v a r i e d t h r u s h e s (Table V I I ) , and 2) a p o s i t i v e c o r r e l a t i o n between the l e n g t h s o f b o u t s and t h e l e n g t h s of t h e f o l l o w i n g r e c u r r e n c e i n t e r v a l s . The g e n e r a l p a t t e r n i n which r o c k wrens s i n g ABCADBA...IGIHJIK...PSOSQPS...BACABDA has been d e s c r i b e d as i n t e r m e d i a t e between the ABCBADC p a t t e r n o f v a r i e d t h r u s h e s (or t h e r a t h e r s i m i l a r ABCDABCD of Swainson's thrushes) and the AAA...BBB... p a t t e r n of western meadowlarks (Kroodsma 1975). C o n s i d e r f o u r components of t h e p a t t e r n : 1) Song t y p e s a r e not u s u a l l y r e p e a t e d i m m e d i a t e l y . 2) Each one i s sung a few t i m e s , a l o n g w i t h s e v e r a l o t h e r s , and then i s not used a g a i n f o r a 140 c o n s i d e r a b l e l e n g t h of t i m e ; 3) During the p e r i o d when a song type i s used, t h e r e c u r r e n c e i n t e r v a l s g e t i n c r e a s i n g l y l a r g e r . . 4) Song t y p e s t h a t o c c u r c l o s e t o g e t h e r tend t o be d i s s i m i l a r t o each o t h e r (Kroodsma 1975). I n terms o f a model of c o n t r o l , t h e f i r s t component i n d i c a t e s t h a t s e l f i n h i b i t i o n o f song u n i t s i s g r e a t e r t h a n i n h i b i t i o n between u n i t s ; t o g e t h e r t h e second and t h i r d suggest t h a t s e l f i n h i b i t i o n g r a d u a l l y i n c r e a s e s (as shown i n F i g u r e 33b) and t h e n wanes a t a s low r a t e ; t h e f o u r t h i n d i c a t e s t h a t t h e magnitude of i n h i b i t i o n between u n i t s i s r e l a t e d t o t h e s i m i l a r i t y o f t h e songs t h e y produce. e I s u s p e c t t h a t a l t h o u g h a model based on t h e s e i d e a s would mimic t h e f e a t u r e s l i s t e d above, i t would s t i l l not a c c o u n t f o r t h e r a t h e r p r e c i s e way i n which r o c k wrens c y c l e t h r o u g h t h e i r r e p e r t o i r e s . From Kroodsma 8s (1975) d e s c r i p t i o n , i t appears t h a t one s e t of song t y p e s p r e d i c t a b l y f o l l o w s a n o t h e r — - - i n a mannner r e m i n i s c e n t of the f i x e d o r d e r i n which Swainson's t h r u s h e s s i n g t h e i r songs: Thus, where a Swainson's t h r u s h s i n g s ,;..AB... but never ..;BA..., a r o c k wren may s i n g .:.; C DEDF;.. but never FGEHC... E e c a l l t h a t t h i s k i n d o f b e h a v i o r can be accounted f o r by a model i n which t h e i n h i b i t i o n between song u n i t s f o l l o w s a p a r t i c u l a r a s y m m e t r i c a l p a t t e r n (Table X V I I ) , but t h e r e may w e l l be a l t e r n a t i v e models t h a t a r e more a p p r o p r i a t e . Rock wrens o c c a s i o n a l l y s i n g a song t y p e t w i c e i n s u c c e s s i o n (...AA...), and i n such c a s e s the i n t e r v a l between the two songs i s l o n g e r than when an o t h e r song t y p e i n t e r v e n e s (.;;. ABA... j . To paraphrase Kroodsma (1975), song B i n t h i s 141 example appears t o reduce t h e s e l f i n h i b i t i o n of u n i t A, a l l o w i n g i t t o produce a n o t h e r song sooner t h a n i t would have o t h e r w i s e . I n o t h e r words, B f a c i l i t a t e s A, T h i s c l e a r l y r e f u t e s t h e model I have been a d v o c a t i n g , s i n c e the model assumes o n l y i n h i b i t o r y c o n n e c t i o n s between song u n i t s * I have not been a b l e t o f i n d s i m i l a r e v i d e n c e o f f a c i l i t a t i o n e i t h e r i n my own d a t a c o l l e c t e d from v a r i e d t h r u s h e s or i n o t h e r p u b l i s h e d accounts o f s i n g i n g b e h a v i o r , b u t we s h o u l d n o t i g n o r e the p o s s i b i l i t y t h a t f a c i l i t a t i o n i s o f more widespread importance i n the c o n t r o l of s i n g i n g . In t h i s r e v i e w I have taken t h e t h e o r y d e v e l o p e d s p e c i f i c a l l y t o e x p l a i n t h e c o n t r o l o f s i n g i n g i n v a r i e d t h r u s h e s and e l a b o r a t e d i t t o account f o r p a t t e r n s o f s i n g i n g i n o t h e r s p e c i e s . Some of t h e s e e l a b o r a t i o n s can be c r i t i c i z e d as i n a d e q u a t e t o account f o r a l l observed p a t t e r n s . Some can be c r i t i c i z e d as f a r f e t c h e d . But, as Fodor (1975) has s a i d , "Even a r e m o t e l y p l a u s i b l e t h e o r y i s b e t t e r t h a n no t h e o r y at a l l " , and my hope i s t h a t t h i s one w i l l a t l e a s t serve as a s t i m u l u s f o r f u r t h e r r e s e a r c h . 142 LITERATURE CITED Altum, B. 1868. Der v o g e l und s e i n l e b e n . W i l h e l m Neimann, Munster. B a i l e y , F. A. 1924. Handbook of b i r d s of t h e western U n i t e d S t a t e s . Houghton M i f f I o n Co., Boston* Bent, A. C. 1949* L i f e h i s t o r i e s of North American t h r u s h e s , k i n g l e t s , and t h e i r a l l i e s : o r d e r P a s s e r i f o r m e s . U.S. N a t l . Mus. B u l l , . 196. : Bertram, B. C. R. 1976. K i n s e l e c t i o n i n l i o n s and e v o l u t i o n . I n P. P. G* Bateson and R. A. Hinde ( e d s . ) , Growing p o i n t s i n ethology,. Cambridge U n i v . P r e s s , Cambridge. B l o u g h , D. S. 1972. R e c o g n i t i o n by the p i g e o n of s t i m u l i v a r y i n g i n two d i m e n s i o n s . J . Expt. A n a l . Behav. 18: 345-367. Brockway, B. F. 1969. R o l e s o f b u d g e r i g a r v o c a l i z a t i o n s i n t h e i n t e g r a t i o n o f b r e e d i n g b e h a v i o r . I n R. A. Hinde ( e d ; ) , B i r d v o c a l i z a t i o n s . Cambridge Univ. P r e s s , Cambridge* C h a t f i e l d , C. and fi. E. Lemon. 1970. A n a l y s i n g sequences o f b e h a v i o u r a l e v e n t s * J . Theor. B i o l 29: 427-445. Dawkins, R. 1976. H i e r a r c h i c a l o r g a n i s a t i o n : a c a n d i d a t e p r i n c i p l e f o r e t h o l o g y . I n p* P. G* B a t e s o n and R* A; Hinde ( e d s . ) . Growing p o i n t s i n ethology,. Cambridge Univ. P r e s s , Cambridge* Dobson, ,C. W. and R. E. Lemon. 1977. Markovian v e r s u s r h o m b o i d a l p a t t e r n i n g i n t h e song of Swainson's t h r u s h . B e h a v i o u r 62: 276-297. F a l l s , J . B. and J * R. Krebs. 1975. Seguences of songs i n r e p e r t o i r e s of western meadowlarks ( S t u r n e l l a n e g l e c t a ) . Can. J . Z o o l . 53: 1165-1178. ~~ T F i s h , W. 2., K. N e l s o n , and D. I s a a c * 1962,; The t e m p o r a l p a t t e r n i n g .of meadowlark song. Amer. Z o o l . 2: 409. ( A b s t r a c t ) F o dor, J,. A. 1975. The language o f t h o u g h t ; Thomas Y. C r o w e l l Co., New York. H a r t s h o r n e , C. 1973. Born t o s i n g . I n d i a n a Univ. P r e s s , B l o o m i n g t o n . Hinde, R. A. 1958. A l t e r n a t i v e motor p a t t e r n s i n c h a f f i n c h song. Anim* Behav* 6: 211-218. 143 Hinde, E. A,. 1970a,. Animal b e h a v i o u r (2nd ed,.). M c G r a w - H i l l Book Co., New York. Hinde, E. A. 1970b. B e h a v i o r a l h a b i t u a t i o n ; I n G. Horn and B,. A. Hinde ( e d s . ) , S h o r t - t e r m changes i n n e u r a l a c t i v i t y and b e h a v i o r . Cambridge U n i v ; P r e s s , Cambridge* Hinde, E. A. and E. S t e e l . 1976. The e f f e c t o f male song on an estrogen-dependent b e h a v i o r p a t t e r n i n t h e female canary ( S e r i n u s c a n a r i u s ) . Hornu Behav. 7: 293-304. \ ,—,—i K o n i s h i , M. 1964. Song v a r i a t i o n i n a p o p u l a t i o n o f Oregon j u n c o s . The Condor 66: 423-436. K o n i s h i , M. and F. Nottebohm. 1969. E x p e r i m e n t a l s t u d i e s i n t h e ontogeny of a v i a n v o c a l i z a t i o n s . I n E: A. Hinde ( e d . ) . B i r d v o c a l i z a t i o n s . Cambridge Univ. P r e s s , Cambridge. K r e b s , J . B. 1977a,; H a b i t u a t i o n and song r e p e r t o i r e s i n t h e g r e a t t i t . Behav. E c o l * S o c i o b i o l . 1: 215-227. Kr e b s , J . B. 1977b. The s i g n i f i c a n c e o f song r e p e r t o i r e s : t h e Beau Geste h y p o t h e s i s . Anim. ;Behav. 25: 475-478. Kr e b s , J . B., E. A s h c r o f t , M. Webber. 1978. Song r e p e r t o i r e s and t e r r i t o r y defence i n t h e g r e a t t i t . Nature 271: 539-Kroodsma, D. E„ 1975,; Song p a t t e r n i n g i n the r o c k wren,; The Condor 77: 294-3031 Kroodsma, D. E. 1976. B e p r o d u c t i v e development i n a female s o n g b i r d ; d i f f e r e n t i a l s t i m u l a t i o n by g u a l i t y of male song. S c i e n c e 192: 574-575. Kroodsma, D. E. 1977. C o r r e l a t e s o f song o r g a n i z a t i o n among No r t h American wrens- Amer., N a t u r . 111: 995-1008. L e i n , M. B. 1978. Song v a r i a t i o n i n a p o p u l a t i o n o f c h e s t n u t -s i d e d w a r b l e r s ( D e n d r o i c a p e n s y l y a n i e a : i t s n a t u r e and s u g g e s t e d s i g n i f i c a n c e . Can. J . Z p o l . 56: 1266-1283. Lemon, B. E. 1968. The r e l a t i o n between o r g a n i z a t i o n and f u n c t i o n of song i n c a r d i n a l s . B e h a v i o u r 32: 158-178. Lemon, B, E. and, C. C h a t f i e l d . 1971. O r g a n i z a t i o n o f song i n c a r d i n a l s . Anim; Behav 19: 1-17. Lemon, B . E . and C. C h a t f i e l d . 1973. O r g a n i z a t i o n , of song of r o s e - b r e a s t e d grosbeaks. Anim. behav. 21: 28-44. L o t t , D. and P. N. Brody; 1966. Support of o v u l a t i o n i n t h e r i n g dove by a u d i t o r y and v i s u a l s t i m u l i . J . Comp. P h y s i o l . P s y c h o l . 62: 311-313, 144 M a c K i n t o s h , N. J : 1974. The p s y c h o l o g y o f a n i m a l l e a r n i n g . Academic P r e s s , London. M a r l e r , P. 1956. The v o i c e of the c h a f f i n c h ; New b i o l o g y 20: 70-87. M a r l e r , P. 1970. A c o m p a r a t i v e approach t o v o c a l l e a r n i n g : song development i n white-crowned sparrows. J . Comp. P h y s i o l . P s y c h o l . 71: 1-25. i M a r t i n , D. J . 1977- Songs of t h e f o x sparrow* I,: s t r u c t u r e o f song and i t s comparison w i t h song i n o t h e r E m b e r i z i d a e . The Condor 79; 209^221,. M o s t o f s k y , D. I . (ed*) 1 965.; S t i m u l u s g e n e r a l i z a t i o n ; S t a n f o r d Univ. P r e s s , S t a n f o r d ; N e l s o n , K,. 1973. Does the h o l i s t i c s tudy o f b e h a v i o r have a f u t u r e ? I n P. P. G. Bateson and P. H,. K l o p f e r ( e d s . ) , P e r s p e c t i v e s i n E t h o l o g y . Plenum P r e s s , New York; Peek, F. W. 197 2. An e x p e r i m e n t a l study of t h e t e r r i t o r i a l f u n c t i o n of v o c a l and v i s u a l d i s p l a y i n t h e male r e d -winged b l a c k b i r d ( A g e l a i u s phoenieeus) . Anim. Behav. 20: 112-118. Peeke, H. V. S. and M. J,. Herz. (eds.) 1973; H a b i t u a t i o n ; Academic P r e s s , New York; P e t r i n o v i c h , L., T. P a t t e r s o n , and H. V. S. Peeke. 1976. R e p r o d u c t i v e c o n d i t i o n and t h e response of white-crowned sparrows ( Z o n o t r i c h i a l e u c o p h r y s n u t t a l l i ) t o song. S c i e n c e 191:206-207. , Robbins, C. S-, B. Bruun^ and H. S,. Zim. 1966.: B i r d s of North America* Golden P r e s s , New York. S i e g e l , S. 1956. Nonparametric s t a t i s t i c s f o r t h e b e h a v i o r a l s c i e n c e s . M c G r a w - H i l l Book Co. , New York;; S l a t e r , P. J - B* 1973. D e s c r i b i n g seguences of b e h a v i o r : I n P; P. G. Bateson and P,. H. K l o p f e r ( e d s . ) , P e r s p e c t i v e s i n e t h o l o g y . Plenum P r e s s , New York. Smith, D. G. 1976. An e x p e r i m e n t a l a n a l y s i s of t h e f u n c t i o n of red-winged b l a c k b i r d song; B e h a v i o u r 56: 136-156. Thompson, R. F., P. M. Groves, T. J . T e y l e r , and E* A. Boemer; 1975. A d u a l - p r o c e s s t h e o r y of h a b i t u a t i o n : t h e o r y and behavior. I n H. V. S.* Peeke and M. J . Herz. ( e d s . ) , HabituationT~Academic P r e s s , New York; ' Thorpe, W. H. 1958. The l e a r n i n g o f song p a t t e r n s by b i r d s , w i t h s p e c i a l r e f e r e n c e t o t h e song of t h e c h a f f i n c h . I b i s 100: 535-570. 145 Todt, D. 1975. Shor t term i n h i b i t i o n of o u t p u t s o c c u r r i n g i n th e v o c a l b e h a v i o u r of b l a c k b i r d s (Turdus merula m. L . ) . J . Comp. P h y s i o l . 98: 289-306. V e r n e r , J . 1975. Complex song r e p e r t o i r e o f male l o n g - b i l l e d marsh wrens i n e a s t e r n Washington. L i v i n g B i r d 14: 263-300. Weizenbaum, J . 1976. Computer power and human reason* W. H. Freeman and Co. , San F r a n c i s c o . W i l k i n s o n , , E. and P. E. Howse. 1975. Time r e s o l u t i o n o f a c o u s t i c s i g n a l s by b i r d s . Nature 258: 320-321. 

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