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Breeding biology of the American Robin (Turdus migratorius) Kemper, Dorothy Lynne 1971

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THE BREEDING BIOLOGY OF THE AMERICAN ROBIN (TURDUS MIGRATORIUS) IN SOUTHWESTERN BRITISH COLUMBIA by DOROTHY LYNNE KEMPER Sc., University of British Columbia, 1967 THESIS SUBMITTED'IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Zoology We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA NOVEMBER, 1971 In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t freely available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of ~%a^<?<^^/ The University of B r i t i s h Columbia Vancouver 8, Canada D a t e ^ J ^ - / , /?7Z i i ABSTRACT The American Robin (Turdus migratorius) i n southwestern B r i t i s h Columbia was studied i n regard to timing of reproduc-t i o n , breeding biology, gonadal changes with time, and the rela t i o n s h i p between photoperiod and gonadal condition. The basic breeding biology i s s i m i l a r to that found f o r other robins i n the eastern and midwestern United States. The only marked difference i s the very high nesting success rate of the robins which I studied. The ov e r a l l success rate was $6.6 percent with $7.& percent of a l l eggs l a i d hatching and 9$.6 percent of these fledging. The onset of the breeding season was marked by the increase i n t e r r i t o r i a l aggressive behaviour of male robins. This increase i s a gradual process taking place over a two month period from early February u n t i l l a t e March. The h i s t o l o g i c a l pattern of testes development and re-gression i n the robin i s the same as that for other temperate zone passerines. The length of the d a i l y photoperiod has a definate effect on the timing of t e s t i c u l a r recrudescence. The average testes weights of robins kept on eight, 12, and 16 hour photoperiods f o r the period of the annual cycle when t e s t i c u l a r development occurs i n the wild were 9.4 mg., 14.6 mg., and 200.5 mg. respectively. As well as stimulating gonadal development increasing photoperiod, as opposed to short day lengths, affects migra-i i i tory behaviour as evidenced by Zugunruhe or night restlessness. i v TABLE OF CONTENTS Page I INTRODUCTION 1 I I METHODS AND MATERIALS 4 I I I RESULTS Non-Breeding Season F i e l d Behaviour 11 Gonadal A n a l y s i s 13 Prelude to the Breeding Season F i e l d Behaviour 16 Gonadal A n a l y s i s 20 Breeding Season F i e l d Behaviour 27 Nesting Success 32 Gonadal A n a l y s i s 34 Onset of the Non-Breeding Season F i e l d Behaviour 37 Gonadal A n a l y s i s 3$ Photoperiod Experiments 41 IV DISCUSSION Nesting Parameters 4$ Breeding Season and Gonadal C o n d i t i o n . . 53 Photoperiod Experiments 5$ V CONCLUSIONS 63 VI LITERATURE CITED 64 V LIST OF TABLES Page I Number of s i g h t i n g s of lone or paired robins versus f l o c k e d r o b i n s on the campus and south Vancouver areas 17 I I The t e s t i c u l a r weight and seminiferous tubule diameter at the c o n c l u s i o n of the experiments f o r the ro b i n s maintained on d i f f e r e n t photoperiods 43 v i LIST OF FIGURES Page 1. Map of the study area on the University of B r i t i s h Columbia campus showing nest s i t e s 5 2. Average t e s t i s weight compared to the average hours of l i g h t per day f o r each half month period during the year 14 3 . Average ovarian weight compared to the average hours of l i g h t per day for each half month period during the year 15 4. Average seminiferous tubule diameter com-pared to the average hours of l i g h t per day f o r each half month period during the year 22 5. The percentage of the t o t a l number of nesting e f f o r t s which were started i n each time period compared to the average hours of l i g h t per day f o r that period 26 6. The relationship between the development of male and female reproductive organs 40 7. The average hourly a c t i v i t y of two groups of robins housed under different photoperiod conditions. 7a. The a c t i v i t y f o r March 9/10 46 7b . The a c t i v i t y f o r March 16/17 46a 7c. The a c t i v i t y for March 24/25 46bb 7d. The a c t i v i t y for March 30/31 46c 7e. The a c t i v i t y f o r A p r i l 3/9 46d 7f. The a c t i v i t y f o r A p r i l 13/14 46e 7g. The a c t i v i t y f o r A p r i l 20/21 46f 7h. The a c t i v i t y f o r A p r i l 27/23 46g 3. The relationship of breeding season to l a t i t u d e 57 v i i ACKNOWLEDGEMENTS I would l i k e to thank Dr. J . M. Taylor f o r her c o n t i n -uing encouragement and guidance. Dr. D. F i s h e r , Dr. R. Drent and Dr. N. L i l e y provided valuable c r i t i c i s m and encourage-ment. Mr. Fred Shaw of Western Peat Moss Limited generously permitted me to c o l l e c t specimens on the company farm i n Richmond and aided me i n those c o l l e c t i o n s . I am indebted to Mr. Ken Reid and Miss Catherine Kemper f o r f i e l d a s s i s t a n c e i n the summers of 1 9 6 $ and 1 9 6 9 . Discussions with other graduate students and f a c u l t y of the U n i v e r s i t y were most h e l p f u l i n f u r t h e r i n g t h i s study. I would a l s o l i k e t o thank my husband without whose encouragement and patience t h i s study would not have been completed. 1 INTRODUCTION The American Robin (Turdus migratorius) i s one of the most abundant and widespread North American passerines. I t ranges throughout the breadth of the North American continent and breeds from the northern portions of the United States northward to the t r e e l i n e i n Canada and Alaska. I t nests i n deciduous and coniferous f o r e s t s and also i n the centre of urban developments. The robin i s a highly adaptable species. S u r p r i s i n g l y , i n spite of i t s r e l a t i v e abundance, very l i t t l e has been published regarding i t s reproductive biology. This project i s twofold: one facet emphasizes the basic reproduc-t i v e biology of the robin i n f i e l d circumstances, and the other, a laboratory study, tests i t s s e n s i t i v i t y to photo-period. The following studies represent the spectrum of work published on the American Robin to date. Two major studies have been completed on the nesting habits and success of the robin. One, by Howell (1942), followed the nesting near Ithaca, New York; the other, by Young (1955), concerned robins near Madison, Wisconsin. The paper by Howell (1942) dealt thoroughly with nest b u i l d i n g , clutch s i z e , development of young, and nesting success. He also described certain as-pects of parental behaviour and gonadal condition. The l a t t e r was at the gross morphological l e v e l only. Young (1955) was primarily concerned with parental behaviour and o v e r a l l nest-ing success. In addition to these two major papers, there 2 are several others that pertain to my own study. Descriptions of a single nesting e f f o r t have been made by Gray (1897); Howe (1898); and Schantz (1939). Hamilton (1935) recorded the growth and development of n e s t l i n g robins. A nest record card survey of breeding season and clutch size done by Howard (1967) provides information on these subjects f o r various l a t i t u d e s on the east coast of North America. Studies of other aspects of robin biology include one on age and longevity (Farner 1945, 1949) and two on winter and spring robin roosts (Black, 1932; Howell, 1940). There are also many notes of observations of abberrant behaviour patterns, p a r t i c u l a r l y those regarding breeding or nesting ( B r a c k b i l l , 1944, 1947; Kress, 1967). The European members of the family Turdidae have received f a r more detailed consideration from o r n i t h o l o g i s t s . Studies of the English Robin (Lack, 1943) and Blackbird (Lack, 1949? Lack and Light, 1941) are primarily oriented to the ecological significance of clutch and brood size i n these species. Nest record card surveys of breeding seasons (Myres, 1 9 5 5 ) , clutch size (Snow, 1955a) and nesting success (Snow 1955b) discuss these topics f o r the Blackbird, Song Thrush, and M i s t l e Thrush. These are the European turdids most c l o s e l y related to the American Robin. While there i s abundant information on nest-ing habits and success there i s very l i t t l e on gonadal condi-t i o n or the eff e c t of photoperiod on the annual cycle of European thrushes. 3 The lack of information on the annual gonadal cycle of the turdids, i n general, and the American Robin, i n p a r t i c u l a r , has been the impetus behind the choice of the American Robin f o r t h i s study. The study i t s e l f deals primarily with the reproductive biology of the robin, including the nesting parameters such as breeding season, nest l o c a t i o n , clutch and brood s i z e , and success. I have examined t h i s with r e f e r -ence to the annual gonadal cycle, and both are considered i n terms of photoperiod. My r e s u l t s on the nesting cycle are compared to those published accounts f o r robins under d i f -ferent conditions of l a t i t u d e , longitude, and habitat. The e f f e c t of photoperiod on gonadal cycle and breeding season i s also compared to the r e s u l t s f o r clo s e l y r e l a t e d passerine species. 4 METHODS AND MATERIALS My study of the nesting habits of the American Robin was located on the University of B r i t i s h Columbia campus, 123°90* west longitude and 49°18' north l a t i t u d e . This work was done from September, 1967, through December, 1969. 'i'he campus provided a 200 acre garden habitat with many deciduous and coniferous trees and shrubs interspersed with large expanses of grass; the area was an i d e a l nesting habitat. During the 1968 breeding season of the robin I located 22 nests, most of which had young that were already hatched. In the 1969 season I made a systematic weekly search of an area of the campus ( f i g . 1) and located 72 nests. To v e r i f y the thoroughness of the search I checked the area again i n October when the deciduous trees had l o s t t h e i r f o l i a g e , leaving the nests exposed. I found f i v e nests not previously recorded. No attempt was made to determine what percentage of nests i n coniferous trees were missed. Each of the nests located during the 1969 season was r e v i s i t e d at three to seven day i n t e r v a l s , the shorter period f o r active nests. Nests were also checked on the expected day of hatching and fledging. The number of eggs and/or young, the dates of laying, hatching, and fledging, the tree species, nest height, nest construction material and the amount of cover afforded the nest were recorded f o r each nest. I determined the l a t t e r subjectively by noting whether the nest was completely exposed, covered from below, from above, from Figure 1. Map of the study area on the University of B r i t i s h Columbia campus. The s o l i d dots indicate nest s i t e s . The scale i s approximately one inch to 2 0 0 f e e t . 6 the side, any two of the l a s t three, or completely concealed. Cover consisted of the trunk, branch or f o l i a g e of the tree or of part of a building. I also observed the behaviour of the parents, t h e i r relationships with each other, with t h e i r young, and with conspecifics. The data from the f i e l d observations were coupled with information obtained from a h i s t o l o g i c a l examination of the gonads of robins c o l l e c t e d on a blueberry-cranberry farm i n Richmond, B r i t i s h Columbia, 123*10' west longitude, 49° 1 3 ' north l a t i t u d e , approximately 30 miles southeast of the campus. The farm, comprising 400 acres, was bordered on three sides by deciduous shrubs, vines and trees and on the fourth side by a hay f i e l d . Road and window k i l l s of robins were also obtained from various sources i n the Vancouver area and the Canadian W i l d l i f e Service contributed six specimens which they c o l l e c t e d i n the v i c i n i t y of Richmond. I col l e c t e d on a weekly basis from March through July and September and October of 1968 and 1969. A t o t a l of 240 birds were sampled: 171 the f i r s t year and 69 the second. Body weight to the nearest 0.5 gram, and t o t a l body length and greatest wing primary length to the nearest m i l l i -meter, were recorded f o r each specimen. With minor exception, each b i r d was dissected within ten minutes of death. The amount (non, s l i g h t , extensive) and l o c a t i o n (subcutaneous, peritoneal) of f a t was noted as well as the obvious presence of parasites. The gonads were removed, 7 measured f o r g r e a t e s t l e n g t h and w i d t h t o t h e n e a r e s t m i l l i -m e ter, and p l a c e d i n B o u i n ' s s o l u t i o n . A l l t e s t e s were c u t open t o i n c r e a s e t h e speed o f p e n e t r a t i o n by t h e p r e s e r v a t i v e . Specimens w h i c h had been dead f o r more t h a n 45 minutes were t r e a t e d as t h e o t h e r s e xcept t h a t no attempt was made t o p r e s e r v e t h e gonads. A f t e r 24 t o 48 h o u r s t h e gonads were t r a n s f e r e d t o 70 p e r c e n t a l c o h o l . They were weighed t o 0 . 1 m i l l i g r a m s on a S a r t o r i u s f o u r d e c i m a l p l a c e a n a l y t i c a l pan b a l a n c e a f t e r t h o r o u g h p r e s e r v a t i o n and d e c o l o u r a t i o n . One t e s t i s f r o m each b i r d was embedded i n p a r a f f i n , s e c t i o n e d a t 7 mu. and s t a i n e d w i t h D e l a f i e l d ' s h a e m a t o x y l i n and e o s i n . The s l i d e s were a n a l y z e d f o r t u b u l e d i a m e t e r and t h e p r e s e n c e o r s t a g e o f development of sperm. The number and s i z e o f f o l l i c l e s were r e c o r d e d f o r each o v a r y . When f o l l i c l e s were o b v i o u s l y p o s t - o v u l a t o r y o r r e g r e s s i n g ( D a v i s , 1 9 4 2 a , 1942b, 1942c) I n o t e d them as s u c h , but g r o s s d i f f e r e n t i a t i o n of d e v e l o p i n g and r e g r e s s i n g ones was not always p o s s i b l e . The c o l l e c t i o n o f m a t e r i a l f o r gonadal a n a l y s i s was c o n t i n u e d d u r i n g t h e n o n - b r e e d i n g season on a monthly b a s i s , t h e purpose b e i n g t o determine t h e h i s t o l o g i c a l c o n d i t i o n i n f u l l r e g r e s s i o n and t o dete r m i n e t h e on s e t o f gonadal d e v e l o -pment. T h i s work was c o r r e l a t e d w i t h f i e l d o b s e r v a t i o n s made d u r i n g t h e w i n t e r months i n t h e e x p e c t a t i o n t h a t con-s p i c u o u s d i f f e r e n c e s between n e s t i n g b e h a v i o u r and w i n t e r b e h a v i o u r o f r o b i n s would p r o v i d e e x t e r n a l c l u e s t o t h e t i m i n g o f t h e s h i f t between t h e a c t i v e and i n a c t i v e s t a g e s of r e p r o -8 duction. Such observations were concerned s o l e l y with the presence of robins i n winter f l o c k s or as single or paired t e r r i t o r i a l b i r d s . F i e l d studies were conducted on an intermittent schedule from November through January, t o t a l l i n g approximately 40 hours during the winter of 1968-69. Begin-ning i n February, a rigorous weekly observation period of a minimum of two and a half hours per week was established. This time was spent searching f o r and/or observing robins. The work was again primarily c a r r i e d out on the campus of the University of B r i t i s h Columbia but i t became necessary on occasion to make observations on the nearby golf courses of south Vancouver, as well, i n order to locate robins. In addition to these f i e l d studies, robins were captured from January and March of each year f o r laboratory experiments on the e f f e c t of photoperiod on the timing of the gonadal cycle. Also I wanted to determine i f a Zugunruhe e f f e c t was produced, since the robin i s known to be a night migrant. The f i r s t set of experiments was conducted from January to A p r i l of 1969 using a t o t a l of 21 birds obtained from the golf courses and Richmond. The birds were housed i n groups of from three to f i v e i n wire cages four feet by two feet by two fe e t . Each cage was provided with one perch, one food dish, and one water dish. The feed was a mixture of a commercial chick s t a r t e r and ground beef. Once one b i r d i n each cage discovered the food the others rapi d l y accepted i t . Both food and water were provided ad l i b i t u m . 9 The cages were enclosed by a heavy black v i n y l p l a s t i c which e f f e c t i v e l y blocks a l l natural l i g h t . Illumination was provided by a single 60 watt cool white flourescent l i g h t f o r each cage. There were three experimental groups on eight, 12, and 16 hours of l i g h t , respectively, i n a 24 hour day. There was some l i g h t leakage to two of the groups. A very small amount of l i g h t , both i n quantity and i n t e n s i t y , pene-trated through several pinholes i n the p l a s t i c and exposed the eight-hour groups, and considerably more sunlight reached the sixteen-hour group. In the l a t t e r case, however, i t occurred only at times when they were already under a r t i f i c i a l l i g h t i n g . For the second set of experiments, which ran from Feb-ruary through May, 1970, 14 birds were co l l e c t e d i n the south Vancouver and campus areas. Six of these were housed in d i v i d u -a l l y i n cages 14 inches by 15 inches by 15 inches f i t t e d with one food dish and one water dish. Each cage also had one perch which was connected to a microswitch such that the switch was tripped each time the b i r d hopped on or off the perch. The switch was connected to an Esterline-Angus 20 channel event recorder. Blocks of three cages were kept together on one photoperiod. Three robins on an eight-hour l i g h t , sixteen-hour dark photoperiod were illuminated and protected against i n t e r f e r i n g l i g h t by the same means as those on the previous experiment. These three were tested against three on natural photoperiod, which ranged from 12 to 16 hours of l i g h t per 24 hour day. 10 Birds mist-netted f o r both experiments were maintained f o r approximately one week under natural l i g h t conditions to allow them to s e t t l e i n and become accustomed to the food. In the second set of experiments t h i s was done i n communal cages and then they were transfered to in d i v i d u a l ones. No attempt was made during either set of experiments to control the temperature. At the end of the experiments a l l the birds were s a c r i -f i c e d and t h e i r condition recorded as f o r the c o l l e c t e d speci-mens. S i m i l a r l y , t h e i r gonads were preserved and prepared f o r h i s t o l o g i c a l examination. 11 RESULTS Non-breeding Season F i e l d Behaviour The American Robin, as observed on the campus area, i s a gregarious species during the non-breeding season, and sightings of lone individuals are rare. Several thousand birds of a l l ages and both sexes share the same evening roost (Black, 1932; Forbush and May, 1955). During the day the birds are found i n smaller groups of highly variable numbers. The groups I observed had from f i v e to approximately 40 mem-bers, with 12 to 15 being the most common number. The f l o c k size was extremely l a b i l e as groups s p l i t and rejoined through-out the day. Once I located a group of robins I could r e l y on f i n d i n g them i n the same place as long as the food supply las t e d , usually one to three days. Their food consisted primarily of soft f r u i t s , e s p e c i a l l y the berries of the Mountain Ash (Sitka sitchensis) and h o l l y (Ilex sp.), although they appeared to eat insects when avai l a b l e . There were several exceptions to t h i s general pattern of gregarious behaviour. In January and February of 1969 I observed lone birds on the University of B r i t i s h Columbia campus on 10 occasions, at which times I d i d not locate any robins i n groups. This was a very severe winter with the ground being frozen and/or snow-covered f o r 41 days, begin-ning December 26. By mid-January food was very scarce since many of the berries had frozen and rotted on the trees. Each 12 lone b i r d seemed to be defending a food source, e i t h e r a bush wit h a few b e r r i e s or garbage. On two occasions I witnessed one r o b i n approach another t h a t was feeding. Both times the f e e d i n g b i r d immediately attacked the other and pursued i t u n t i l i t r e t r e a t e d out of s i g h t . F l o c k s of f i v e to 12 robins were s t i l l present i n the south Vancouver area p r i m a r i l y around h o l l y t r e e s which s t i l l had b e r r i e s and near feeders th a t had been put out by r e s i d e n t s of the area. I autopsied nine of the many b i r d s which died during t h i s p e r i o d . A l l were abnormally l i g h t , averaging 63 .7 gm. which i s approximate-l y 20 gm. lower than normal. They had no f a t deposits and had a dark b i l e - l i k e f l u i d i n the gut which was not present i n b i r d s I autopsied at any other time. Steve Johnson (pers. comm.) observed a s i m i l a r f l u i d i n s t a r l i n g s (Sturnus v u l g a r i s ) dying at t h i s time. When the snow s t a r t e d to c l e a r (February 3 - 1 0 ) the robins regrouped i n t o f l o c k s which d i d not break up again u n t i l March. I a l s o observed an unusual behaviour patt e r n on December 7 and 9 , 1967, both cloudy, r a i n y days. On each occasion I heard a r o b i n g i v i n g the t y p i c a l mating song. In each case i t took place i n mid-afternoon and the b i r d was part of a f l o c k of f i v e to 10 r o b i n s a l l s i t t i n g i n the same t r e e . The s i n g i n g b i r d was showing part of i t s mating season behav-i o u r a l p a t t e r n at an abnormal time and under abnormal s o c i a l c o n d i t i o n s . 13 Gonadal Analysis The reproductive state of the robins which I coll e c t e d during the f a l l and winter i s represented i n Figure 2 f o r males and Figure 3 f o r females. The gonads are extremely reduced from October through February. The testes of 20 specimens co l l e c t e d during t h i s period averaged 2.4 nig. i n weight, ranging from 1.0 to 4.3 mg. They are a pale grey to black i n colour i n contrast to t h e i r opaque white colour i n the breeding season. H i s t o l o g i c a l examination showed no mature or developing sperm present. Seminiferous tubule diameter averaged 23.34 mu. ranging from 7.50 to 24.68 mu. The l e f t ovary during t h i s same period ranged from 17.8 to 33.1 mg. i n weight, averaging 23.7 mg. I t i s a very small compact organ with a l l f o l l i c l e s l e s s than 0.5 mm. i n diameter and none showing any increase i n si z e over the others. CO O CO u o DC 9 10 u P . 11 -p Xi 12 13 14 15 16 17 800 .5 600 -p . ^400 .^200 •P CO CU 0 Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 2. Average testis weight compared to the average hours of light per day for each half month period during the year. >> CC Q U CD CU •p Xi hG •H >A 4H O CO o 9 10 11 12 13 14 15 16 17 18 • 600 to e •p M •H cu 300 > o 0 Dec Jan Feb Mar Apr May Jun J u l Aug Sep Oct Nov Dec Figure 3. Average ovarian weight compared to the average hours of l i g h t per day f o r each half month period during the year. 16 Prelude to the Breeding Season  F i e l d Behaviour In February and March the s o c i a l o r g a n i z a t i o n begins to s h i f t from the nomadic f l o c k s t r u c t u r e towards the aggressive, t e r r i t o r i a l mating behaviour. Table I compares the number of s i g h t i n g s of lone or t e r r i t o r i a l b i r d s t o those of f l o c k s . The s h i f t i s a gradual one, not only f o r the species as a whole but a l s o f o r every i n d i v i d u a l . The f i r s t i n d i c a t i o n of the change i n behaviour i s a decrease i n the number of l a r g e f l o c k s . A f t e r February 1st I never saw a group of more than 15 robins on the campus, except i n the evening r o o s t s , and :. a f t e r March 1st never more than f i v e . The f e e d i n g h a b i t s and a s s o c i a t e d behaviour of the robins a l s o changes at t h i s time. They spend an i n c r e a s i n g l y greater amount of time f e e d i n g on earth worms and i n s e c t s than on b e r r i e s so they are on the ground more f r e q u e n t l y than i n t r e e s . Robins, when feeding on the ground at any time of the year, e x h i b i t a spacing behaviour. P r i c e (1933) describes the t e n a c i t y of c e r t a i n i n d i v i d u a l s f o r a p a r t i c u l a r part of the area on which the group i s feedi n g . For the two b i r d s which he could recognize the t e r r i t o r i e s averaged 400 yards square. The r e s i d e n t chased away any other r o b i n which i n -truded onto i t . I have observed the same so r t of behaviour during the non-breeding season but r e s t r i c t e d to a much sm a l l e r area, 50 square yards maximum. These b i r d s never d i s p l a y e d the behavioural patterns which are commonly as s o c i a t e d with 17 Dates Single or Groups of Hours From Paired 3 or More C i v i l Dawn # Sightings # Sightings to Dusk Jan 1-15 2*(©,2) 2*(2,0) 9.5 Jan 16-30 3*(0,3) 2*(1,1) 10.0 Jan 31-Feb 14 4*(2,2) 5*(4,1) 10.6 Feb 15-Mar 1 4*(1,3) 2*(2,0) 11.5 Mar 2-16 4 7 12.5 Mar 17-31 9 5 13.3 Apr 1-15 16 1 14.5 Apr 16-30 14 0 15.3 May 1-15 20 1 16.3 May 16-30 17 0 17.0 May 31-Jun 14 14 1 17.6 Jun 15-29 21 0 18.0 Jun 30-Jul 14 5 0 17.6 Jul 15-29 5 3 17.0 Jul 30-Aug 13 3 7 16.25 Aug 14-28 0 0 15.3 Aug 29-Sep 12 0 2 14.5 Sep 13-27 1 1 13.5 Sep 28-0ct 12 0 6 12.5 Oct 13-27 0 2 11.5 Oct 28-Nov 11 0 2 10.6 Nov 12-26 0 4 10.0 Nov 27-Dec 11 0 2 9.5 Dec 12-27 1 3 9.3 Dec 27-31 Table I. Numbers of sightings of lone or paired robins versus flocked robins on the campus and south Vancouver areas, based on 10 hours of searching per 14-day period. Numbers in parenthesis after asterisk denote breakdown of totals into 1968 and 1969 seasons, respectively. 18 defending a nesting t e r r i t o r y , such as singing from a con-spicuous perch and f i g h t i n g other trespassing robins. Nor did the chases ever r e s u l t i n f i g h t s , since the intruder always l e f t and often both birds involved would just move away from each other. This behaviour could more properly be considered an i n d i v i d u a l distance rather than a true t e r r i t o r y , such as that established early i n the breeding season. This spacing while ground feeding i s not evident when the group i s feeding i n trees. In the l a t t e r case a l l the members of one group w i l l feed i n the same or adjacent trees. Changes i n spacing while ground feeding are another i n d i c a t i o n of the onset of breeding behaviour. The males patrol a l a r g e r feeding area and become more aggressive i n t h e i r defense of i t . The resident b i r d chases the intruder further, pecks at the intruder and gives a loud, harsh c a l l , none of which occurred e a r l i e r i n the winter when robins were ground feeding. For example, on March 12 I observed a group of f i v e robins (three males and two females) feeding on a lawn appro-ximately 30 yards by 20 yards on the campus. One of the males attacked any other robin which came within f i v e yards of him. He pecked at the intruder rather than just chasing him and pursued him out of the group feeding area. As soon as the attacking male returned to h i 3 feeding area the ousted b i r d would return to the group area. Two days l a t e r I saw a group of four birds (three males and one female) on t h i s same area with one male occupying the same part of i t as did the t e r r i -19 t o r i a l male described above, except that he was defending about a 1 5 0 square yard area, almost three times the s i z e of that defended on the previous occasion. During March males s t a r t ot leave the groups e n t i r e l y . At f i r s t they depart f o r one or two hours i n the early morning and evening, at which time they sing the mating or t e r r i t o r i a l c a l l as described by Howell ( 1 9 4 2 ) and Young 1 1 9 5 5 ) , and they are extremely aggressive. This singing i s usually done i n or near what i s to become the male's t e r r i t o r y . The habitat of the nesting area i s either of the mixed garden type near buildings with shrubs, bushes and trees interspersed with grassed areas, or i n a densely wooded area that i s bordered by large grass and f i e l d areas. As the male chooses h i s t e r -r i t o r y he spends increasingly more time on i t and defends i t to the point of f i g h t i n g with other males that intrude with-i n i t . Whenever one male enters another's t e r r i t o r y the resident male immediately gives a loud harsh c a l l and f l i e s at the intruder. I f the l a t t e r f l e e s , as I observed i n three of 1 6 cases, the owner pursues him f o r several yards and then lands. The intruder stops a few feet away. I f the intruder does not f l e e (13 of 1 6 confrontations) they attack each other with feet and b i l l s as described by Howell ( 1 9 4 2 ) . These f i g h t s l a s t three minutes at most, being terminated when one b i r d moves several yards away. None of the battles I witnessed resulted i n any physical damage other than the l o s s of a few 20 feathers. Simple chases, without f i g h t s , occur throughout the breeding season but I witnessed only one f i g h t a f t e r mid-May and that was on July 3. The t e r r i t o r i a l males do not spend a l l day on t h e i r t e r r i t o r y but spend part of i t on group feeding areas. These are generally large, grassy areas such as small parks, f i e l d s , or large lawns that are not within breeding t e r r i t o r i e s . The males also return to the large communal roosts at night. As the males establish t h e i r t e r r i t o r i e s the females also leave the groups, i . e . the groups are d i s i n t e g r a t i n g . They choose a male and/or h i s t e r r i t o r y and a i d him i n p a t r o l -l i n g the t e r r i t o r y and i n chasing intruders. Like the male they do not spend a l l t h e i r time on the t e r r i t o r y but do some of t h e i r feeding on the group areas and roost with the other robins. The birds have reached t h i s stage by early A p r i l and are ready to begin the breeding cycle. I t i s important to note (Table I) that t h i s s h i f t takes place over a long period of time, and that the hours of l i g h t per day have increased from just under 12 to almost 14. Gonadal Analysis A dramatic increase i n the size of the gonads begins i n March, a f t e r the d a i l y photoperiod exceeds 12 hours per day. For the f i r s t h a l f of March the testes weights average 52.9 mg., ranging from 3.5 to 313.2 mg. f o r 11 specimens ( f i g . 2). Of these 11, f i v e were coll e c t e d i n the Richmond area i n 1969 and average only 9.5 mg., whereas the remaining six were c o l -21 lected on the campus i n 1968 and average 89.3 mg. i n weight. The seminiferous tubule diameter also increases greatly during the f i r s t h a l f of March ( f i g . 4). The males co l l e c t e d i n the Richmond area had an average tubule diameter of 32.82 mu. ranging from 25.65 to 38 .40 mu. None of these specimens contained mature or developing sperm. Those c o l l e c t e d on campus s t i l l had no developing sperm even though the tubule diameters averaged 65.95 mu., almost twice the diameter of those col l e c t e d i n Richmond. The range i n diameter f o r the campus groups was from 37.28 to 107 .70 mu. The o v e r a l l average of seminiferous tubule diameter i n robins from both areas was 50.89 mu. During the second half of March the testes increased markedly i n weight to an average of 257.1 mg. f o r eight specimens, ranging from 10.6 to 487.8 mg. ( f i g . 2). The dichotomy between samples col l e c t e d i n Richmond and on the campus was s t i l l evident, f o r the two males from Richmond averaged 18.5 mg. (range of 10.6 to 26.5 mg.) and s i x from the campus averaged 336.7 mg. (range of 20.2 to 487.8 mg.). A l l of the testes of birds taken on the campus during the second half of March showed developing sperm and one con-tained a few mature sperm. H i s t o l o g i c a l examination of the Richmond specimens showed no developing sperm.„ This d i f f e r -ence was r e f l e c t e d i n the tubule diameters which averaged 141.69 mu., ranging from 129.45 to 154.20 mu. f o r campus birds, and 45.27 mu., ranging from 41.78 to 48.75 mu. f o r those from 9 CD Q 10 u £ 11 JS 12 fafl ;1 13 CH O CO U o 14 15 16 17 18 200 CP I i 5 0 CO O -P CU CU cuLDOC C H • H Q e CU co 50 0 Dec Jan Feb Mar Apr May Jun J u l Aug Sep Oct Nov Dec Figure 4. Average seminiferous tubule diameter compared to the average hours of l i g h t per day for each half month period during the year. 23 Richmond. The ovaries also increase i n size during March but the change i s not as sharp nor as early as that of the testes. Nor i s there a noticable difference i n amount of ovarian de-velopment between females c o l l e c t e d i n Richmond and those c o l l e c t e d on the campus. Eight specimens col l e c t e d during the f i r s t h a l f of March averaged 33.6 mg. i n ovarian weight, ran-ging from 25 . 4 to 4 6 . 6 mg. ( f i g . 3). None of these showed any f o l l i c u l a r development. For 10 specimens collected i n the second h a l f of March the ovaries averaged 85.1 mg. i n weight, ranging from 48.3 to 117.6 mg. Of these a l l but one had some f o l l i c l e s which were enlarging and three had one f o l l i c l e each that had increased to at l e a s t 2.0 mm. i n diameter. The rapid increase i n gonadal size continues throughout A p r i l , although i t i s not as marked i n the f i r s t h a l f of the month as i t i s i n the second portion. The average t e s t i s weight f o r the f i r s t half of the month i s 300.5 mg., ranging from 8.1 to 611.3 mg. ( f i g . 2). Again, the Richmond birds seem slower to develop than those from the campus. The aver-age weight f o r the testes of eight of the former i s 199.8 mg., ranging from 8.1 to 361,3 mg. The campus birds average 362.6 mg. f o r 13 t e s t i c u l a r specimens, ranging from 123.0 to 611.3 mg. The males c o l l e c t e d on campus during the f i r s t h a l f of A p r i l a l l had developing spermatocytes and six showed mature 24 sperm. The diameter of the seminiferous tubules had not increased noticeably since that at the end of March ( f i g . 4), averaging 130.19 mu. and ranging from 106.09 to 168.54 mu. A l l those with mature sperm were greater than 120.00 mu. i n diameter. The birds c o l l e c t e d i n Richmond during t h i s same period, however, showed no mature sperm. They averaged 88.00 mu. i n diameter, ranging from 30.00 to 121.65 mu. A l l those over 85.00 mu. i n diameter d i d show developing sperm, i n c l u -ding spermatids. By the end of A p r i l the d a i l y l i g h t phase of the photo-period has increased to 15.5 hours, the gonads have taken an-other sharp increase i n s i z e , and differences i n the develop-ment of males from Richmond and the campus disappeared. The testes now average 494.2 mg. i n weight, ranging from 292.4 to 941.5 mg. f o r 11 specimens. The seminiferous tubule d i a -meters range from 120*97 to 148.8 mu., averaging 131.14 mu. A l l testes contained some mature sperm. The average ovarian weight f o r 15 samples taken i n the f i r s t h a l f of A p r i l was 148.6 mg., ranging from 27.9 to 556.0 mg. ( f i g . 3). Of these, six showed no f o l l i c u l a r development and f i v e had one f o l l i c l e greater than 2.0 mm. i n s i z e . One ovary, weighing 90.8 mg., had a f o l l i c l e measuring 2.8 mm. while another ovary, weighing 556.0 mg., had one measuring 2.9 mm. Those showing no development weighed from 27.9 mg. to I38.6 mg. In the second h a l f of A p r i l the ovaries increased to 25 2 5 2 . 1 3 mg. i n weight on the average, ranging from 4 0 . 5 to 5 3 5 . 1 mg. f o r eight specimens. Of these one, at 4 0 . 5 mg., showed no development, and the r e s t had from 1 to 12 f o l l i c l e s c l e a r l y enlarged. Of the l a t t e r , the l a r g e s t was 4 . 3 mm. i n diameter on an ovary weighing 53 5 .1 mg. and having nine developing f o l l i c l e s . Thus, during A p r i l the gonadal s t a t e of the males and, to a l e s s e r extent, that of the females i n d i c a t e s readiness f o r breeding. The b i r d s are i n f u l l breeding plumage. The males have a j e t - b l a c k head, yellow b i l l and b r i g h t red b r e a s t , w i t h the back and wing contour f e a t h e r s being black. The females have the same yellow b i l l but are somewhat p a l e r i n a l l other c o l o u r a t i o n . Both sexes e x h i b i t the same plumages as described by Forbush and May ( 1 9 5 5 ) and Godfrey ( 1 9 6 6 ) . They are p a i r i n g and commencing the n e s t i n g c y c l e ( f i g . 5 ) . March A p r i l May June 5-16 17-23 29-9 10-21 22-3 4-15 16-27 23-ft Q-j>0 21-? 3- J u l y 4 ,15-;26 ,29 a 9 & 10 cu P 11 S 12 CM 13 o ? 14 § 15 16 17 20 CO -P CO <D «H CD O -P ^ "1 0 P CO i u C P CD CO o cu cx, 0 figure 5. The percentage of the t o t a l number of nesting e f f o r t s which were started i n each time period compared to the average hours of l i g h t per day fo r that period. 2 7 Breeding Season F i e l d Behaviour Pair formation i s an exceedingly undemonstrative pro-cedure f o r robins. The male has already established his t e r r i t o r y and seems to make no e f f o r t to a t t r a c t the female. I never observed movements by the males which could be const-rued as courtship display or courtship feeding, although B r a c k b i l l ( 1 9 4 4 ) describes a rare occurrence of inverted courtship feeding. Of 1 4 copulations which I witnessed the male always chased the female f o r several seconds to a min-ute and then mounted her. No special desplay followed copu-l a t i o n . At t h i s stage of the n e s t l i n g cycle the pair i s feeding both within and without t h e i r nesting t e r r i t o r y and i s returning to communal roosts at night. Both sexes are involved i n choosing the nest s i t e ( B r a c k b i l l , 1 9 4 7 ) . Contrary to the evidence of other workers (Howell, 1 9 4 2 ; loung, 1 9 5 5 ) there was no preference f o r evergreen over deciduous trees at the beginning of the nesting season or f o r the season as a whole. The t o t a l of 2 8 ever-green and 4 4 deciduous tree nests was not s i g n i f i c a n t l y d i f f e r e n t using either a chi-square or a one-sample runs t e s t . The height of the nests ranged from 1 . 5 to 3 0 feet above ground l e v e l , the average f o r 7 2 nests being 1 0 . 7 - 6 . 8 f e e t . There did not seem to be any preference f o r high or low nests at any p a r t i c u l a r time of the year. The nest s i t e seemed to be selected, however, on the basis of the amount of 28 cover provided. Of the 72 nests, seven were completely exposed, one was hidden only from below, 22 were protected from above, one from the side, 23 from two directions and 18 were com-p l e t e l y hidden. Both Kolmogorov-Smirnov and Chi-square tests show a s i g n i f i c a n t difference amoung these groups at 0.05 l e v e l . This indicates that robins prefer nest s i t e s which of f e r cover. Once the s i t e was chosen the female began nest construc-t i o n . She alone did the bui l d i n g but the male was often nearby. Total time from s t a r t to f i n i s h averaged 7.6 days, ranging from f i v e to 21 days. The female f i r s t b u i l t a shallow mud bowl reinforced with small twigs and/or straw. This was elaborated with more twigs, straw, grass, s t r i n g etc. into the open cup thickwalled shape that i s characteris-t i c of the robin's nest. In the f i n a l stage of construction the female l i n e s the nest with grass and/or shredded paper and shapes i t to f i t her body. The female does not necessarily work every day but may pause f o r several days, usually between stages (Young, 1955). I have noticed no change i n nests f o r from three to f i v e days but never longer unless the nest had been deserted. The l a t t e r was the case i n 10 of 48 nests and none of these had reached the l i n i n g stage. After the nest i s completed egg layin g may begin imme-di a t e l y or may be delayed as much as a week. Generally, the female lays one egg per day on consecutive days sometime i n the morning. I found only one instance of a missed day, t h i s 29 between the second and t h i r d days of a three-egg c l u t c h . The female does not begin t o incubate u n t i l she l a y s the l a s t egg, but from that time on she stays on or near the nest 24 hours per day. She w i l l be o f f the nest f o r a maximum of 20 minutes at any given time, but u s u a l l y f o r only three or f o u r minutes. She remains on the nest at night w h i l e the male r e t u r n s to the communal r o o s t . I twice observed what I assumed to be males (on the b a s i s of t h e i r plumage c o l o u r a t i o n ) s i t t i n g on the nest, although other workers (Howell, 1942; Young, 1955; Howe, 1898) report t h a t only the females incubate. In one of the cases the female was nearby f e e d i n g , when she returned the presumed male l e f t the nest immediately and she s e t t l e d on i t . In the other case no other r o b i n was seen i n the v i c i n i t y . For those nests f o r which I know the exact date of l a y i n g and hatch i n g the egg in c u b a t i o n p e r i o d l a s t e d from 12.5 to 13.5 days. In ten cases one egg hatched a day l a t e r than the r e s t . During the n e s t l i n g period both parents are i n constant daytime attendance. For one to three days immediately f o l l o w -i n g h a t c h i n g the male i s almost the sole p r o v i d e r of food, since the female s t i l l spends the m a j o r i t y of her time incuba-t i n g the young, e s p e c i a l l y i f the weather i s cool or r a i n y . On may 16, 1969, the weather was overcast with an almost cons-tant d r i z z l e and the temperature was approximately 60 F when I observed a female brooding her three one-day-old o f f s p r i n g . Over a three-hour o b s e r v a t i o n a l period she stayed w i t h them f o r 10 to 20 minute i n t e r v a l s , averaging 13 minutes. She l e f t 30 the nest f o r a maximum of three minutes at a time and fed while she was away but never brought food to the nest. The male brought food to the young six times while I was observing them. When the female was off the nest he stood r i g h t beside i t but did not afford the nestlings any protection from the r a i n . As the young grow older, consequently developing more feathers and l a r g e r appetites, both the male and the female spend a l l day feeding them. The food consists primarily of earthworms, c a t e r p i l l a r s and adult and l a r v a l insect stages. The porportion of any one item i n the diet depends on what-ever i s most p l e n t i f u l at the time of greatest demand. For example, i n 1969 there was an extremely abundant crop of tent c a t e r p i l l a r s on campus whose appearance coincided with the hatching of the young of f i r s t nesting e f f o r t s that season. These nestlings were fed c a t e r p i l l a r s almost exclusively. The female continues to incubate the young at night u n t i l they leave the nest. The male, however, stays with h i s family only u n t i l dusk when he departs, presumably f o r the communal evening roost. He i s apparently not i n the v i c i n i t y of the nest since he does not respond to the alarm c a l l of the female, whereas during the daytime the male always appears immediately i f the female gives the c a l l . This pattern of care continues u n t i l the young are ready to leave the nest or are forced out. The average period of time the young remained i n the 31 nest was 12 days, ranging from 11 to 14 days. The natural fledg i n g time appears to be closer to 13 days, f o r any d i s -turbance aft e r 11 days seems to cause the young to leave the nest. The was the case i n the four instances of 11-day n e s t l i n g periods that I observed, f o r during my e f f o r t s to ascertain the presence and number of young they l e f t the nest. When the young fledge t h e i r heads, backs, and upper wing surfaces are a grey-brown colour; t h e i r b i l l s are a d u l l grey-black with, at most, a trace of pale yellow near the head. They frequently s t i l l have t u f t s of down feathers protruding from t h e i r juvenile plumage, es p e c i a l l y i n the head region and they have a very short t a i l . They are not f u l l y capable of caring f o r themselves when they leave the nest regardless of whether they take 11 or 14 days to fledge. They are incapable of f l i g h t s of more than a few feet and, because of t h e i r short t a i l s , have poor d i r e c t i o n a l a b i l i t y and s t a b i l i t y . Nor can the fledgings feed themselves f o r the f i r s t several days; hence they follow t h e i r parents and beg f o r food. For the f i r s t few days the family remains very close to the nest s i t e . Other investigators (Howell, 1942; Young, 1955) report that the young may return to the nest f o r the f i r s t and perhaps second night. I found no evidence of t h i s , although I searched eight nests just p r i o r to sunset of the day on which the young fledged. They could, however, have returned l a t e r . As the young learn to feed themselves the family group s t a r t s to break up and the female prepares another nest f o r a second clutch. The male continues to watch over the young f o r approximately one more week, by which time the female may be incubating another clutch. From the end of the f i r s t nesting e f f o r t i n mid to l a t e May u n t i l August I observed large groups of robins, composed e n t i r e l y of juveniles. A l l such groups occurred apart from areas of nesting a c t i v i t y and my observations of them were confined to the v i c i n i t y of the blueberry farm i n Richmond. Nesting Success The average clutch size f o r the 1969 season was 3.04i 0.59 eggs, ranging from two to four, f o r 27 nests that I followed from the stage of nest construction to that of fledging of the young. Of these, 2.67+1.12 hatched and 2.63^ 1.13 fledged. This i s a f a i r l y high success rate. The decrease from egg to n e s t l i n g stage represents the l o s s , i n every case, of the entire brood. The drop between n e s t l i n g and fledging stages was due to a p a r t i a l loss of one brood. In 1968 I traced eight nests from the egg stage through to fledging. The average clutch size f o r these was 3.25 £ 0.71 eggs, and there was 100 percent recruitment to the nest-l i n g and fledging stages. A Mann-Whitney U t e s t applied to the clutch sizes f o r both years showed no s i g n i f i c a n t d i f f e r -ence between them. 33 Success can a l s o be measured i n terms of t o t a l numbers of eggs l a i d , hatched, and young f l e d g e d . In 1969 82 eggs were l a i d i n the 27 nests f o l l o w e d f o r the e n t i r e n e s t i n g p e r i o d . Of these, 72 young hatched and 71 f l e d g e d . This gives an o v e r a l l success percentage of $6.6. Of a l l the eggs l a i d $7.$ percent hatched and of these 9$.6 percent f l e d g e d . I t i s i n t e r e s t i n g to note, however, th a t of 53 nests observed during b u i l d i n g only 5$.5 percent produced any young. Of the other 41.5 percent, 12 ne s t s , or 22.6 percent never had more than one egg l a i d i n them. Hence, females had spent time and energy b u i l d i n g nests t h a t were never pr o d u c t i v e . Of the 10 nests which f a i l e d during the n e s t i n g c y c l e one was the r e s u l t of predation by a Cooper's Hawk ( A c c i p t e r  c o o p e r i i ) which took the female. One other f a i l u r e could be a t t r i b u t e d to competition with a p a i r of Brewer's B l a c k b i r d s (Euphagus cyancephalus) that harrassed the female r o b i n d a i l y w h i l e she was b u i l d i n g and l a y i n g eggs. She never completed i n c u b a t i o n of them. The reasons f o r the f a i l u r e s of the other eight nests are unknown. Human i n t e r f e r a n c e was a f a c t o r i n four of the 12 nests which were not used a f t e r b u i l d i n g . The f a c t o r s a f f e c t i n g the remainder are unknown. An attempt was made to determine i f any of the p h y s i c a l f a c t o r s of n e s t i n g such as nest s i t e and t i m i n g had any e f f e c t on success. Nest height d i d not have any s i g n i f i c a n t e f f e c t (based on a Mann-Whitney U t e s t using the normal d i s t r i b u t i o n which produced a value of z~0.027). There was a l s o no 34 d i f f e r e n t i a l e f f e c t between nesting i n either an evergreen or a deciduous tree; (the chi-square r e s u l t i n g from a 2 X 2 contingency table was 0.013, not s i g n i f i c a n t ) . Nor was cover an important fa c t o r (determined by a chi-square test done by lumping the three categories that included cover from one d i r e c t i o n only; the chi-square was 5.8460). Whether a nest was started early or l a t e i n the season also had no e f f e c t (shown by a one-sample runs test and a chi-square test on a 2 X 2 contingency t a b l e ) . For the l a t t e r an e a r l y / l a t e c l a s s i f i c a t i o n was used based on the grouping of nesting a c t i v i t y shown i n Figure 5. A l l nests started before the May 28 - 30 break are c l a s s i f i e d as "early" nests and a l l those a f t e r as " l a t e " . It i s also i n t e r e s t i n g to note ( f i g . 5) that no nests are started a f t e r mid-July although the d a i l y photoperiod i s s t i l l i n excess of 16 hours of l i g h t . Gonadal Analysis The gonadal condition reaches i t s peak and then declines r a p i d l y during the breeding season. The t e s t i c u l a r weight ranges from 476.4 mg. to 901.7 mg. and averages 762.8 mg. f o r 18 specimens co l l e c t e d i n the f i r s t half of May, during which the d a i l y l i g h t period exceeds 16 hours. For the second half of May the average f o r 10 males was 807.7 mg., ranging from 6 9 6 . 7 to 880.7 mg. During t h i s month the seminiferous tubules reach t h e i r greatest diameter. The average diameter i s 151.70 mu. f o r the f i r s t half of the month, ranging from 3 5 1 3 2 . 6 0 to 1 6 4 . 3 3 mu. For the l a t t e r part of the month the average i s 1 5 7 . 8 3 mu., ranging from 1 5 2 . 7 0 to 1 6 4 . 0 3 mu. A l l specimens had mature sperm present. During June the testes s t a r t to regress i n terms of both weight and s i z e ; however, a l l specimens s t i l l show mature sperm present and there i s l i t t l e change i n the structure of the seminiferous tubules. I t i s also during t h i s month that the t o t a l hours of l i g h t per day reaches i t s maximum of just under 1 8 hours. The testes average 5 5 4 . 2 mg. weight, ranging from 2 9 5 . 4 to 8 6 5 . 3 mg., f o r 1 0 specimens col l e c t e d i n the f i r s t h a l f of June. For the same period the semini-ferous tubule diameter averaged 1 4 7 . 3 9 mu., ranging from 1 2 8 . 8 5 to 1 7 4 . 3 8 mu. In the second half of the month the six testes collected averaged 4 8 3 . 1 mg., ranging from 3 3 7 . 9 to 7 9 1 . 4 mg. The average tubule diameter was 1 4 2 . 1 5 mu., ranging from 1 2 9 . 6 0 to 1 4 7 . 1 5 mu. The ovaries showed very marked siz e changes during the height of the breeding season. In the f i r s t part of May the average weight f o r the ovary from each of seven specimens was 8 1 3 . 9 mg., ranging from 3 4 1 . 6 to 1 2 6 5 . 5 mg. A l l but one specimen had more than f i v e enlarged f o l l i c l e s , with 1 0 being the largest number. The largest f o l l i c l e measured 1 2 . 4 mm. This was on the 1 2 6 5 . 5 mg. specimen which also had one post ovulatory f o l l i c l e . For the second h a l f of May eight ovaries averaged 7 3 3 . 2 mg., ranging from 1 0 0 . 3 to 2 5 4 4 . 9 mg. Again a l l ovaries showed developing f o l l i c l e s , the number ranging 3 6 from two on the l i g h t e s t ovary to nine on one weighing 522.8 mg. The heaviest specimen had the largest f o l l i c l e (diameter 12.3 mm.). This specimen had only four developing f o l l i c l e s but a l l were greater than 9.0 mm. i n diameter, and t h i s b i r d also had an egg i n the lower part of the oviduct. In June the ovaries, as well as the testes, begin to regress. Each ovary averages 317-5 mg. i n weight, ranging from 73.5 to 1726.9 mg. f o r the eight specimens collected during the f i r s t half of the month. This drops to 89.7 mg. average f o r f i v e specimens collected i n the second half of June which ranged from 66.5 to 196.0 mg. Two of the specimens collected during early June showed no enlarged f o l l i c l e s . With the exception of that of the 1726.9 mg. specimen, the largest f o l l i c l e was 2.6 mm. i n diameter. The large ovary had three enlarged f o l l i c l e s , ranging from 8.4 to 12.6 mm. in diameter. By the end of the month only two specimens showed any f o l l i c u l a r enlargement and that consisted of three f o l l i c l e s (two on one, one on the other), the largest of which was 2.7 mm. i n diameter. 37 Onset of the Non-Breeding Season  F i e l d Behaviour During the breeding season most p a i r s have attempted to nest at l e a s t twice and some, three times. The l a t t e r are s t a r t i n g t h e i r l a s t attempt i n J u l y . Some p a i r s reuse the same nest and some b u i l d new ones but remain i n much the same area or t e r r i t o r y even i f they change t h e i r nest s i t e . As the young from the f i n a l n e s t i n g e f f o r t f l e d g e both parents stay w i t h them f e e d i n g and c a r i n g f o r them u n t i l they are completely capable of c a r i n g f o r themselves. Even then the young stay w i t h both parents, so small groups are evident i n areas which were f o r m e r l y r e s t r i c t e d t o breeding b i r d s . As more p a i r s complete n e s t i n g more groups are i n evidence. This occurs throughout J u l y and e a r l y August w h i l e the d a i l y hours of l i g h t are s t i l l i n excess of 16 but l e s s than the maximum of 18 reached i n June. Some of these f a m i l y groups may j o i n together but the groups g e n e r a l l y remain s m a l l , ranging from f i v e t o 12 mem-bers. For the l a s t part of August no robins were observed on campus. They returned i n September i n somewhat l a r g e r groups, as high as 20 members, and f a m i l y groups could no longer be d i s t i n g u i s h e d . From t h i s time on the behaviour of the robins was t y p i c a l of the w i n t e r , or non-breeding season, patterns described p r e v i o u s l y . 3 8 Gonadal A n a l y s i s By J u l y f a r fewer n e s t s a r e b e i n g s t a r t e d and t h e gona-d a l c o n d i t i o n s a re r e g r e s s i n g r a p i d l y . D u r i n g t h e f i r s t h a l f of t h e month t h e average t e s t i s w e i g h t i s 3 7 3 . 1 mg., r a n g i n g f rom 1 0 4 . 1 t o 5 5 0 . 5 mg. f o r seven specimens. Two o f the specimens showed mature sperm. The r e s t had a few sperm p r e s e n t but t h e s e m i n i f e r o u s t u b u l e s t r u c t u r e was degenera-t i n g . The average t u b u l e d i a m e t e r was s t i l l q u i t e l a r g e , 1 3 9 . 7 1 mu., r a n g i n g from 1 2 5 . 6 3 t o 1 5 4 . 4 3 mu. In t h e second h a l f o f t h e month t h e t e s t i s weighed 9 4 . 3 mg., r a n g i n g f r o m 1 4 . 4 t o 2 9 1 . 6 mg. f o r seven specimens. On m i c r o s c o p i c e x a m i n a t i o n none o f t h e specimens showed any sperm p r e s e n t o r d e v e l o p i n g . The average t u b u l e d i a m e t e r had d e c r e a s e d t o 6 6 . 5 5 mu., r a n g i n g f r o m 3 $ . 2 5 t o 1 1 6 . 1 9 mu. I n J u l y t h e o v a r i e s become f u l l y r e g r e s s e d . None of t h e 1 2 specimens c o l l e c t e d d u r i n g the month showed any e n l a r g e d f o l l i c l e s . The average w e i g h t of t h e o v a r y of s i x specimens c o l l e c t e d i n t h e f i r s t h a l f o f t h e month weighed 5 7 . 6 mg., r a n g i n g f rom 3 1 . 8 t o 7 0 . 1 mg. I n t h e l a t t e r p a r t o f J u l y the a verage w e i g h t was 3 9 . 8 mg., r a n g i n g f r o m 2 5 . 6 t o 8 2 . 3 mg. Throughout August seven male specimens were c o l l e c t e d . The average t e s t i c u l a r w e i g h t was 4 1 . 6 mg., r a n g i n g f r o m 1 7 . 4 t o 8 4 . 5 mg. The t u b u l e d i a m e t e r averaged 5 7 . 2 0 mu., r a n g i n g f rom 3 5 . 4 6 t o 7 2 . 9 1 mu. w i t h no sperm b e i n g p r e s e n t i n th e s e m i n i f e r o u s t u b u l e s . 39 In September eight specimens showed a testes weight which averaged 21.7 mg., ranging from 8.6 to 44.6 mg. The seminiferous tubule diameter from t h i s period was 32.51 mu. , ranging from 18.72 to 42.66 mu. The ovaries averaged 20.6 mg. weight ranging from 10.8 to 36.0 mg. f o r six specimens col l e c t e d i n August. In Sep-tember the average was 22.6 mg. based on nine specimens and ranged from 12.2 to 31.4 mg. Figure 6 shows the r e l a t i o n s h i p between the development and regression of the male and female reproductive systems f o r the entire breeding season. 40 240 4$0 720 720 I? 430 to 240 0 1 March A p r i l M a y June J u l y August Figure 6. The r e l a t i o n s h i p between the development of male and female reproductive organs. 41 Photoperiod Experiments Two sets of c o n t r o l l e d photoperiod experiments were conducted, one from January to A p r i l of 1969 and the other from February t o May of 1970. Both were designed to demons-t r a t e the e f f e c t , i f any, of i n c r e a s i n g photoperiod on gonadal development and on migratory behaviour as evidenced by Z'ugunruhe or night r e s t l e s s n e s s . In the f i r s t experiment 21 robins were maintained, i n groups of three to f i v e b i r d s , on three d i f f e r e n t l i g h t regimes. One group had an eight-hour l i g h t , 16-hour dark day, l e s s l i g h t than would be normally r e c e i v e d at the l a t i t u d e of my study area at any time of the year. Another group was maintained on a 12-hour l i g h t , dark c y c l e , s i m i l a r to the photoperiod experienced on campus i n the l a t t e r part of February. The t h i r d group r e c e i v e d 16 hours of l i g h t and eight of darkness i n each 24 hours. This compares to the photoperiod of e a r l y May, during the height of the breeding season on campus. I o r i g i n a l l y intended to use only males f o r both these experiments, however, when the robins were s a c r i -f i c e d at the c o n c l u s i o n of each experiment s e v e r a l of the males proved to be females. During the f i r s t experiment s i x r o b i n s (four males and two females) were kept on the short day l e n g t h ; eight robins (seven males and one female) on the 12-hour d a y l i g h t and seven rob i n s ( s i x males and one female) on the long day c y c l e . 42 Actual experimental conditions began February 1, 1969 after a l l the birds had been i n c a p t i v i t y under natural photoperiod conditions f o r at least one week. The birds were s a c r i f i c e d on A p r i l 19, 1969 when the natural photoperiod was approxi-mately 15-hours l i g h t , nine-hours dark. The changes i n t e s t i c u l a r development on the different photoperiods are summarized i n Table I I . Those males on the short day length treatment had an average t e s t i c u l a r weight of 9.4 mg., ranging from 4.1 to 20.7 mg. The average semini-ferous tubule diameter was 39.3$ mu., ranging from 37.80 to 41.25 mu. The males on 12-hour days had an average t e s t i c u l a r weight of 14.6 mg., ranging from 5.0 to 33*9 mg. and a mean tubule diameter of 45 .03 mu., ranging from 30.9$ to 72.83 mu. Neither of these two groups of males showed any h i s t o l o g i c a l t e s t i c u l a r development. The average t e s t i c u l a r weight of those males on 16-hour daylight was 200.5 mg., ranging from 77-$ to 313.$ mg. with a tubular diameter averaging 110.25 mu., ranging from 82.95 to 143.1 mu. The testes of t h i s group did have developing spermatocytes but no sperm. These res u l t s can be compared to the gonadal development of birds i n the wild state. Four males, collected i n January at the same time that the experimental birds were captured, had an average t e s t i c u l a r weight of 2.9 mg., ranging from 2.0 to 3-9 mg. and an average tubule diameter of 16.3$ mu., ranging from 7.58 to 21.90 mu. The t e s t i c u l a r state of robins c o l l e c -ted in Richmond and on the campus during A p r i l , i . e . at the Photoperiod (Hours) F i r s t Experiment 8 12 16 Second Experiment $ n a t u r a l Testes Average 9.4 14.6 200.5 25.0 366.9 Weight (mg.) Range 4.1- • 5.0- 77.8- 17.0- 271.4-20.7 33-9 313.3 32.9 414.8 Seminiferous Average 39-38 45.03 110.25 44.14 124.65 Tubule Diameter Range 37.80-- 30.98- 82.95- 36.22- 121.13-(mu.) 41.25 72.83 143.1 52.05 126.68 Table I I . The t e s t e s weight and seminiferous tubule diameter at the conclusion of the experiments f o r the rob i n s maintained on d i f f e r e n t >photoperiods. 44 • same time that the captive birds were s a c r i f i c e d , has been given previously, as has the gonadal condition of those collected i n May when the photoperiod was comparable to the 16-hour l i g h t that the experimental birds received. The females also showed different amounts of gonadal development under diffe r e n t photoperiod conditions. The two birds on short day length had ovarian weights of 26.8 mg. and 40.9 mg. and showed no f o l l i c u l a r development. The ovary of the one female on a 12.-hour l i g h t , 12-hour dark cycle weighed 56.8 mg. and s i m i l a r l y showed no development. The ovary of the female on long day length, however, weighed I83.I mg. and had nine enlarging f o l l i c l e s , two of which were over 2.0 mm. i n diameter. Like the males the development of the captive females can be compared to that of females collected i n Richmond and on the campus. The second set of experiments was designed to show the effect of photoperiod on behaviour, p a r t i c u l a r l y migratory behaviour, as well as on gonadal development. For these experiments, run the following year from March 6 to May 5 using birds captured i n la t e February, the robins were housed i n d i v i d u a l l y . This was done primarily to permit the measur-ment of each individual's night a c t i v i t y and also to reduce any e f f e c t , on both a c t i v i t y and gonadal development of aggre-ssion or the s o c i a l dominance of one b i r d i n a cage over the others. For t h i s experiment only two photoperiods were main-45 tained: one was an eight-hour l i g h t , 16-hour dark cycle ex-perienced by two males and one female, and the other was the natural photoperiod of the University of B r i t i s h Columbia f o r the duration of the experiment. Three males were kept on the l a t t e r treatment. The two males on an eight-hour photoperiod had testes weights of 32.9 mg. and 17.0 mg., averaging 25.0 mg. at the conclusion of the experiment. Their tubule diameters measured 52.05 mu. and 36.22 mu., averaging 44.14 mu. (Table I I ) . No development of secondary spermatocytes or spermatids was evident. The female i n t h i s treatment had an ovary which weighed 28.3 mg. and had no developing f o l l i c l e s . The three males kept under natural photoperiod condi-tions had an average t e s t i c u l a r weight of 366.9 mg., ranging from 271.4 to 414.8 mg. The seminiferous tubule diameter averaged 124.65 mu., ranging from 121.13 to 126.58 mu. These specimens did show developing spermatocytes and spermatids. The a c t i v i t y of the two groups of robins i s shown i n Figures 7a to 7h. The period from 4 pm one night to 8 am the following morning represents the dark phase of the birds on short photoperiod. The average of the a c t i v i t y of these birds i s shown by the s o l i d l i n e . For the three males on natural photoperiod the dark phase of t h e i r d a i l y cycle i s shown at the top of each figure and the average of t h e i r a c t i v i t y by the dotted l i n e . The figures are f o r one day of each week fo r the eight weeks of the study. The night a c t i v i t y or 46 Figure 7. The average hourly activity of two groups of robins housed under different photoperiod conditions. The dotted line represents those exposed to natural photoperiod with the line at the top of the graph showing the period of darkness for these birds. The solid line represents the activity of birds kept under short photo-period which were in.darkness from 4 pm to 8 am each day. 7a. The activity for March 9/10. 4-6 a u 500 2 4 6 8 10 12 2 4 6 8 10 12 midnight Figure 7b. The a c t i v i t y f o r March 16/17. 46 b 46 c 2 4 6 8 10 12 2 4 6 8 10 12 midnight Figure 7d. The activity for March 30/31. 46 e 500 2 4 6 8 10 12 2 4 6 8 10 12 midnight Figure 7f. The a c t i v i t y f o r A p r i l 14/14. 46 f 46 g i 1 2 4 6 $ 10 12 2 4 6 8 10 12 midnight Figure 7h. The a c t i v i t y f o r A p r i l 27/2$. 47 Zugunruhe of the birds on natural photoperiod, which varied from 12.2$ hours at the onset of the experiment to 16 hours of light per day at the end of the experiment, is strikingly evident. 48 DISCUSSION My study of the American Robin (Turdus mi g r a t o r i u s ) concerns i t s e l f with two main aspects of the reproductive b i o l o g y of the s p e c i e s . One c o n s i d e r a t i o n i s a study of the n e s t i n g parameters of these b i r d s . This i n c l u d e s the r o l e s of the parents from the onset of the breeding season u n t i l the l a s t brood i s reared, the p h y s i c a l f a c t o r s of n e s t i n g such as l o c a t i o n , and the n e s t i n g success. The other aspect i s the r e l a t i o n s h i p of the i n c r e a s i n g day lengths of s p r i n g to gon-adal recrudescence and the t i m i n g of the breeding season. This r e l a t i o n s h i p was s t u d i e d both by f i e l d observation and l a b o r a t o r y experiments. The study was somewhat complicated by the presence of three subspecies of r o b i n (T. m. Migrato-r i u s , T. m. caurinus, and T. m.propinquus) i n the Vancouver area. U n f o r t u n a t e l y a workable d e s c r i p t i o n of the d i f f e r e n t subspecies and t h e i r h a b i t s i s not a v a i l a b l e (Coues, 1903; Forbush and May, 1955; Godfrey, 1966) so I was not able to determine whether or not a given b i r d was a migrant or r e s i -dent and to which subspecies i t belonged. Nesting Parameters The p a t t e r n of n e s t i n g a c t i v i t i e s which I recorded f o r the American Robin i s b a s i c a l l y the same as t h a t described by Howell (1942), Young (1955) and Howard (1967). These researchers, however, stud i e d populations which were migra-t o r y , so t h e i r observations begin w i t h the a r r i v a l of b i r d s 49 in spring and the establishment of nesting territories by the male. Since my work deals with a partially resident popula-tion of robins I was able to observe the early indications of breeding behaviour. I used the shift from gregarious winter behaviour to t e r r i t o r i a l behaviour as a marker for the onset of the breeding season. There are accounts of winter behaviour in robins (Black, 1 9 3 2 ; Price, 1 9 3 3 ) but none des-cribing the shift from social flocking behaviour to pairing. The European turdids, especially the English Robin (Erithacus rubecula), show a t e r r i t o r i a l behaviour year round (Lack, 1 9 4 8 ) . The territory established in autumn is in a different place from that established in spring, but Lack ( 1 9 4 8 ) gives no indication of when or how the breeding t e r r i -tory is established. It does seem, however, that the winter territory of the English Robin i s more distinct and permanent than the individual spacing behaviour which I noted while American Robins were ground feeding during winter. The spring, or breeding, territory of the English Blackbird (Turdus  merula) i s not established until after pair formation occurs (Lack and Light, 1 9 4 1 ) . The latter, however, takes place in early February, so i t may not coincide with gonadal recru-descence whereas territory establishment may. Unfortunately there is no information on the gonadal cycle of this species. The Blackbird on the Shetland Isles of Britain winters in loosely aggregated groups near, and occasionally in, the breed-50 ing area (Venebles and Venebles, 1952). The winter birds are not normally t e r r i t o r i a l or aggressive towards one another. The authors do report non-functional "sexual chasing" occur-ring from January until the start of the breeding season but point out that pairing and territory establishment do not occur until the end of February and March. My records show that for the years studied there was a gradual breakdown of winter flocking behaviour in the robin with a parallel increase in intraspecific aggressive ter r i t o -r i a l behaviour. This occurred from the beginning of February until early April, although few permanent territories were established before the latter half of March. During this period the hours of light per day increased from 10.6 to 14.5. Once the territories are established the activities of the males and females in terms of defence, nest site locating, and nest construction are the same at the University of British Columbia as those described by others (Howe, 1898; Howell, 1942; Young, 1955). Howell (1942) did note a significant difference between the number of nests constructed in ever-green compared to deciduous trees at the beginning of the nesting period. My data did not show a difference between these two site types at any time of the breeding season but i t did show that the robins preferred nest sites which offered some cover. The roles of the males and females which I observed for egg-laying, incubation, care of nestlings and care of fl e d -51 gings proceeded according to the pattern observed by others (Howe, 189$; Hamilton, 193 5; Schantz, 1939; Howell, 1942; Young, 1955) with the exception that on two occasions I wit-nessed males (sexed on the basis of plumage colouration) i n -cubating eggs. Howell (1942) quotes others as having recorded such behaviour although he did not observe i t . Since I did not c o l l e c t the birds i n question I cannot be absolutely cer-t a i n of t h e i r sex. In one of- the two cases a much paler-coloured bir d was nearby and took over the incubation. For t h i s nest then either the male was a s s i s t i n g with the incu-bation or there were two females sharing the nest. This behaviour of the males may be more corre c t l y c a l l e d brooding rather than incubation as there i s some question as to whe-ther or not the male i s capable of warming the eggs (Ryves, 1943). Both the length of the incubation and ne s t l i n g periods and the a c t i v i t i e s connected with them are much the same for the robins I observed as those observed by others (Howe, 1$9$; Hamilton, 1935; Howell, 1942; Young, 1955). S i m i l a r l y the average clutch size does not vary s i g n i f i c a n t l y from the 3.04— 0.05 found by Young (1955) or the usual three or four eggs per clutch noted by Howell (1942) and Howard (1967). The l a t t e r investigator found a seasonal v a r i a t i o n i n clutch s i z e , using data from nest record cards, which was s i g n i f i c a n t l y d i f f e r e n t f o r the periods of May and July. Similar changes i n clutch size have been discovered by nest record card analy-52 s i s f o r s e v e r a l B r i t i s h t u r d i d s (Snow, 1955a ). No such s i g n i -f i c a n t v a r i a t i o n with season i s apparent i n the b i r d s which I s t u d i e d but t h i s could be due to the r e s t r i c t e d number of nests observed. The n e s t i n g success was the only other n e s t i n g parameter f o r which my r e s u l t s d i f f e r e d markedly from other published records. The o v e r a l l success r a t e of 8 6 . 6 percent from eggs to f l e d g i n g s i n my study i s much higher than the average 49 percent observed by Young (1955) or the 6 5 . 2 percent shown by Howell ( 1 9 4 2 ) . My r e s u l t s are a l s o higher than the ex-pected success f i g u r e s f o r both open and hole n e s t i n g b i r d s , 43 percent and 55 to 76 percent r e s p e c t i v e l y , as c a l c u l a t e d by Nice ( 1 9 3 7 ) . The i n c u b a t i o n period had the greatest f a i l -ure r a t e i n a l l r e p o r t s but t h i s i s a l s o the p e r i o d i n which there i s the most discrepancy between my r e s u l t s and the others. The robins I observed had a hatching success of 8 7 . 8 percent, whereas Young (1955) observed only 58 percent and Howell (1942) 7 3 . 9 percent. The r a t e s f o r hatching to f l e d g i n g success were 9 8 . 6 percent (my r e s u l t s ) 8 8 . 2 percent (Howell, 1942) and 78 percent (Young, 1 9 5 5 ) . The high recruitment r a t e which I found f o r robins on the campus of the U n i v e r s i t y of B r i t i s h Columbia could w e l l be a l o c a l phenomenon and not representa-t i v e of the robins of southwestern B r i t i s h Columbia as a whole. In order t o determine i f t h i s was so or not I checked my r e s u l t s with those of the B r i t i s h Columbia nest record card survey (Drent, pers. comm.). This data f o r robins i n 53 southern coastal gardens and parks indicates a 72 percent hatching success and 1 0 0 percent fledging success. The surveys of one person in the Vancouver area give an overall success figure of 75 percent for nests in 1 9 6 9 . The results for British Columbia as a whole, however, show a hatching success of 6 4 . 5 percent and a fledging success of 6 4 percent for an overall recruitment of.59 percent. These figures indicate that the southwestern corner of British Columbia, especially the area with gardens and parks, can expect a higher than average, for this species, success rate. The area of the campus which I studied is relatively free.from cats, one of the main residential predators of robins, and from racoons, squirrels and birds of prey, important woodland predators. There i s also l i t t l e human interference or vandalism as there are few students and virtu a l l y no young children present on campus during most of the breeding season of the robin. This habitat would be comparable to that studied by Kendeigh ( 1 9 4 2 ) for which he found an overall success rate of 71 percent with a hatching and fledging success rate of 36 percent and $3 per-cent respectively. Breeding Season and Gonadal Condition At the onset of the breeding season, as the previously described shift in social behaviour pattern i s taking place, the photoperiod is increasing from 1 2 hours of light per day at the beginning of March to over 1 4 hours at the beginning 54 of A p r i l when most t e r r i t o r i e s are e s t a b l i s h e d . None of the ovar i e s c o l l e c t e d at t h i s time have f o l l i c l e s s u f f i c i e n t l y -developed t o ovulate. The t e s t e s do not show any increase i n s i z e u n t i l the f i r s t of March and are at only h a l f t h e i r p o t e n t i a l weight by m i d - A p r i l . The seminiferous tubules have, however, enlarged almost completely by m i d - A p r i l , with some specimens possessing mature sperm. The d i f f e r e n c e i n t e s t e s development between b i r d s c o l l e c t e d i n Richmond, which were, at t h a t time of year, g e n e r a l l y found i n groups, and those c o l l e c t e d on campus, which were g e n e r a l l y alone or i n groups of only two or thr e e , i s marked. The Richmond rob i n s could be males which had not, as yet, e s t a b l i s h e d t e r r i t o r i e s or they could be migrants who would be moving f u r t h e r north be-f o r e s e t t i n g up t e r r i t o r i e s . This would agree with Wolfson's (1942) f i n d i n g s on the Oregon Junco (Junco oreganus) that the t e s t e s of r e s i d e n t b i r d s recrudesce e a r l i e r and at a f a s -t e r r a t e than those of migrants. In e i t h e r case the estab-lishment of a t e r r i t o r y and/or p a i r i n g w i t h a female may be necessary t o s t i m u l a t e f u r t h e r gonadal development. Compari-son of the t e s t i c u l a r c o n d i t i o n of each group of b i r d s i n d i -cates that the onset of breeding behaviour i n the American Robin can be c o r r e l a t e d with detectable developmental changes i n the h i s t o l o g y of the t e s t e s . The a c t u a l n e s t i n g p e r i o d , as defined by the s t a r t of nest b u i l d i n g u n t i l the l a s t brood leaves the nest, runs from l a t e March u n t i l August. Egg-laying occurs from e a r l y A p r i l 55 until early July, with a peak in mid-April and a smaller one in early June ( f i g . 5 ) . Howell (1942) working with a migrant population at Ithaca, New York, found that nesting territory establishment did not take place until the last week of March and that eggs were layed from early April until the f i r s t of August. Young (1955) discovered an egg-laying season of April 1 2 t h to July 20 th at Madison, Wisconsin, based on the time of laying of the f i r s t egg of a clutch. He found a peak in nest-ing activity in the third week in April. Howe (1$9$) describes a nest construction period from early April until the third week of May in eastern Massachusettes. This, when compared to the results of a nest card survey of the same area (Howard, 1967) indicates that he may have succumbed to the well-noted human f a i l i n g (Myres, 1955) to search less d i l l i g e n t l y for nests in the latter part of the season. The genus Turdus in Great Britain has a breeding season lasting from March to July (Myres, 1 9 5 5 ) . The Blackbird may ini t i a t e nesting as much as two weeks later in woodland habi-tats than in gardens at the same latitude (Myres, 1 9 5 5 ) . Myres (1955) also demonstrated a delay in the start of the breeding season as one moves northward and a dependence on warm weather conditions before nesting w i l l begin in March and early A p r i l . The thrushes of the Shetland Islands have a breeding season from April until early August (Venebles and Venebles, 1952) although the number of nests started after July 1 s t is much reduced. The season for the Blackbird in 56 New Zealand (Bull, 1946) runs from late August to early-December, comparable to February through June in northern latitudes, with the majority of the activity occurring from mid-September to late October. The timing of the breeding season of various turdids at different latitudes is summar-ized in Figure 8. It i s clear, from this information, that the timing of the breeding season i s related to latitude, and hence, to photoperiod, but not exclusively dependent upon i t . Other factors such as temperature, habitat, and food availability may ameliorate the situation on a local basis. The histology of the gonads of those male robins collec-ted on the campus of the University of British Columbia reveal that some are in breeding condition by the end of March and a l l are spermatogenic by midrApril, the time at which the majority of f i r s t nests are begun. Spermatogenesis is main-tained until the end of June, and then the testes begin to regress very rapidly. The same pattern i s shown (Howell, 1942) for the male robin at Ithaca, New York. The development of the ovary is not as straightforward. Females in f u l l repro-ductive condition with eggs ready to ovulate were found only in the May sample. It i s , however, obvious (fig. 5) that females must have been laying eggs from April to July. The lack of ovarian specimens to substantiate the nesting cycle of the female i s probably partially due to the rapid develo-pment and regression of f o l l i c l e s (Meyer _et aJL, 1947; Davis, 57 H G F -3-D B + Jan Feb Mar Apr May Jun Jul Aug Figure The relationship of breeding season to latitude. The area within brackets indicates the time span in which 90 percent of breeding occurs. The references are as follows: A. Bull, 1946 B. Howe, 189S C. Howell, 1942 D. Howard, 1967 E . Young, 1955 F. This study G. Myres, 1955 H. Venables and Venables, 1952 58 1942b ; Romanoff, 1943) and also to the dependence of the fe-male on stimulation by some local condition that initiates ovarian development (Davis and Davis, 1 9 5 4 ; Farner, 1964) rather than photoperiod (Wolfson, 1 9 4 2 ) . The precise factors that stimulate f o l l i c u l a r enlargement are unknown but i t i s possible that the readiness of the female is a controlling factor in the timing of the breeding season. Photoperiod Experiments I conducted two sets of experiments to determine i f increasing photoperiod would stimulate gonadal development in the American Robin, particularly testicular development, as was indicated by my f i e l d observations. A positive re-lationship between the two factors has already been established for several other species of birds including two junco species (Junco hyemalis and J. oreganus) (Rowan, 1 9 2 9 ; Wolfson, 1959) and the White-crowned Sparrow (Zonatrichia leucophrys gambelii) (Farner and Mewaldt, 1952 ; Farner and Wilson, 1 9 5 7 ; Farner, I964; Farner et £^1, 1 9 6 6 ) . Both experiments which I conducted showed a positive correlation between the hours of light and recrudescence of gonads. In neither case, however, did the captive birds attain a gonadal condition equal to that of wild robins experiencing similar hours of daylight. The birds which I maintained on short day length, eight hours of light per day, had average testicular weights that were greater than the average for the robins which I collected 59 during the winter, even though the wild birds were experien-cing photoperiods in excess of eight hours per day. The noise issued by robins which were experiencing longer photo-periods but which were housed beside the short day birds may have had an effect on the development of the latter. Also the intensity of the light used in the experiments, which I did not measure, may have been important in this regard (Burger, 1943; Bartholomew, 1949)- The experimental robins on 12 hour days showed an average testicular weight that was similar to that of wild robins captured in early March, when the photo-period is about 12 hours. The testes of both captive and wild birds at 12 hours of light showed only very slight devel-opment beyond the stage achieved by winter birds, indicating that the photoperiod must exceed 12 hours for any stimulatory effect to occur. Birds kept on 16 hours of light per day showed definite testicular enlargement and development although not to the same extent as wild birds collected when the photoperiod was about 16 hours. These latter birds had an average testicular weight twice that of the captive birds. The wild birds had mature sperm present in their seminiferous tubules whereas the captive robins had only developed to the spermatid stage. The testicular condition of the captive robins was closer to that of wild birds collected at about 15 hours of light per day photoperiod than to those collected at 16. Retarded development of captive birds as compared to 60 birds in natural conditions has been noted by others (Scott and Middleton, 1968 for the Brown-Headed Cowbird (Molothrus ater); Miller, i960 for Zonotrichia leucophrys). This d i f f -erence might be attributed to light intensity, type of food available, restricted activity, and social stimulus from con-specifics, especially birds of the opposite sex. The envir-onmental stimulus of a suitable breeding territory may also be an important factor. The effect of two of these factors, social interactions and light intensity, i s further exemplified by the differing results of the two experiments. In the f i r s t set of experi-ments each cage held several birds, one of which appeared to be dominant. This bird would always eat f i r s t and often chased the other birds around the cage. The dominant robins, although not significantly different from others on the same treatment, weighed more and had the heaviest testes at the end of the experiment. Whether the social conditions served to stimulate the one bird or retard the others or both is not known. However, for the second set of experiments the robins were housed individually and these birds showed greater gonadal de-velopment on both light regimes than did the robins under the same light conditions in the previous experiment. This difference was very noticable for the 16 hours of light per day birds which showed an average testicular weight of 200.5 mg. in the f i r s t experiment and 366.0 mg. in the second. For the second experiment not only were the birds 61 caged separately but they were a l s o exposed to n a t u r a l r a t h e r than a r t i f i c i a l l i g h t . From these r e s u l t s i t i s c l e a r t h a t a photoperiod of greater than 12 hours per day over the period of my e x p e r i -ments, i s necessary to i n i t i a t e gonadal development. I t i s more d i f f i c u l t to determine i f the l i g h t p eriod must reach a c e r t a i n amount before f u l l spermatogenesis or ovarian development i s reached. C e r t a i n l y the c o l l e c t e d b i r d s d i d not reach such a s t a t e u n t i l there was 1$ hours of l i g h t p p e r day, but i t i s impossible to say whether or not they would have developed completely i f kept at a r a t i o greater than 12 hours l i g h t : 12 hours dark but l e s s than 15 hours l i g h t : 9 hours dark. This may depend on the genetic makeup of the species ( M i l l e r , I960). Recent s t u d i e s by Menaker ( i n press) i n d i c a t e t h a t i t i s not the l e n g t h of the d a i l y l i g h t phase t h a t i s im-portant but the time of day at which l i g h t occurs. In other words, a b r i e f f l a s h of l i g h t repeated d a i l y at a point i n the c i r c a d i a n rhythm when the species i s photoreceptive i s a l l that i s necessary t o s t i m u l a t e gonadal development. Wolfson's data (1953) w i l l probably, on r e a n a l y s i s , corroborate Menaker's hypothesis. I f such a hypothesis i s c o r r e c t , then f o r the robins which I studied the photoreceptive period would have to be at some point i n the c i r c a d i a n rhythm s h o r t l y a f t e r darkness would normally occur on a n a t u r a l 12 hour l i g h t day. The e f f e c t of p h o t o s t i m u l a t i o n at t h i s time may then be cumulative u n t i l f u l l breeding c o n d i t i o n i s a t t a i n e d . 62 It is also, however, evident from my results that fac-tors other than photoperiod play a role in determining the precise timing of the breeding season. The relationship, i f any, of photoperiod to the maintainence of reproductive gonadal condition throughout the breeding season and to the gonadal regression at the end of the season i s untested for the robin. During the second experiment I also measured Zugunruhe or night restlessness. This i s an indication of migratory behaviour in species which migrate at night. I wanted to know i f the relationship between increasing photoperiod, gon-adal recrudescence and spring migration shown for other species (Rowan, 1 9 2 6 ; Wolfson, 1 9 4 2 , 1959 ; Farner, 1 9 5 5 , Farner et a l , I 9 6 0 ) would also occur in the robin. Since act-i v i t y would be monitored from the onset of the experiment such a study would also indicate how rapidly the increased photoperiod would become effective. The results showed that a definite relationship between increased photoperiod and migratory behaviour exists. Virtually no night activity was demonstrated by robins on eight hour light photoperiods whereas those on long days showed Zugunruhe nightly beginning five days after the start of experimental conditions. 63 CONCLUSIONS The breeding b i o l o g y of the Amercian Robin of south-western B r i t i s h Columbia, as observed i n t h i s study, i s b a s i c a l l y the same as t h a t of t h i s species s t u d i e d elsewhere i n North America. The f a c e t of the breeding b i o l o g y of the r o b i n which i s presented f o r the f i r s t time i s the gradual na-tu r e of the onset of the breeding season as evidenced by the change i n i n t r a s p e c i f i c t e r r i t o r i a l behaviour. F i e l d observations and l a b o r a t o r y experiments both i n -d i c a t e that photoperiod i s a major f a c t o r c o n t r o l l i n g the onset of gonadal development and hence t i m i n g of the breeding season. Gonadal recrudescence w i l l not occur u n t i l the photoperiod exceeds twelve hours per day. The l a b o r a t o r y experiments f u r t h e r demonstrate that long day lengths alone are not s u f f i c i e n t to stimulate f u l l reproductive response. Other f a c t o r s such as s o c i a l stimulus from c o n s p e c i f i c s of both sexes, and environmental s t i m u l i may modify the r a t e of reproductive development. Much greater a t t e n t i o n needs t o be given to the r o l e of the female as regards the t i m i n g of the breeding season. 64 LITERATURE CITED Bartholomew, G. H. 1949. The effect of l i g h t i n t e n s i t y and day length on reproduction i n the English Sparrow. B u l l . Museum Comp. Zool. 101: 433-476. Black, J . D. 1932. A winter robin roost i n Arkansas. Wilson B u l l . 44: 13-19. B r a c k b i l l , H. 1944. Normal and inverted courtship feeding by the robin. Auk 61: 138-139 1947. Choice of nest-site i n the American Robin. Wilson B u l l . 59: 116. B u l l , P. C 1946. Notes on the breed ing cycle of the Thrush and Blackbird i n New Zealand. Emu 46: 198-208. Burger, J . W. 1943* Some effects of coloured i l l u m i n a t i o n on the sexual a c t i v a t i o n of the male S t a r l i n g . J . Exp. Zool. 94: 161. Coues, E. 1903. Key to North American Birds. Dana Estes and Co., Boston. Davis, D. E. 1942a. The bursting of avian f o l l i c l e s at the beginning of a t r e s i a . Anat. Rec. 82: 153-165. 1942b. The regression of the avian postovulatory f o l l i c l e . Anat. Rec. 82: 297-307. 1942c. The number of eggs l a i d by cowbirds. Condor 44: 10-12. Davis, J . and B. S. Davis. 1954. The annual gonad and thyroid cycles of the English Sparrow i n Southern C a l i f o r n i a . Condor 56: 328-345. Farner, D. S. 1945- Age groups and longevity i n the American Robin. Wilson B u l l . 57: 56-74. 1949. Age groups and longevity i n the American Robin: comments, further discussion, and certain re-vi s i o n s . Wilson B u l l . 61: 6 8 - 8 1 . 1955. The annual stimulus f o r migration: ex-perimental and physiological aspects, p. 198-237. In A. Wolfson (ed.) Recent Studies i n Avian Biology. Univ. I l l i n o i s Press, Urbana. 65 Farner, D. S. 1964 . Time measurment i n vertebrate photoper-iodism. Amer. Nat. 9 $ : 375-3^6 . , B. K. F o l l e t t , J . R. King, and M. L. Morton. 1966. A quantitative examination of ovarian growth i n the White-Crowned Sparrow. B i o l . B u l l . 1 3 0 : 6 7 - 7 5 . , and L. R. Mewaldt. 1952 . The r e l a t i v e roles of photoperiod and temperature i n gonadal recrudescence in male Zonotricha leucophyrs gambelii. Anat. Rec. 1 1 3 : 6 1 2 - 6 1 3 . , A. Oksche, F. I. Kamemoto, J . R. King, and H. E. Cheyney. i 9 6 0 . A comparison of the effect of long d a i l y photoperiod on the pattern of energy storage i n migratory and non-migratory finches. Comp. Biochem. Physiol. 2 : 125-142. , and A. C. Wilson. 1 9 5 7 . A quantitative exam-ination of t e s t i c u l a r growth i n the White-Crowned Sparrow. B i o l . B u l l . 1 1 3 : 254-267. Forbush, E. H., and J. B. May. 1955. A Natural History of American Birds of Eastern and Central North America. Bramhall House, New York. Godfrey, W. E. 1 9 6 6 . The Birds of Canada. Queen's P r i n t e r , Ottawa. Gray, R. 1#97. Nesting of the robin Merula migratoria. Wilson B u l l . 9 : 2 9 . Hamilton, W. J . J r . 1 9 3 5 . Notes on nestling robins. Wilson B u l l . 4 7 : 1 0 9 - 1 1 1 . Howard, D. V. 1 9 6 7 . Variation i n breeding season and clutch-size of the robin i n northeastern United States and the Maritime provinces of Canada. Wilson B u l l . 7 9 : 432-440. Howe, R. H. J r . 1#9$. Breeding habits of the American Robin, (Merula migratoria) i n eastern Massachusettes. Auk 1 5 : 162-167. Howell, J. C. 1 9 4 0 . Spring roosts of the robin. Wilson B u l l . 5 2 : 1 9 - 2 3 . 1942 . Notes on the nesting habits of the Ameri-can Robin. Amer. Midland Nat. 2$ : 529-603-66 Kendeigh, S. C. 1942 . Analysis of losses i n the nesting of birds. J . W i l d l . Mgmt. 6 : 1 9 - 2 6 . Kress, S. W. 1 9 6 7 . A Robin nests i n winter. Wilson B u l l . 7 9 : 245-246. Lack, D. 1 9 4 3 . The L i f e of the Robin. London. 194$ . Notes on the ecology of the Robin. I b i s 9 0 : 252-279. 1 9 4 9 . Family size i n certain thrushes (Turdidae). Evolution 3: 57-66 . , and W. Light. 1 9 5 1 . Notes on the spring t e r r i t o r y of the Blackbird. B r i t . Birds 3 5 : 47-53-Menaker, M. i n press. Meyer, R. K., C. Kabat ? and I. 0 . Buss. 1947- Early involu-tionary changes i n the postovulatory f o l l i c l e s of the Ring-Necked Pheasant. J . W i l d l . Mgmt. 1 1 : 43-49-M i l l e r , A. H. i 9 6 0 . Adaptation of breeding schedule to l a t i -tude. Proc. X l l t h Int. Ornith. Congr. ( H e l s i n k i ) . 513-522. Myres, M. T. 1955- The breeding of Blackbird, Song Thrush and Mistl e Thrush i n Great B r i t a i n . Pt. I. Breeding seasons. Bird Study 2: 2-24-Nice, M. M. 1937- Studies i n the l i f e history of the Song Sparrow. I. Trans. Linn. Soc. N. Y. 4 : 1-247-Pr i c e , J . B. 1933- Winter behaviour of two semi-albino Western Robins. Condor 3 5 : 52-54-Romanoff, A. L. 1943- Growth of an avian ovum. Anat. Rec. $ 5 : 261-267 . Rowan, W. 1 9 2 6 . On photoperiodism, reproductive p e r i o d i c i t y , and the annual migrations of birds and certain f i s h e s . Proc. Boston Soc. Nat. History 3 $ : 147-189. 1 9 2 9 . Experiments i n bi r d migration. I. Manipulation of the reproductive cycle: seasonal h i s t o l o g i c a l changes i n the gonads. Proc. Boston Soc. Nat. History 3 9 : 151-208. Ryves, B. H. 1 9 4 3 . An investigation into the roles of males i n r e l a t i o n to incubation. B r i t . Birds 3 7 : 1 0 - 1 6 . 6 7 Schantz, ¥. E. 1 9 3 9 . A detailed study of a family of Robins. Wilson B u l l . 5 1 : 1 5 7 - 1 6 9 -Scott, D. M., and A. L. A. Middleton. 1968. The annual t e s t i -cular cycle of the Brown-Headed Cowbird (Molothrus  at e r ) . Can. J . Zool. 4 6 : 7 7 - 8 7 . Snow, D. W. 1 9 5 5 a . The breeding of the Blackbird, Song Thrush, and M i s t l e Thrush i n Great B r i t a i n . Pt. I I . Clutch-si z e . Bird Study 2: 7 2 - 8 4 . 1 9 5 5 b . The breeding of Blackbird, Song Thrush, and M i s t l e Thrush i n Great B r i t a i n . Pt. I I I . Nesting success. Bird Study 2: 1 6 9 - 1 7 8 . Venables, L. S. V., and U. M. Venables. 1 9 5 2 . The Blackbird i n Shetland. Ibis 9 4 : 636-653 . Wolfson, A. 1 9 4 2 . Regulation of spring migration i n juncos. Condor 4 4 : 2 3 7 - 2 6 3 . 1 9 5 3 . Gonadal and f a t response to a 5 :1 r a t i o of l i g h t to darkness i n the White-Throated Sparrow. Condor 5 5 : 1 8 7 - 1 9 2 . 1 9 5 9 . The role of l i g h t and darkness i n the regu-l a t i o n of spring migration and reproduction i n bird s , p. 6 7 9 - 7 1 6 . In R. B. Withrow (ed.) Phot operiodism. AAAS, Washington, D. C. Young, H. 1 9 5 5 . Breeding behaviour and nesting of the Eastern Robin. Amer. Midland Nat. 5 3 : 3 2 9 - 3 5 2 . 

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