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Some aspects of the economics of territoriality in North American hummingbirds Armstrong, Doug P. 1986

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SOME ASPECTS OF THE ECONOMICS OF TERRITORIALITY IN NORTH AMERICAN HUMMINGBIRDS by  DOUG P. ARMSTRONG B.Sc.  University  of Guelph, 1983  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard  THE UNIVERSITY OF BRITISH COLUMBIA February 1986 ©  Doug P. Armstrong, 1986  In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree a t the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make it  freely  a v a i l a b l e f o r r e f e r e n c e and study.  I further  agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying o f t h i s  thesis  f o r s c h o l a r l y purposes may be granted by t h e head o f my department o r by h i s o r her r e p r e s e n t a t i v e s . understood t h a t copying o r p u b l i c a t i o n o f t h i s for financial  gain  ZoOLOG-V  The U n i v e r s i t y o f B r i t i s h 1956 Main Mall Vancouver, Canada V6T 1Y3 Date  FEBROARV  thesis  s h a l l n o t be allowed without my  permission.  Department o f  It i s  2H  t  Columbia  written  i i  ABSTRACT  T e r r i t o r i a l i t y is  one  of  the  t e r r i t o r i a l i t y , formulation thesis  examples  and  played  two  feeding to  North  clearest  strategies  applicable  post-breeding  has  testing  follows  foraging based  and  of  of  are  an  theories  lines  of  used  territories? of  hummingbirds  energy-based important  on  feeding  role  in  the  t e r r i t o r i a l i t y .  inquiry.  by  t e r r i t o r i a l i t y  of  American  F i r s t ,  hummingbirds Second,  are  breeding  as  This  what  special  holding  energy-  energy-based well  as  models  non-breeding  hummingbirds?  for  I  use  a  simulation  a  non-breeding  visited  locations  suggest  that  visited  significantly bouts.  An  model  birds  in  each  patch of  a  avoiding  is  v i s i t e d .  nectar  patches of  a v a i l a b i l i t y  bout, of  they  flowers  empty  They  of  The  short-term  can  possible  benefit  foraging  they  the  hummingbird  might  benefit  that  examine  foraging.  birds  important  for  patterns  while  during  by  to  t e r r i t o r i a l  while  recently  model  a l l  therefore that  are  avoiding  recently  simulation  results  by  flowers  avoiding  would  not  benefit  on  previous  v i s i t e d avoidance  or  nearly  create easy  benefits  to  of  flowers  a l l  flowers  coarse-grained exploit  later  on. Results Calliope of  of  a  field  Hummingbirds  t e r r i t o r i a l i t y  males.  foraging  that on  the  males  nearby  on  indicate  cannot  Throughout  indicated  study  breeding that  account 2 month  a  solely  for  breeding  could  have  undefended  areas  t e r r i t o r i a l i t y  the  than  energy-based behavior  season,  obtained by  of  flower  energy foraging  of  male model these  sampling faster on  by  their  territories.  During June, flowers blooming on the t e r r i t o r i e s  were sparse and/or c o n t a i n e d very l i t t l e not have obtained enough energy  from them even to compensate f o r  the c o s t of hovering while f o r a g i n g . v i r t u a l l y a l l their time.  n e c t a r , and males c o u l d  Consequently,  f o r a g i n g away from t h e i r  they  did  t e r r i t o r i e s a t that  During May, when nectar a v a i l a b i l i t y on the t e r r i t o r i e s  was a t i t s peak, males d i d not l e a v e , territories  i n response  expand,  or  shift  their  to experimental e x c l o s u r e of a l l flowers  blooming on them. I  develop  promiscuously containing  a  model  optimal  breeding male that defends  no  energy  models  on the premise  that  which  a  size  breeding  T h i s model i s analogous  of energy-based  based  territory  that a male's  maximizes  his  away from h i s t e r r i t o r y .  that the s i z e  optimal  territory  is  immediate r e p r o d u c t i v e success.  A  An experiment  performed  s i z e w i l l be  while f o r a g i n g on a C a l l i o p e  d i d not support t h i s p r e d i c t i o n , and  intruders.  to  size  of h i s t e r r i t o r y may have been l i m i t e d  a b i l i t y to d e t e c t  design  f e e d i n g t e r r i t o r i a l i t y , but i s  by the r a t e at which he can o b t a i n energy  male's t e r r i t o r y  territory  in  p r e d i c t i o n of t h i s model i s that a male's t e r r i t o r y limited  for a  sources, as d i d the C a l l i o p e males d u r i n g  June at my study s i t e . existing  of  suggested  only by h i s  iv  T A B L E OF CONTENTS  ABSTRACT  i i  LIST  OF T A B L E S  vi  LIST  OF F I G U R E S  v i i  ACKNOWLEDGEMENTS Chapter  1  GENERAL Chapter  ix  INTRODUCTION  1  2  FORAGING STRATEGIES  FOR N E C T A R I V O R O U S  BIRDS  HOLDING FEEDING T E R R I T O R I E S  7  The M o d e l Flower  14 d i s t r i b u t i o n and  Hummingbird Foraging Simulation  nectar  production  16  energetics  18  tactics  19  Results  Adjusting  the  21  patch-leaving  The e f f e c t  of  nectar  The e f f e c t  of  territory  rule  production  21 characteristics  structure  32  Discussion Chapter  23  35  3  ECONOMICS OF B R E E D I N G T E R R I T O R I A L I T Y I N M A L E C A L L I O P E HUMMINGBIRDS  40  Study  44  Part  I.  Area Profitability  Methods  and  energy  production  of  territories  51 51  Results  57  Comparative  p r o f i t a b i l i t y of defended and undefended  areas  57  Territory  energy  production  and  metabolic  requirements of b i r d s Discussion  64 67  Part I I . Responses of males to experimental e x c l o s u r e of t e r r i t o r y  nectar sources  69  Methods  69  Results  70  Discussion  72  Chapter 4 WHAT DETERMINES THE SIZES OF BREEDING TERRITORIES HELD BY MALE CALLIOPE HUMMINGBIRDS?  78  The Model  80  Access to females  80  Cost of defense  81  Methods  83  Results  86  Discussion  90  Chapter 5 CONCLUDING REMARKS Literature Cited  96 100  vi  LIST OF TABLES  Table  3.1.  Flower  visitation  rates  by  male  Calliope  Hummingbirds on 4 p l a n t s p e c i e s Table 3 . 2 .  T o t a l number of flowers censused on  d u r i n g each week i n June  60 territories 61  LIST OF FIGURES  F i g u r e 2.1. Flowchart r e p r e s e n t a t i o n of the model Figure  2.2.  Energy  15  reserves of model b i r d s a t the end of  the day as a f u n c t i o n of  patch-leaving  rules,  and of  patch and flower memories Figure  2.3.  The  influence  22 of  nectar p r o d u c t i o n  r a t e on  f o r a g i n g e f f o r t of model b i r d s  25  F i g u r e 2.4. The e f f e c t of memory on the  ability  of  model  b i r d s t o accumulate e n e r g e t i c r e s e r v e s Figure  2.5.  production support Figure  The e f f e c t of temporal d i s t r i b u t i o n on the minimum t o t a l p r o d u c t i o n  of nectar  required  to  model b i r d s with no memory and p e r f e c t memory .. 29  2.6.  The  influence  foraging  effort  threshold  i n 4 hours  Figure  27  when  2.7. The e f f e c t  of  empty  nectar p r o d u c t i o n flowers  reach  r a t e on  a  volume  '  30  of a flower volume t h r e s h o l d on the  minimum nectar p r o d u c t i o n  r a t e r e q u i r e d t o support  model  b i r d s with no memory and p e r f e c t memory  31  F i g u r e 2.8. The r e l a t i v e u t i l i t y of patch memory and flower memory  34  F i g u r e 3.1. P o s i t i o n s of t e r r i t o r i e s 1 through 6 on 1  June  1985 Figure  45 3.2. Phenology of t e r r i t o r i a l i t y  i n the meadow, and  approximate timing of r e p r o d u c t i v e a c t i v i t i e s F i g u r e 3.3. Nectar volumes of flowers blooming on  47 and o f f  viii  territories Figure and  over the course  3.4. Concentration  of the breeding  of nectar  season  58  i n flowers blooming on  off territories  59  F i g u r e 3.5. Comparative p r o f i t a b i l i t y of  flowers  blooming  on and o f f t e r r i t o r i e s ~, F i g u r e  3.6.  Estimated  62 daily  energy  production  on  territories  65  F i g u r e 3.7. Estimated  p r o p o r t i o n of d a i l y  nectar  production  removed by i n s e c t s  66  F i g u r e 3.8. Changes i n p o s i t i o n s and s i z e s  of  territories  throughout May  71  F i g u r e 3.9. Changes i n time budgets of males i n response t o e x c l o s u r e of Ribes bushes on t h e i r Figure  4.1.  on t e r r i t o r y  F i g u r e 4.2. Comparison of t e r r i t o r y and  73  Design of experiment to examine the e f f e c t of  energy a v a i l a b i l i t y  did  territories  d i d not  have  access  size defense when  84 the  to an a r t i f i c i a l  male feeder  outside h i s t e r r i t o r y F i g u r e 4.3. Changes i n the behavior he had access  t o a feeder  87 of t e r r i t o r y  owner when 89  ix  ACKNOWLEDGEMENTS I wish to thank my  s u p e r v i s o r Lee  Gass  share of time, enthusiasm, and f i n a n c i a l 1/2  years.  Steve  Paton,  for  his  generous  support over the past 2  S t a f f a n Tamm, Glenn S u t h e r l a n d , and  Gayle Brown a l l c o n t r i b u t e d to making the V i v a r i u m a s u p p o r t i v e , g e n e r a l l y p l e a s a n t , and s u i t a b l y s i l l y G.,  Don Ludwig, Jamie Smith, and  draft  of  this  suggestions.  One  P.,  T.,  Staffan  thesis,  r e s e a r c h atmosphere.  Judy  and  Myers  offered  a  read  an  number  Glenn S.,  earlier  of  or more chapters were a l s o reviewed  L.  Doreen  Sieben p r o v i d e d l o g i s t i c  never  Lavigne, and Ron  support i n the f i e l d  have  Dave Brooks.  tolerance  Noakes,  the E.H.  University Anthony,  T h i s r e s e a r c h was of  B.  White,  i n t e r e s t i n g responses.  an NSERC  Postgraduate  Columbia  Graduate  Gass.  Keremeos.  S i and  i n the form  This  research  been conducted i f not f o r the enthusiasm f o r  R.  Guelph  by  Dave  and e s p e c i a l l y Tom  Nudds  possible  Black, and G. I wish to  by  Green to my  thank  them  for  F i n a n c i a l support was p r o v i d e d by  Scholarship,  Fellowship,  of  u l t i m a t e l y made  intrusions into their private l i v e s ; their  Pulliam.  And Peter Schumaker o f f e r e d s t a t i s t i c a l a d v i c e .  ecology imparted to me at  the  Steve  Peter Arcese, Lynn Carpenter, Steve  of c o o k i e s , lemonade, and r i d e s i n t o would  useful  by  Lima, Diana Tomback, David Paton, Graham Pyke, and Ron Don  Lee  a  University  of  British  and NSERC grant 67-9876 to  CL.  1  CHAPTER 1 GENERAL INTRODUCTION  Hummingbirds ( T r o c h i l i d a e ) are a d i v e r s e family of at 319  species  (Johnsgard 1983), a l l found i n North,  South America. use  of  morphological,  nectar  as an energy source,  physiological,  that  are  other  f a m i l i e s of b i r d s depend  sunbirds  interpreted  (Nectariniidae),  degree  of  Adaptations  and  behavioral  on  honeycreepers  feeding. floral  a s s o c i a t e d with nectar  of  hummingbirds to  to  nectar  bird-mediated  feeding,  the  feeding  (Grant  Grant  1968;  1978;  Kodric-Brown  1979;  1978;  Feinsinger  and Waser  d i v e r s i t y of both hummingbirds and  hummingbird/plant  communities  1976,1978a,1978b; F e i n s i n g e r and Brown  and of  affect  Bowers  bird/plant relationship.  plant/animal  1983).  Brown The  and  great  food p l a n t s i n t r o p i c a l  et  Colwell 1985).  aspects  of  i n t e r e s t i n the s t r u c t u r e of  (Wolf  hummingbirds many  Stiles  their  stimulated considerable  and  p o l l i n a t i o n , have made  coevolution  exploitation  and  do.  plants  characteristics  nectar:  However, none of these e x h i b i t  of  1985;  Three  (Drepanididae),  f o r the study  al.  extensive  by a s e r i e s of  t h i s r e l a t i o n s h i p a v a l u a b l e one  America has  and  characteristics  to nectar  extensively  specialization  that hummingbirds  ornithophilous  and  as adaptations  honeyeaters (Meliphagidae). same  Central,  These b i r d s are c h a r a c t e r i z e d by t h e i r  floral  least  that  al. 1978; As  1976;  Feinsinger  Feinsinger  well,  the  facilitate  et  unique nectar  of t h e i r b i o l o g y beyond  the  2  The small  most obvious  size.  Hummingbird slightly  c h a r a c t e r i s t i c of  Individual (Mellisuga  less  than  species  g,  g i g a s ) , which averages 20.2 median was  weight  5.0  of 191  g.  d e n s i t y , and  Bee  to the Giant Hummingbird  (Patagonia  (Brown  and  Bowers  1985).  The  small  size,  in  combination  with  l a r g e breast muscles, high muscle m i t o c h o n d r i a l  them  to  exploit  including  1960,  in f l i g h t .  1975),  Hovering,  in  a wide v a r i e t y of flowers that  would be u n a v a i l a b l e to b i r d s that perched birds,  Cuban  averages  s p e c i a l i z e d wing anatomy (Greenewalt  allows  other  the  their  probably  g  which  allows hummingbirds to hover motionless turn,  from  is  s p e c i e s l i s t e d by Brown and Bowers (1985)  Their  proportionately  range  helenae),  2  hummingbirds  while  foraging.  No  those of the three other n e c t a r i v o r o u s  f a m i l i e s , are capable of s u s t a i n e d h o v e r i n g . As a consequence of t h e i r metabolic  rates  that  are  small  higher  size, than  those  v e r t e b r a t e s except  f o r some shrews (Johnsgard  their  consumption,  oxygen  thermoregulation physiologists Dawson 1967;  energy  have been of p a r t i c u l a r  (Pearson  1950;  Calder and King  1983).  i n t e r e s t to 1963;  been  by  (Pearson  1954;  Lasiewski  1962;  Calder  Carpenter  and Hixon  1985;  Tooze  energy  have other  Therefore, and  comparative and  Similarly, environments  of i n t e r e s t , p a r t i c u l a r l y during p e r i o d s of e n e r g e t i c  s t r e s s induced  1978;  any  Lasiewski  Hainsworth 1974).  energy r e g u l a t i o n by hummingbirds i n t h e i r n a t u r a l has  of  utilization,  Lasiewski  1974;  hummingbirds  regulation  nesting,  is  partly  migration,  or  1975; and  inclement  weather  Hainsworth and Wolf Gass  1985).  This  achieved by p h y s i o l o g i c a l means.  3  However, the behavior of the b i r d s i s a l s o c r i t i c a l  in  insuring  that they do not s u f f e r e n e r g e t i c s h o r t f a l l . Hummingbirds'  behavior  i n f l u e n c e s both the r a t e s at which  they l o s e energy  through metabolic expenditures, and  at  acquire  which  they  energy  from  floral  the  nectar.  rates Energy  expenditures depend on the thermal m i c r o c l i m a t e s i n which  birds  reside,  rates  at  and  on the a c t i v i t i e s  which b i r d s  acquire  i n which they engage.  energy  depend  both  on  The  the  foraging  s t r a t e g i e s they employ, and on the d i s t r i b u t i o n and abundance of available  nectar.  Birds'  timing of f o r a g i n g bouts  foraging  (Hainsworth  strategies and Wolf  dictate  1983),  their  and  their  c h o i c e s as to which i n d i v i d u a l flowers and/or patches of flowers they  visit  (Pyke  Montgomerie  et  profitability  1978a,  al. of  1981;  1984;  birds'  Gass  and  Stephens  foraging  and  Montgomerie Paton  environments  1981;  1986). is  The  at  least  p a r t i a l l y a consequence of f a c t o r s beyond t h e i r c o n t r o l , such as the nectar p r o d u c t i o n r a t e s of influence  flowers.  Birds  can,  however,  t h i s p r o f i t a b i l i t y by defending nectar resources  from  competitors. There are at l e a s t  two  reasons  why  hummingbirds  should  sometimes  defend energy  sources.  F i r s t , because of t h e i r  metabolic  rates,  may  a  shortfall,  they  run  high  risk  of  rapid  energetic  so that they need constant access to energy  sources.  Second, because flowers are s t a t i o n a r y and conspicuous,  they are  e a s i e r to defend  than other types of food sources.  American s p e c i e s , t e r r i t o r i e s are defended the breeding season  (Stiles  1970,  and  Among  North  by a d u l t males d u r i n g  r e f e r e n c e s w i t h i n ) , and  by  4  males,  females,  and  p a r t i c u l a r l y during Stiles In  1970;  juveniles migration  outside  the  (Pitelka  breeding season,  1951;  Paton and Carpenter 1984,  Armitage  1955;  and r e f e r e n c e s w i t h i n ) .  almost a l l cases, these t e r r i t o r i e s c o n t a i n nectar  producing  flowers on which the owners feed. Territoriality  of  non-breeding hummingbirds i s one of the  c l e a r e s t examples of energy-based and Houston  1984).  feeding t e r r i t o r i a l i t y  The presence or absence of t e r r i t o r y defense  by non-breeding Anna's Hummingbirds (Calypte anna) at sites  can  be  (Ewald and  dictated  Carpenter  territories  by m a n i p u l a t i o n of energy  1978;  defended  production  of  Ewald  by  (Selasphorus r u f u s ) are energy  migrating  closely the  1980),  related  flowers  and Brown  Gass  1983).  Hixon  territoriality and  testing  al.  thesis  territoriality  1984,  follows  with  two  not  and  the  up  this  density  the  1978;  addressing  the  and  (Gass et a l . ^  Montgomerie  and  Consequently, hummingbird  work  l i n e s of i n q u i r y . for  of  to  the  formulation  (Schoener  on  1983;  1985).  hummingbird  Chapter 2 suggests  animals  holding  feeding  uses a s i m u l a t i o n model to p r e d i c t whether or  these s t r a t e g i e s should be used by t e r r i t o r i a l  Chapters 3 and 4  sizes  1986; Jones and Krummel  some s p e c i a l f o r a g i n g s t r a t e g i e s territories,  availability  Hummingbirds  has p l a y e d an important r o l e i n  1983; Lima  specified  Rufous  of recent models of t e r r i t o r i a l i t y  Hixon et a l . This  et  and  they c o n t a i n  1976; Gass 1979; Kodric-Brown 1981;  (Davies  report  the  results  of  an  hummingbirds.  empirical  study  issue of whether or not energy-based models are  a p p l i c a b l e t o breeding as w e l l as non-breeding t e r r i t o r i a l i t y  in  5  hummingbirds. The  model  postbreeding  in  Chapter  migratory  Because  birds  the  based  on  territories  of (Gass  i s used to evaluate the b e n e f i t s to  hummingbird of a v o i d i n g r e c e n t l y v i s i t e d effectively  i n t r u d e r s , t h e i r own of  is  Rufous Hummingbirds i n C a l i f o r n i a  and S u t h e r l a n d 1985), and territorial  2  defend  f o r a g i n g may  spatial variation  their  a  locations.  territories  against  account f o r a l a r g e p r o p o r t i o n  i n nectar a v a i l a b i l i t y among f l o w e r s .  The s i m u l a t i o n r e s u l t s suggest that while b i r d s might b e n e f i t by avoiding flowers v i s i t e d recently during a foraging would  not  as Armstrong  et a l .  Chapter  3  Columbia.  T h i s chapter i s c u r r e n t l y  examines  (Stellula  whether  an  energy-based  calliope)  in  territories  held  the  by  them  Pitelka  (Pitelka  1951;  1970,  1971).  hummingbird breeding success  birds  at is  documents an example of cannot  be  Throughout  accounted  and  However,  territoriality  season, of  Legg  North  differences  breeding American between  1956; W i l l i a m s o n  rigorous  time a  the  empirical  immediate  confounding  breeding for  Calliope  solely  by e n e r g e t i c  on non-  reproductive  factor.  territoriality  1956;  work.  has so f a r been c o n f i n e d t o the  which not  compared  male  possible  can  i n t e r i o r of B r i t i s h  have  hummingbirds, and have suggested  Stiles  press  model  of a group of breeding male  Several descriptive studies  non-breeding  in  (1986).  for t e r r i t o r i a l i t y  Hummingbirds  and  they  b e n e f i t s i g n i f i c a n t l y by a v o i d i n g patches of flowers  v i s i t e d on p r e v i o u s bouts.  account  bout,  that  Chapter 3 clearly  considerations.  the 2 month breeding season, f l o w e r s blooming on  the  6  territories areas. at  were l e s s p r o f i t a b l e than those on nearby undefended  In May,  i t s peak,  territories on those  when nectar a v a i l a b i l i t y  males  did  not  leave,  on the t e r r i t o r i e s  expand,  i n response to experimental  4  develops  success.  of  immediate  defended  suggests  that  the  optimal  sizes  of  with no  f o r a g i n g away from  experiment  using  one  territorial  hypothesis,  but d i d suggest  their  male  did  his  territory.  that access to energy  On  can  territories.  An  not support  this  artificial  the b a s i s of t h i s may  influence  of males i f i t a f f e c t s t h e i r  territories.  food  that the male became more s u c c e s s f u l  at e x p e l l i n g i n t r u d e r s when provided with an from  for  territories  by males are i n f l u e n c e d by the r a t e s at which they  o b t a i n energy while  hold  size  reproductive  T h i s model, which a p p l i e s to t e r r i t o r i e s  resources,  success  their  e x c l o s u r e of a l l flowers  a model of optimal t e r r i t o r y  breeding males based on maximization  suggest  shift  territories.  Chapter  away  or  was  the  feeder  observation, I reproductive  a b i l i t i e s to e s t a b l i s h  and  7  CHAPTER 2 FORAGING STRATEGIES FOR NECTARIVOROUS  BIRDS  HOLDING FEEDING TERRITORIES  If their if  food resources are foraging  they  are  effort to  requirements.  limited,  i n areas  meet  Because  r e s o u r c e s changes over  time,  spatial  of  distribution  term  single  animal's  The at  and  home  spatial  any  Caching  changes  time  may  their  is  distribution, animal  an o b v i o u s  Given locations  that  of food reflect  of t h e i r to  but  food  shift  the  include  also  include  effort  within a  i n an a n i m a l ' s that  observed  animal's  in  own  itself  significant  the s p a t i a l  may  hundred  seems i n t u i t i v e l y r e a s o n a b l e t h a t  local  rodents their  food.  When  depressions in of i t s food  history.  and n u t c r a c k e r s  seed caches  activity.  a l s o a l t e r the  distribution  i t s recent foraging  range  influencing  as the abundance, of a v a i l a b l e cause  home  granivorous  example o f a n i m a l s  seed c a c h i n g t i t s  of s e v e r a l  nutritional  Such s h i f t s  of f o r a g i n g  However, f o r a g i n g  f o o d a v a i l a b i l i t y , and t h u s reflect  available  range.  f o r a g e s , i t may  r e s o u r c e s may  i s most  need  foraging.  allocation  partially  as w e l l  may  concentrate  and  vast distances,  b e h a v i o r , such as that  birds,  food  distribution  animals  distribution  food d i s t r i b u t i o n s .  an  in  must  energetic  spatial  seasonal m i g r a t i o n s spanning short  i n which  their the  animals  can  remember  ( S h e t t l e w o r t h 1983), i t  at least  some a n i m a l s might  be  8  adept  at  remembering and a v o i d i n g f o r a g i n g l o c a t i o n s that they  have r e c e n t l y d e p l e t e d . could  remember  recently  spatiotemporal s c a l e are  As w e l l ,  i t i s possible  visited  1981).  the l a s t  patch,  s e v e r a l patches i t has v i s i t e d .  i t might  food (Green 1984).  A forager with  1981) patch  be  foraging i n a  A b i t r e f e r s t o a p o r t i o n of discrete  quantity  memory  on  sub-  l e s s than a whole b i t .  both  of  s a i d to have c o g n i t ive  these  spatiotemporal  maps (Menzel and Wyers  of both the patches i n i t s home range and the b i t s i n the in  which  i t i s currently  foraging.  would c o n s i s t of the f o r a g e r ' s a p r i o r i and Wolf 1979; Lima 1983) of the amount individual  bits,  and  of  the  These c o g n i t i v e maps  e x p e c t a t i o n s (Hainsworth of  depleted  or  nearly  other l o c a t i o n s . locations  If  depleted, i t does  food  available  q u a l i t y of whole patches.  forager expects that l o c a t i o n s i t has v i s i t e d  of  memory  A b i t , t h e r e f o r e , cannot be f u r t h e r  d i v i d e d , and a forager cannot v i s i t  could  long term  While  a patch that may or may not c o n t a i n a s i n g l e  scales  items  maintain a short term memory of which b i t s of  that patch i t has a l r e a d y v i s i t e d .  of  Food  i n patches w i t h i n an animal's home range,  and thus an animal might maintain a r e l a t i v e l y of  foragers  " l o c a t i o n s " on more than one  (Gass and Montgomerie  often d i s t r i b u t e d  that  recently  so,  i t s pattern  w i l l be systematic (Kamil 1978).  of  If a  will  i t should p r e f e r e n t i a l l y  in  be  visit  visiting  That i s , the number  b i t s r e v i s i t e d per patch w i l l be smaller and the average  i n t e r v a l between r e v i s i t s t o patches w i l l be longer  than  time would  be expected by chance. Although  systematic  visitation  may be a consequence  of a  9  forager  maintaining  accomplished within.and  a  without  cognitive such  a  map,  map.  harvester  about food a v a i l a b i l i t y  (Pyke  1978b).  An  i t s path,  and  l o c a t i o n s i n which food has visit. and  depleted  be  visitation  both  by a forager that had  i f i t were an  in  such  a  efficient  i s a forager way  that i t  t h e r e f o r e g e n e r a l l y encounters  been  replenished  The common c h a r a c t e r i s t i c shared  f o r a g e r s that maintain  also  e f f i c i e n t harvester  that sequences i t s f o r a g i n g movements crosses  could  Systematic  among patches c o u l d be achieved  no e x p e c t a t i o n s  rarely  it  since  i t s last  by e f f i c i e n t  harvesters  c o g n i t i v e maps i s that they can a v o i d  patches or b i t s of patches p r i o r to encountering  Consequently, I w i l l ,  f o r the sake of b r e v i t y , r e f e r  to  either  c o u l d not, by d e f i n i t i o n , v i s i t  a sub-  type of forager as having Because  a  forager  them.  "memory".  component of a b i t , any d i s c r i m i n a t i o n between d e p l e t e d and nondepleted b i t s of a patch must be done a constitute  " b i t memory".  a v o i d a patch a p r i o r i patch a p o s t e r i o r i A  forager  that  Ydenberg  frequently.  therefore  In c o n t r a s t , a forager c o u l d e i t h e r  ("patch memory"), or choose t o leave that  if itsinitial used  intake were  low.  only an a p o s t e r i o r i p a t c h - l e a v i n g  rule,  such as the g i v i n g - u p time r u l e 1982;  p r i o r i , • and  r a t e of food  (Krebs  et  a_l.  1974;  1984), might encounter r e c e n t l y v i s i t e d  However, i f i t s r u l e were e f f e c t i v e ,  McNair patches  i t might spend  only a short amount of time i n such patches before moving on new ones. even  Nevertheless,  i f i t d i d use  to  memory c o u l d be b e n e f i c i a l t o a forager an  memory c o u l d save time spent  e f f e c t i v e patch-leaving  rule.  t r a v e l i n g t o d e p l e t e d patches,  Patch and  10  time  spent  visiting  those patches. of the patch  empty or n e a r l y empty b i t s before l e a v i n g  B i t memory c o u l d save time spent  revisiting  bits  i n which i t i s c u r r e n t l y f o r a g i n g .  Whether or not a forager should use e i t h e r type  of  memory  depends on whether those b e n e f i t s outweigh any a s s o c i a t e d c o s t s . Regardless  of  whether  c o g n i t i v e map or by storage  and  memory  i s achieved  by  harvesting  efficiently,  i t will  processing  of  information.  maintaining a require  A long term c o s t of  memory would be i n v o l v e d i f t h i s storage and p r o c e s s i n g r e q u i r e d the animal for  t o maintain a d d i t i o n a l n e u r a l c a p a c i t y  other  a s p e c t s of i t s behavior.  of  simultaneously observant  other (Gass  functions 1985).  required  A short term cost would be  i n v o l v e d i f t h i s storage and p r o c e s s i n g reduced accuracy  not  the  the speed and/or  animal  must  perform  For example, i t might become l e s s  of p o t e n t i a l p r e d a t o r s  (Milinski  1984; Lawrence  1985)  or other a s p e c t s of i t s environment. It  is likely  that the magnitude of the b e n e f i t s of memory,  and t h e r e f o r e the l i k e l i h o o d that they w i l l will  exceed  the c o s t s ,  depend on the c h a r a c t e r i s t i c s of the p a r t i c u l a r animal.  expect  that the b e n e f i t s are most l i k e l y t o  be  significant  I if  the animal has the f o l l o w i n g c h a r a c t e r i s t i c s .  1.  I t s f o r a g i n g c r e a t e s l o c a l resource d e p r e s s i o n s . can happen i f the animal's in  relation  resources.  to  the  rate  This  r a t e of food intake i s high of  renewal  of  i t s food  11  2.  I t s foraging i s concentrated within a r e l a t i v e l y home  range,  so  that  there  is a  small  reasonably  high  p r o b a b i l i t y of r e t u r n i n g to a r e c e n t l y d e p l e t e d a r e a .  3.  I t maintains its  f a i r l y e x c l u s i v e use of food resources i n  home range, so that i t s own f o r a g i n g accounts f o r  a l a r g e p r o p o r t i o n of food removed.  4.  I t e x p l o i t s food resources that are Otherwise,  there  may  be  fairly  a constant  resources i n t o r e c e n t l y d e p l e t e d areas  immobile.  i n f l u x of these from  areas  of  higher abundance. The some  f i r s t three c h a r a c t e r i s t i c s may be shared, a t l e a s t t o  degree,  territories. the  by  a  variety  of  animals  that  However, a group of t e r r i t o r i a l  hold  feeding  f o r a g e r s that f i t s  above c h a r a c t e r i s t i c s p a r t i c u l a r l y w e l l i s that composed of  members  of  the  hummingbirds, Territories hummingbirds  four  families  sunbirds, of  both  (Gass  support  et_ a l .  (Gill  and  and  Wolf  1975)  1983) c o n t a i n j u s t  their t e r r i t o r i e s  enough  from  t h e i r energy from immobile f l o w e r s .  and hummingbirds  by honeycreepers  (Kamil  to  In a l l cases,  I t has a l r e a d y been  1977)  (Sutherland, i n prep.)  nectar  i n t r u d e r s , and  that r e v i s i t s t o i n f l o r e s c e n c e s by sunbirds  clusters  and  1976; Gass 1979; Kodric-Brown and  found  flower  birds:  honeyeaters.  the b i r d s ' d a i l y e n e r g e t i c requirements.  b i r d s v i g o r o u s l y defend obtain  nectarivorous  honeycreepers,  sunbirds  Brown 1978; Hixon et a l .  of  ( G i l l and Wolf  and  revisits  to  1978) occur l e s s o f t e n  12  than would be expected by chance.  I t i s n ' t c l e a r to what  t h i s i s a r e s u l t of memory, and t o what extent  i t i s a r e s u l t of  b i r d s making a p o s t e r i o r i d e c i s i o n s to leave depleted i n f l o r e s c e n c e s or flower c l u s t e r s . that  sunbirds  forage  at  However,  appeared t o be h a r v e s t i n g  different no  Gill  heights  evidence  of  on  and Wolf  patches of  (1977)  found  i n that they tended t o  successive  harvesting  extent  was  foraging found  bouts.  for either  honeycreepers or hummingbirds.  I know of no s t u d i e s  d i r e c t l y t e s t e d the p o s s i b i l i t y  that b i r d s a v o i d r e v i s i t s on any  spatiotemporal In  s c a l e by m a i n t a i n i n g  this  paper,  nature and extent hummingbird.  I  use  therefore  a s i m u l a t i o n model to p r e d i c t the  hope  to  for a  provide  of  n e c t a r i v o r e s or other  previously  visited  hummingbirds s p e c i f i c a l l y general  for several  territorial locations.  r a t h e r than  reasons.  territorial  and  Second,  hummingbirds  (Pitelka  .1980; Hixon e t a l .  birds.  1942; Gass e_t a l .  to  what  territorial foragers  in  f o r those reasons given  important  nectarivorous  or  modeled  above, memory c o u l d be p a r t i c u l a r l y other  theoretical  f o r a g e r s do show memory I  First,  territorial  a  background f o r f u t u r e s t u d i e s t h a t examine whether extent  have  a c o g n i t i v e map.  of the b e n e f i t s of memory  I  that  for  hummingbirds  previous  s t u d i e s of  1976; Gass 1979;  Hixon  1983; Paton and Carpenter 1984) have g r e a t l y  enhanced our understanding of feeding t e r r i t o r i a l i t y  i n general,  and  experimental  have  system. data  revealed  these b i r d s t o be a convenient  T h i r d , I c o u l d base my  that e x i s t  "relationships  assumptions  f o r hummingbirds. in  the  model  on  the  extensive  A l l parameters and f u n c t i o n a l  a r e e i t h e r based on l a b o r a t o r y or  13  f i e l d data f o r Rufous allometric species the  Hummingbirds  relationships  based  (Montgomerie 1979).  model  apply  to  a  In  the  assumptions may  specific  territorial a f f e c t my The  a  interpolated  number  of  system,  they  I  indicate  and  can  ways  f i t the c h a r a c t e r i s t i c s of  foragers,  from  hummingbird  be  or f u t u r e e m p i r i c a l work  discussion,  not  on  are  Fourth, because the p r e d i c t i o n s  compared with r e s u l t s of past system.  or  discuss  how  of  easily  on  that  in  which  my  other  types  of  these d e v i a t i o n s  might  predictions. predictions  of  the  model  relate  to  two  possible  day  a hummingbird  b e n e f i t s of memory:  1.  A decrease i n the amount of time per must  forage  in  order  to  meet  its  energetic  requi rements.  2.  A  decrease  hummingbird's  in  the  minimum  territory  amount  must  of  nectar  a  produce f o r i t to meet  those requirements.  I examine the b e n e f i t s of both patch memory and where  flowers  information costs  of  correspond  to  bits  to assess e i t h e r the maintaining  short  memory,  and  compare the magnitudes of c o s t s and make  qualitative  the b e n e f i t s w i l l  predictions be h i g h e s t ,  in  the  term  model. or  therefore benefits.  flower  intuitive  I have no  the  long  I cannot I  do,  as to the c o n d i t i o n s and  memory,  term  directly however,  under which  predictions  as  to  14  whether these b e n e f i t s a r e l i k e l y  The The  model simulates  to exceed the c o s t s .  Model  a hummingbird f o r a g i n g i n i t s t e r r i t o r y  over the course of a s i n g l e day ( F i g . of  2.1). I assume that  the day are a v a i l a b l e f o r f o r a g i n g , and t h a t the r e s t of the  time i s spent s l e e p i n g . reserves.  Each b i r d  At  dawn,  initiates  model  its first  from a c e n t r a l l y l o c a t e d perch t o any territory.  B i r d s remove a l l nectar  birds  have  decisions  made at f l o w e r s ) .  of  the  patches  from each flower  Those b i r d s with  the  patch it  same a p o s t e r i o r i p a t c h - l e a v i n g  i s depleted;  flies  to  i f a b i r d decides  a new patch.  in i t s  they  visit  discussion  flower  are able t o a v o i d r e v i s i t i n g at l e a s t some f l o w e r s . use  no f a t  feeding bout by f l y i n g  (but see Pyke 1978a; Gass and Montgomerie 1981 f o r a of  14 h  memory  A l l birds  r u l e to determine i f a  that a patch  is  depleted,  B i r d s with patch memory are able to  a v o i d r e v i s i t i n g at l e a s t some patches. Model adjusting  birds the  regulate  length  Bouts always terminate  their  in  perch,  intake.  f a t storage  when 40 u l of nectar have been  prep.).  energy r e q u i r e d t o harvest energy  of  of time they r e s t between feeding  T h i s volume per bout i s t y p i c a l (Sutherland,  rates  f o r c a p t i v e Rufous  However, bouts vary that n e c t a r ,  and  harvested.  Hummingbirds  therefore  i n net  At the end of each bout, a b i r d r e t u r n s t o i t s  and uses at l e a s t  perch  bouts.  i n the amount of  part  of  i t s net  energy  produce f a t which i t s t o r e s f o r the coming n i g h t . its  by  intake  to  I t remains on  u n t i l any remaining energy i s used up by i t s r e s t i n g  • S e l e c t patch • Fly t o patch yes Fly back to  • Select  s a m e patch  • Fly  flower  Patch depleted?  to f l o w e r  Drink all n e c t a r in f l o w e r no <^  4 0 ul h a r v e s t e d y e t ? y>—  yes • Fly to perch • Store  fat  • Perch  until  energy  remaining  from  bout burned no  F i g u r e 2.1.  -<(  14 h y e t ?  last off  ^-  yes  Flowchart r e p r e s e n t a t i o n of the model.  16  metabolism, and then i n i t i a t e s a new f e e d i n g bout by f l y i n g to the patch i n which  i t terminated the l a s t  bout.  If they can, model b i r d s s t o r e energy at a the  amount  but  not  rate  at  exceed  their  whether  or  expected  1981).  nocturnal  requirements  real  (see  One measure of a model b i r d ' s success  not i t i s able t o meet t h i s energy requirement.  Those that cannot do so w i l l have n e g a t i v e 24 h energy A  which  of f a t accumulated a t the end of the day w i l l meet,  Hainsworth e_t a l . is  back  budgets.  hummingbird that c o u l d not meet i t s energy requirements  over a number of days would need to expand  i t s territory,  find a  new l o c a t i o n , or somehow reduce i t s energy e x p e n d i t u r e .  Another  measure of success i s the amount of time that model b i r d s foraging  over  the  course  of the day.  spend  Among model b i r d s that  s a t i s f y t h e i r energy requirements, the most s u c c e s s f u l are those that minimize the amount success  of  time  they  spend  foraging.  The  a model b i r d depends on the amount and d i s t r i b u t i o n  of nectar i n i t s t e r r i t o r y foraging  of  and  on  the  effectiveness  of i t s  tactics.  Flower d i s t r i b u t i o n and nectar p r o d u c t i o n I based model t e r r i t o r i e s on those h e l d by m i g r a t i n g Rufous Hummingbirds  in  the  d u r i n g J u l y and August Sutherland  1985).  (Aquilegia  formosa)  producing  spurs.  and t e r r i t o r i e s  G r i z z l y Lake area of northwest (Gass e t a l .  California  1976; Gass 1979;  Gass  and  These t e r r i t o r i e s averaged 239 red columbine flowers,  each  of  which  Flowers were d i s t r i b u t e d  averaged  406  m  2  in  has  5  nectar  i n d i s c r e t e patches,  area.  I  use  a  model  17  territory  of  900  flowers.  number of columbine flexibility patches.  spurs  Flowers of  can few  be  a  real  territory,  among  "coarse-grained"  In a l l cases, t e r r i t o r i e s are square  d i s t a n c e between patches Regardless  allows  or " f i n e - g r a i n e d " t e r r i t o r i e s  are the i n d i v i d u a l patches.  patches.  and  i n t o d i f f e r e n t numbers of  distributed  l a r g e patches  of many small patches. as  in  i n d i v i d i n g the t e r r i t o r y  territories  shape  T h i s number both approximates the  and  of  the  I can a l s o vary both  density  of  in the  flowers  within  flower spacing, I assume that  patches  are d i s c r e t e and d i s t i n c t to the hummingbird. To examine how total  nectar  model b i r d s '  foraging t a c t i c s  p r o d u c t i o n r e q u i r e d to support  the nectar p r o d u c t i o n r a t e per flower r a t h e r the  number of flowers per t e r r i t o r y .  influence  them, I then  the  manipulate  manipulating  I assume that a l l flowers  produce nectar at the same rate (but see F e i n s i n g e r 1978a, for-  a  discussion  of  variability  i n nectar p r o d u c t i o n ) .  model begins at dusk, at which time a l l flowers are total  nectar  production  r e q u i r e d to support later.  or  below  the  a model b i r d , flowers are a l s o  when  nectar  r e q u i r e d to support at  without also  at  that  minimum l e v e l empty  24  h  their  at  the minimum l e v e l  them, the model c o u l d  be  run  p r o d u c t i o n l e v e l without  the  If  is  birds f a i l i n g examine  production  The  empty.  Because model b i r d s consume a l l nectar produced on  territories  days  is  1983  c h a r a c t e r i s t i c s of nectar p r o d u c t i o n :  of  several  nectar accumulating  to meet t h e i r e n e r g e t i c consequences  for  requirements.  varying  two  and I  other  18  1.  The  proportion  higher  of  nectar  nocturnal  produced  nocturnally.  A  p r o d u c t i o n r a t e r e s u l t s i n a higher  s t a n d i n g crop at dawn.  2.  The  effect  of  production decrease  nectar  rates.  standing  CL.  observations). production  Gass,  R.D.  I simulate  at  a  as  this  by  threshold  typical  nectar  columbine, fill  (CA.  unpublished  stopping  nectar  accumulated  sucrose  volume  time).  which  is  b i r d s have i d e n t i c a l e n e r g e t i c requirements.  I  f o r columbine  35%  they  Montgomerie,  ( e q u i v a l e n t to a t h r e s h o l d f i l l i n g  I assume that a l l nectar i s  on  Some flowers, i n c l u d i n g  their production rates  Redsell,  crop  by  weight  (Gass 1974).  Hummingbird e n e r g e t i c s All  model  assume that b i r d s have a constant body weight of 3.5 temperature i s a constant  15°C,  and  that the a l t i t u d e  Using these v a l u e s , wing morphology Rufous  Hummingbirds  equations, and W,  (Tooze  measurements  1984),  and  respectively. + 0.11d  distance  Duration of forward  (m)  +  0.25  perching, and  f l i g h t s i s given  by  0.12v,  W,  (1979)  W,  i n the model a r r a y .  each flower i s given by t = 1.22  captive  Montgomerie's  0.5  (Gass 1974), where t i s time  flown  i s 2400 m.  from  I estimated that h o v e r i n g , f l y i n g forward,  s l e e p i n g would c o s t 1.0 Watts (W),  0.80  g, that the  0.125 t  =  (s) and d i s E u c l i d e a n Time spent hovering at where  v  is  nectar  19  volume to  (ul).  r e l a t i o n s h i p approximates  handle a columbine  intercept and or  This  to  is  insert  nearly  and  and  empty  energy  assume t h a t  4.5  torpid.  energy  they  efficient.  conversion Therefore,  intake during  feeding  its  energy  nocturnal of  intake  perches before 0.089, t h e  Foraging  the  bird  wide within  of  energy  100%  hovering  time  are  from  14  a b i r d ' s net  that  i t s time  that  the  rate  can  fat  not h  their forage.  is  100%  r a t e of  0.089 W,  amount by  do  I  n e u t r a l 24  so  to  long,  night.  and  h they  sugar  10 h  the  to maintain  f o r the  The  If  of  empty  sleeping,  are  l o n g as  requirement.  spends  nights  enough n e c t a r  0.089 W  bout.  of  itself, to  f a t to l a s t  bouts always exceeds  next  visits  The  energy  it will  meet  which a  bird's  l e n g t h of  time i t  does  not  exceed  foraging.  tactics  patch-leaving  patch.  W and  i f they  as  s  e x c e e d s 0.089 W d e t e r m i n e s  Patch-leaving  patch  0.125  whole n i g h t  of  1985).  costs.  must h a r v e s t  averages  1.22  required  to p o s i t i o n  Therefore,  least  worth  Therefore,  gain  I assume t h a t  rate  kJ  b i r d s spend t h e  energy budgets, net  c o s t at  time  Sutherland  for a bird  its bill.  sleeping costs  need  and  required  withdraw  flowers  that  birds  become  time  .(Gass  in a d d i t i o n to t r a v e l  Given model  the  spur  the  on  the  I  felt  rule.  rule.  That  basis that  of  All  model b i r d s use  i s , they their  t h i s was  a  decide nectar  when intake  reasonable  when  to  leave  a  to  posteriori leave-  rate within  assumption  v a r i e t y of a n i m a l s have been shown t o use patches to decide  an  them  their (Krebs  each that  since  a  experience et  al.  20  1983).  I  nectar  assume that a b i r d w i l l  intake  threshold.  in  Each  that  patch  leave a patch i f i t s r a t e of  drops  b i r d monitors  below  Cowie  (1977)  givinq-up  i t s r a t e of intake by averaging  the nectar volumes i t has obtained from visited.  some  referred  the  to  last  few  flowers  t h i s type of sample as a  "memory window"; however, I w i l l use the term  sampling  window to  a v o i d c o n f u s i o n with patch memory and flower  memory.  Whenever  the average another  f a l l s below the g i v i n g - u p t h r e s h o l d , a b i r d moves to  patch.  threshold  For s i m p l i c i t y ,  I assume that both the g i v i n g - u p  and the s i z e of the sampling  window a r e constant  over  the course of the day. Patch memory and flower memory. benefits  of  memory,  I wished  regardless  of  to  examine  whether  i t involved  m a i n t a i n i n g a c o g n i t i v e map or e f f i c i e n t h a r v e s t i n g . complication  of  foraging  route  (Anderson  1983).  without  altering  with patch indicate  those  recently. from  and  the may  latter also  mechanism  influence  i s that  flower  memory  individual  However, a  an  model  birds'  memories  T h e r e f o r e , i n the model, b i r d s maintain  locations  cognitive  they  have  maps  excluding  they can remember v i s i t i n g .  Perfect  patch memory equals one l e s s than the number of patches  i n the  territory.  Therefore,  patches  in the same  sequence  memory  those  that  v i s i t e d most  They s e l e c t patches and flowers at random,  consideration  animal's  i t s overall travel costs  I wished to manipulate travel costs.  the  b i r d s with p e r f e c t memory v i s i t repeatedly.  Similarly,  perfect  flower  equals one l e s s than the number of flowers per patch.  I  assume that b i r d s r e t a i n t h e i r memory of flowers i n a patch only  21  until a  they v i s i t  new  which  another patch.  feeding  in  initiates  bout a f t e r p e r c h i n g , i t r e t u r n s t o t h e p a t c h i n  i t ended t h e l a s t bout  flowers  However, when a b i r d  that  patch.  and  retains  Because  i t s memory  of  the  m o d e l b i r d s do n o t c h o o s e  new  patches or flowers with respect t o t h e i r current l o c a t i o n s , average  travel  time between p a t c h e s and between f l o w e r s w i t h i n  p a t c h e s i s n o t a f f e c t e d by c h a n g e s benefits flying  of  memory  are  that  in  tactics.  The  t h e y save time and energy  spent  t o d e p r e s s e d p a t c h e s and f l y i n g  nearly  empty  the  flowers.  I  explore  foraging  t o a n d h a n d l i n g empty these b e n e f i t s  or  i n the next  section.  Simulation Results  Adjusting the patch-leaving rule The  o p t i m a l c o m b i n a t i o n of g i v i n g - u p t h r e s h o l d and s a m p l i n g  window d e p e n d s on n e c t a r a v a i l a b i l i t y ,  s i z e of p a t c h e s , and  model b i r d ' s p a t c h a n d f l o w e r memory.  Therefore, I d i d not f e e l  that  I  could  directly  compare t h e s u c c e s s o f m o d e l b i r d s  d i f f e r e n t memory c a p a c i t i e s u n d e r used  a  set  combination.  a variety  I n s t e a d , f o r each s i t u a t i o n ,  t h e o p t i m a l p a t c h - l e a v i n g r u l e by i t e r a t i n g model,  changing  the  s a m p l i n g window by respectively In nocturnal  0.2  giving-up  cases  in  and  with i f I  I found  through runs of  threshold  ul/flower  ( f o r an e x a m p l e ,  those  of c o n d i t i o n s  the  the  and t h e s i z e of t h e  one  flower  increments  see F i g . 2 . 2 ) .  which  f a t r e q u i r e m e n t o f 4.5  birds  could  k J by t h e e n d  accumulate of  the  the  day,  I  22  NO  PATCH  PERFECT  MEMORY  PATCH  MEMORY  Size of Sampling Window (flowers) Stored Fat  -2.0  0.0  2.0  40  (kJ)  Figure 2.2. M o d e l b i r d s ' e n e r g y r e s e r v e s a t t h e end o f t h e day a s a f u n c t i o n o f t h e i r p a t c h - l e a v i n g r u l e s , and o f t h e i r p a t c h and f l o w e r m e m o r i e s . The maximum amount o f f a t s t o r e d is 4.5 kJ. I f t h e a v e r a g e n e c t a r volume i n t h e l a s t few f l o w e r s a b i r d h a s v i s i t e d ( t h e s a m p l i n g window, o r "memory window") i s l e s s t h a n t h e g i v i n g - u p t h r e s h o l d , i t moves t o a new p a t c h . In t h i s p a r t i c u l a r example, e a c h f l o w e r p r o d u c e s 4.3 u l / d a y a t a c o n s t a n t r a t e , and h a s no volume t h r e s h o l d . A l l flowers i n the t e r r i t o r y a r e r e g u l a r l y spaced 1 m a p a r t . Similar results are obtained for a variety of other condit ions.  23  chose over  the p a t c h - l e a v i n g r u l e that minimized time spent the course  meet  the  of the day (to w i t h i n 2%).  n o c t u r n a l requirement,  that maximized the amount of kJ/day). no  not  I chose the p a t c h - l e a v i n g  rule  f a t accumulated  flower.  encountering  In the next  window in  intermediate  one  of  1  different  was  However,  of  produced  patch-leaving  or  nearly  Therefore,  I use  a  flower and a g i v i n g - u p t h r e s h o l d of 0.4  a l l cases. degrees  empty  s e c t i o n , I compare only the success  either  if  type  l e a v i n g r u l e s sometimes worked b e t t e r . section  0.2  was that i f b i r d s had e i t h e r  of b i r d s with no memory and p e r f e c t memory.  ul/flower  (to w i t h i n  the best p a t c h - l e a v i n g r u l e under a l l c o n d i t i o n s  was t o leave a patch a f t e r  sampling  they  of p r e v i o u s l y v i s i t e d patches or f l o w e r s , or p e r f e c t  memory of both,  empty  could  One g e n e r a l i t y I observed  memory  If  foraging  with  birds  of memory, other  had patch-  F i g u r e 2.8 i n the  simulations  rules,  model  each  using of  a  which  third  number  of  maximized  accumulated f a t .  The  e f f e c t of nectar p r o d u c t i o n  characteristics  For a l l s i m u l a t i o n s i n t h i s s e c t i o n , t e r r i t o r i e s c o n s i s t of 25  patches of 36 flowers each, and flowers a r e r e g u l a r l y spaced  1 m apart over flowers continued is  the  had  the e n t i r e t e r r i t o r y . no  threshold  I initially  accumulated  volume,  assumed  that  and that  they  t o accumulate nectar u n t i l emptied by the b i r d . simplest case because d a i l y t e r r i t o r y nectar  i s not a f f e c t e d by whether or not the b i r d s keep the t h r e s h o l d volume.  This  production  flowers  under  24  I  observed  that  i f daily  t e r r i t o r y nectar p r o d u c t i o n was  sufficiently  low that model b i r d s spent a  their  foraging,  time  requirements production  then  at the end of was  below  they the  large  d i d not  day  proportion  meet  their  energy  If  nectar  ( F i g . 2.3).  some minimum l e v e l , b i r d s d e p l e t e d t h e i r  t e r r i t o r i e s before the end of the day, and thus c o u l d energy  regardless  of  how  much  birds  could  accumulate  spent more than about of  the day.  nocturnal  energy  their  25% of t h e i r  Therefore,  among  requirement,  not  gain  time they spent f o r a g i n g . I f  t e r r i t o r y nectar p r o d u c t i o n was even at which  of  the  minimum  level  4.5 kJ of f a t , they  at  never  time f o r a g i n g over the course  birds  that  could  meet  their  there was never a l a r g e range of  d i f f e r e n c e i n time spent f o r a g i n g .  I compared  the  time  spent  f o r a g i n g by a b i r d with no memory and one with p e r f e c t memory at the  minimum  t o t a l nectar p r o d u c t i o n l e v e l a t which a b i r d  no memory c o u l d meet  i t s nocturnal  requirement  with  ( F i g . 2.3).  Regardless of r e l a t i v e day and n i g h t p r o d u c t i o n r a t e s , the t o t a l number of v i s i t s t o flowers over the course of the day was about 6000  for birds  with no memory and 1500 f o r b i r d s with p e r f e c t  memory, and the a b s o l u t e d i f f e r e n c e  i n time spent  foraging  was  10-15%. The production  above  comparison  assumes  that  territory  i s s u f f i c i e n t l y h i g h that a b i r d with no  meet i t s energy  requirements.  nectar  memory  can  The more pronounced advantage f o r  b i r d s with p e r f e c t memory was that the minimum n e c t a r p r o d u c t i o n level  at  which  f a t was lower  they c o u l d accumulate  than  for birds  with  their no  r e q u i r e d 4.5 kJ of  memory  ( F i g . 2.4).  F i g u r e 2.3. The i n f l u e n c e of nectar production r a t e on foraging e f f o r t when (a) n o c t u r n a l nectar production rate = 5x daytime production r a t e , (b) night r a t e = day r a t e , and (c) n i g h t r a t e = 20% day r a t e . In a l l cases, flowers are empty at the previous dusk and have no volume t h r e s h o l d . The proportion of the t o t a l nectar produced that i s a v a i l a b l e at dawn i s 78%, 42%, and 13% f o r ( a ) , ( b ) , and (c) respectively. Numbers adjacent to curves i n d i c a t e the t o t a l nectar produced per flower per day ( u l ) . Solid l i n e s and closed c i r c l e s i n d i c a t e model b i r d s with no patch memory or flower memory, whereas broken lines and open circles indicate b i r d s with p e r f e c t memory of both. For a l l l i n e s above the shaded area, b i r d s with no memory are unable to meet t h e i r nocturnal energy requirement. The shaded area i n d i c a t e s the d i f f e r e n c e i n time spent f o r a g i n g among b i r d s with different memory c a p a c i t i e s at the minimum nectar production l e v e l at which a b i r d with no memory can meet i t s n o c t u r n a l energy requirement. In a l l cases, the t e r r i t o r y has 25 patches of 36 flowers each, and flowers are r e g u l a r l y spaced 1 m a p a r t .  -I  2  1—  4  1  1  1  6  8  10  TIME OF DAY ( h )  -I—  12  27  Figure 2.4. The e f f e c t of memory on a b i l i t y to accumulate e n e r g e t i c r e s e r v e s , under three nectar production regimes. The regimes shown a r e the same as f o r F i g 2.3. The shaded area i n d i c a t e s the range of t o t a l nectar p r o d u c t i o n at which a b i r d with p e r f e c t memory can s u r v i v e the n i g h t , but a b i r d with no memory cannot. T e r r i t o r y s t r u c t u r e as i n F i g . 2.3.  28  Therefore,  a b i r d with p e r f e c t memory c o u l d hold a s m a l l e r ,  productive  territory.  production nectar  The r e l a t i v e magnitude of day and night  r a t e s had two i n f l u e n c e s .  production  nectar  (Fig.  production  nectar p r o d u c t i o n resulted 2.4c).  First,  2.4,  was  Fig.  diurnal,  2.5).  even  a  i n a p r e c i p i t o u s drop i n d a i l y  When  I  simulations,  placed of  perfect  memory  I assumed that nectar I f flowers  meet  i t s nocturnal  production  r e q u i r e d t o support that  increased.  a  required  filled  and  those  with  no  proportionate  difference  bird  between  birds  no  memory  the  absolute  with  perfect  was 22% ( F i g . As  2.6). T h i s well,  i n the minimum nectar production  one with p e r f e c t memory i n c r e a s e d  no t h r e s h o l d volume t o 30% when flowers suggests  with  requirement,  that was r e q u i r e d t o support  This  these  i n 4 h, and nectar was  a  memory  For  r a t e was i d e n t i c a l  compares t o a d i f f e r e n c e of 12% i n F i g u r e 2.3b.  2.7).  survival  f a t accumulation ( F i g .  production  energy  d i f f e r e n c e i n time spent f o r a g i n g  and  h  a t h r e s h o l d accumulated volume on f l o w e r s ,  day and n i g h t .  flower  most  s l i g h t drop i n t o t a l  15% l e s s than  produced a t the minimum r a t e at which  per  crop at  a b i r d with no memory ( F i g . 2.5).  benefits  memory  total  when  However, r e g a r d l e s s of r e l a t i v e day and night  to .support  could  Second,  below the minimum r e q u i r e d f o r 24  b i r d with p e r f e c t memory was about  during  minimum  (and t h e r e f o r e the standing  r a t e s , the minimum t o t a l nectar p r o d u c t i o n  the  the  r e q u i r e d by b i r d s decreased as the p r o p o r t i o n  of nectar produced overnight dawn) i n c r e a s e d  less  that i f flowers  a bird  with  no  the rate  memory  from 15% when flowers had filled fill  in  4  h (Fig.  i n 4 h, a b i r d  with  29  Figure 2.5. The e f f e c t of temporal d i s t r i b u t i o n of nectar production on the minimum total production required to s u p p o r t a b i r d w i t h no memory (solid line) and one with perfect memory (broken line). T e r r i t o r y s t r u c t u r e as in Fig. 2.3.  30  2  A  6  TIME  OF  8  10  12  DAY ( h )  F i g u r e 2.6. The i n f l u e n c e of nectar production rate on foraging e f f o r t when empty flowers reach a volume t h r e s h o l d in 4 hours. Numbers adjacent to curves i n d i c a t e nectar production r a t e s (ul/h) while flowers are producing. These r a t e s are s i m i l a r d u r i n g night and day. Because flowers are not always producing, the d a i l y average r a t e s w i l l be l e s s than these v a l u e s . The shaded area i n d i c a t e s the d i f f e r e n c e in time spent f o r a g i n g among b i r d s with d i f f e r e n t memory c a p a c i t i e s at the minimum nectar production level required to support a b i r d with no memory. Compare with F i g . 2.3b. Interpretation of l i n e s and symbols as i n F i g . 2.3. T e r r i t o r y s t r u c t u r e as i n F i g . 2.3.  31  O  LLl  '  z  4  1  1  1  —  8 12 16 FLOWER FILLING TIME ( h )  i  -  20  Figure 2.7. The e f f e c t o f a f l o w e r v o l u m e t h r e s h o l d on the minimum n e c t a r p r o d u c t i o n rate (see F i g . 6) required to support a bird with no memory (solid line) a n d one with p e r f e c t memory (broken l i n e ) . Territory structure as in Fig. 2.3.  32  p e r f e c t memory c o u l d s u r v i v e with a t e r r i t o r y one  with  they  no  memory.  contained  less  30%  smaller  than  Because flowers produced nectar only i f than  the  volume  threshold,  the  total  produced per day depended not only on the nectar p r o d u c t i o n when  flowers were producing,  kept  under that t h r e s h o l d .  more  flowers  territories  below  the  produced  rate  but a l s o on how many flowers b i r d s  B i r d s with p e r f e c t memory c o u l d keep threshold  more  nectar  volume,  and  thus  their  than those of b i r d s with no  memory.  The  e f f e c t of t e r r i t o r y s t r u c t u r e The p r e v i o u s  birds  with  perfect  patches have over section,  I  s e c t i o n i l l u s t r a t e s the advantages that memory  those with  compare  the  of both patches and flowers w i t h i n no  memory  number  and  size  of  volume  threshold  either.  In  this  r e l a t i v e b e n e f i t s depend  patches,  patches and between flowers w i t h i n For s i m p l i c i t y ,  of  r e l a t i v e b e n e f i t s of patch and flower  memory, and determine whether these the  model  on  or on the d i s t a n c e between patches.  I assumed that flowers had  no  accumulated  and that flowers produced nectar a t i d e n t i c a l  constant  r a t e s d u r i n g day and n i g h t .  For  a l l simulations,  used  nectar  4.3  ul/flower.day,  a  corresponds  production  to  22.9  rate  of  kJ/territory.day.  In  nature,  rate  Montgomerie 1981; Montgomerie and Gass 1981). because  energetically  birds during  with the  no  memory  day,  but  could could  just  which  feeding  t e r r i t o r i e s of Rufous Hummingbirds average about 25 kJ/day and  I  (Gass  I chose t h i s break  even  not accumulate any  33  reserves  to l a s t  utility  of  varied  territories  2.4b).  number  all  the  and  flowers  I c o u l d vary the the  than  The  patch  relative  I  utility  size  were  patch memory i n  of  patches  spatial  g r a i n of  of the f l o w e r s .  2.8c),  was  could  when  However, even  patches of only 9 flowers  each  b i r d s were able to forage q u i t e e f f e c t i v e l y  with  On  not  to  greatest  no patch memory by a v o i d i n g only 8 p r e v i o u s l y v i s i t e d each patch.  In  2.8a,b,c; see a l s o F i g .  t e r r i t o r i e s were d i v i d e d i n t o many small patches.  (Fig.  territories  much more b e n e f i c i a l  (Fig.  100  in  r e g u l a r l y spaced 1 m  of patch memory  when t e r r i t o r i e s c o n s i s t e d of  the  day.  d i s t a n c e between any  memory  evaluated  in flower and  short term flower memory was  birds  2.2).  increases  the  which  changing  a l l cases, the  in  That way,  without  (Fig.  f a t accumulated d u r i n g  first  apart.  night  incremental  terms of t o t a l I  the  the other hand,  even  break  even  birds  with  no  flowers i n  flower  memory  e n e r g e t i c a l l y r e g a r d l e s s of  their  patch memory. I i n c r e a s e d the d i s t a n c e between patches and distance  between  increase  the  territory  flowers  relative  with  25  within  utility  patches  of  d i s t a n c e between patch c e n t e r s ,  decreased  patches to see of  patch  i f this  memory.  900  to m  2  0.33  m.  a  36 flowers each, I t r i p l e d  the  from 6 m to 18  m,  T h i s a l s o i n c r e a s e d the t o t a l area  to 5476 m  observed  2  in  two  Kodric-Brown and  which i s n e a r l y twice long-term Brown 1978;  would  Using  and  reduced from 1  the d i s t a n c e between flowers w i t h i n patches to o n e - t h i r d , m  the  field  the t e r r i t o r y  studies  Gass 1979).  6 - f o l d , from  The  area  ever  (Gass et a l .  1976;  resulting  pattern  34  F i g u r e 2.8. The r e l a t i v e u t i l i t y of patch memory and flower memory when t e r r i t o r i e s have (a) 4 patches of 225 f l o w e r s , (b) 25 patches of 36 f l o w e r s , (c) 100 patches of 9 flowers (all flowers spaced 1 m apart i n each c a s e ) , and (d) 25 patches of 36 flowers (0.33 m between flowers within patches, 18 m between patch c e n t e r s ) . Each flower produces 4.3 ul/day at a s i m i l a r r a t e d u r i n g day and n i g h t , and has no volume t h r e s h o l d . In a l l cases, short term flower memory i s much more b e n e f i c i a l t o model b i r d s than i s patch memory.  35  of  f a t accumulation  (Fig. the  2.8d) i s very benefits  distance  of  as  a f u n c t i o n of patch and flower memory  s i m i l a r t o that of Figure  patch  memory  were  8b.  Therefore,  n e g l i g i b l e even when the  between patches was extremely l a r g e .  Di s c u s s i o n Although I have d e a l t with t e r r i t o r i a l hummingbirds case  example,  territorial  my  predictions  may  f o r a g e r s whose feeding p a t t e r n s  l o c a l i z e d depressions  assumptions  of  the  model  particular  forager.  I  First,  assumed  that  have  activity.  Second,  I  variety  of  significant,  The degree to which  three  closely  general among  by  their  that  food  was  assumptions. i n model  own  foraging  distributed in  patches that were d i s t i n c t t o the b i r d s , and that b i r d s within  a  single  depleted. leaving  patch  until  decided  that  allowed them t o recognize  which they were f o r a g i n g was of  specific  depleted.  assumptions  I  patch  These d i c t a t e the  costs  various  of  model  birds'  times of empty and f u l l  From the s i m u l a t i o n s ,  I  was  patch-  that the patch i n  have  also  made  a  based on l a b o r a t o r y and f i e l d  s t u d i e s of hummingbirds.  handling  that  foraged  T h i r d , I assumed that b i r d s used an e f f e c t i v e rule  series  they  the  of the  flowers  induced  assumed  my  characteristics  a l l variance  hummingbirds' t e r r i t o r i e s was  a  depends on how  f i t the made  to  produce  i n food a v a i l a b i l i t y .  the p r e d i c t i o n s may be e x t r a p o l a t e d  I  apply  as  relative  energetic  a c t i v i t i e s and t h e i r  relative  flowers.  predict  that  short  term  flower  memory may be s u f f i c i e n t l y b e n e f i c i a l to outweigh any a s s o c i a t e d  36  costs.  I  p r e d i c t , however, that longer term patch memory w i l l  have n e g l i g i b l e b e n e f i t s . that  there  term  memory  i s never any payoff of  territories. variation among  I c a u t i o n that t h i s  patterns  Real  of  spatial  territories  that  is  not  imply,  f o r hummingbirds t o r e t a i n variation  may  have  i n p r o f i t a b i l i t y , both among  patches,  does  induced  a  in  great  individual  by sources  long their  deal  flowers  other  of and  than owners'  foraging.  These i n c l u d e d i f f e r e n c e s i n nectar c o n c e n t r a t i o n and  production  r a t e due to d i f f e r e n c e s i n p l a n t s p e c i e s and  makeup, and  differences  i n s o i l q u a l i t y and other a b i o t i c f a c t o r s ,  f o r a g i n g by i n t r u d e r s .  that  hummingbirds  experimentally information  guide  s u c c e s s i v e days.  Gass  learned  enriched  to  genetic  In  and and  patches, their  the  Sutherland  (1985)  remembered and  that  selection  laboratory,  of  found  locations  they  used  patches,  hummingbirds  of this  even learn  on and  remember the c h a r a c t e r i s t i c s of s p a t i a l d i s t r i b u t i o n s of feeders (Gass  1985;  Sutherland,  there i s no payoff  in  prep.).  I p r e d i c t , however, that  f o r hummingbirds to r e t a i n long  term  memory  of p a t t e r n s caused by t h e i r own f o r a g i n g . The  simulations  suggest"  that an important  consequence of  short term memory may be c r e a t i o n and maintenance of p a t t e r n s of v a r i a t i o n that territories  later  flowers while nearly  allow  on.  If  foraging i n a  a l l flowers  unprofitable. immediate  foragers  rate  As a of  before result, energy  to  effectively  exploit  model b i r d s c o u l d a v o i d patch,  they  foraging they intake,  not  could in only  that  their  revisiting  empty patch  achieved  a l l or became a  high  but a l s o c r e a t e d a c o a r s e -  37  g r a i n e d p a t t e r n of v a r i a t i o n  i n which the  among  High  patches  was  high.  within  patches  forager  to decide  to  quickly redirect their  rule  was  Therefore, be  only  half  uses i t s  that  allowed  f o r a g i n g to p r o f i t a b l e  patches.  In a d d i t i o n , t h e i r e n e r g e t i c c o s t per time while patches  that  flying  what i t was while hovering  between  at f l o w e r s .  the c o s t of v i s i t i n g d e p l e t e d patches turned  negligible,  should  when to leave them (Green  1984), and model b i r d s used a p a t c h - l e a v i n g them  in quality  v a r i a n c e among patches  i n c r e a s e the r a t e of h a r v e s t i n g by any experience  variance  out t o  and thus the b e n e f i t s of patch memory were a l s o  negligible. T h i s high c o s t of hovering patches was based on evidence species  of  hummingbirds  i n comparison t o f l y i n g  between  from l a b o r a t o r y s t u d i e s on s e v e r a l  (Montgomerie  1979).  However,  this  r e l a t i v e l y high cost of f o r a g i n g observed i n hummingbirds may be somewhat a t y p i c a l .  For example, most b i r d s ( i n c l u d i n g the other  n e c t a r i v o r e s ) walk, hop, or perch while energetic  for a  number  between of  patches  other animals.  memory may be of r e l a t i v e l y g r e a t e r value of  thus  c o s t of f o r a g i n g i s l e s s than that of forward  In a d d i t i o n , the d i s t a n c e greater  f o r a g i n g , and  may  be  flight.  relatively  Consequently,  f o r some  the  other  patch types  animals. For  efficient  territorial  hummingbirds,  f o r a g i n g i n general may be  the that  primary  benefit  i t allows  them  of to  s a t i s f y t h e i r e n e r g e t i c requirements i n t e r r i t o r i e s that produce less nectar. territories  Hummingbirds may t h e r e f o r e be able t o hold that a r e l e s s expensive to defend.  smaller  The s i m u l a t i o n s  38  suggest that the magnitude of consistent  over  production predict  range  characteristics  that  i t will  r a t e s of flowers (Fig.  a  this  of  benefit  territory  (Fig.  somewhat i f nectar  decrease as a f u n c t i o n of t h e i r  vary.  forager's energy  do  production  standing  Dill  (1978)  optimal  intake)  and  Hixon  territory size  animals'  (1980)  crop  ( i n terms  Gill  of  recently v i s i t e d  potential  i f we  were  i n t r u d e r s surrounding an  costs  the b e n e f i t of a v o i d i n g as  to  competitor  density  experimentally  remove  animal's  territory,  I t seems reasonable that  be p r o p o r t i o n a l t o the amount of i n f o r m a t i o n  likely  that  a c o g n i t i v e map or by e f f i c i e n t the  On perfect  previously  i t is  therefore  the  amount  of  information of  costs  that must be  Regardless of whether memory i s  i n c r e a s i n g f u n c t i o n of the number avoided.  (Wolf  as w e l l as the b e n e f i t s of memory may depend on  s t o r e d and processed.  2.8a),  sunbirds  as e f f i c i e n t l y .  f o r a g e r ' s environment.  by m a i n t a i n i n g  net  increases,  i t would expand i t s t e r r i t o r y , and would  need to forage  might  Therefore,  l o c a t i o n s may i n c r e a s e  Similarly,  p o s s i b l e that  The  1975).  that a  maximizing  w i l l decrease as competitor pressure  and Wolf  increases.  territories  predicted  t h i s e f f e c t has been observed i n t e r r i t o r i a l  1975;  the  I  2.7). Another f a c t o r , however, i s that the i n t e n s i t y of  may  not  fairly  2.8), although  s i z e c o n s t r a i n t on hummingbirds' or other  and  be  s t r u c t u r e s and nectar  5, F i g .  increase  should  achieved  harvesting,  required  locations  that  i t is  w i l l be an must  be  model t e r r i t o r i e s with only 4 l a r g e patches ( F i g . flower  visited  memory  flowers.  consisted  of  With the nectar  remembering production  224  rate I  39  used, model b i r d s needed to remember more than one  time  to  meet  t e r r i t o r i e s with t h e i r energy visited  100  flowers  exploit,  Therefore,  small patches ( F i g . by  do  8  They c o u l d c r e a t e a p a t t e r n of  so  that i t  with a minimum of  using  less  information  if  foraging.  groups of flowers separated my  of t e r r i t o r i e s .  clumps  forage  can  are  organize  of  flowers  or  to  by n a t u r a l landmarks.  is a direct  structureless such  that For  hummingbirds  do  approximate  f u n c t i o n of s t r u c t u r a l  However, i t i s q u i t e  variation.  territories,  they  create  example, tend  plausible  during  to  forage  animals  any in  visitation  (Gass and Montgomerie 1981).  This t a c t i c  However,  depression. hummingbirds'  A  it  may  even  organize  single single  features  that  coarse-grained  nearest-neighbor  costs.  information.  model, I have assumed that the s p a t i a l o r g a n i z a t i o n  of f o r a g i n g behavior  basically  easy  For hummingbirds, patches i n t h i s sense c o u l d  r e f e r e i t h e r to p h y s i c a l l y separated  foraging  was  their territories  d i v i d e d i n t o many small patches w i t h i n which they  In  previously  we might p r e d i c t t h a t animals w i l l be able to  effectively  their  However, on  only  s u f f i c i e n t l y coarse-grained could  at  2.8c), they c o u l d meet  remembering  in each patch.  and  flowers  e n e r g e t i c requirements.  requirements  spatial variation to  their  100  their  patterns feeding  patches  in  of  bout, and  to  sequences w i t h i n them may  minimize  travel  a l s o produce f a i r l y homogeneous l o c a l  possibility  worth  or other animals'  grained patterns s t r a t e g i c a l l y  investigating  is  foraging t a c t i c s create  that coarse-  i n that these p a t t e r n s allow them  to e f f i c i e n t l y e x p l o i t t h e i r t e r r i t o r i e s l a t e r  on.  40  CHAPTER 3 ECONOMICS OF BREEDING TERRITORIALITY IN MALE CALLIOPE HUMMINGBIRDS  Food has always played a territoriality.  Altum  prominent  (1868;  role ' in  translated  Howard (1920) both emphasized the  theories  by .Mayr  importance  of  of  1935) and  an  exclusive  food supply as a f u n c t i o n of t e r r i t o r y defense i n n e s t i n g bir.ds. Although  the e x i s t e n c e of hummingbird t e r r i t o r i a l i t y  in e a r l y l i t e r a t u r e Aldrich to  1939;  consider  i s recorded  ( B u l l o c k 1824; Knowlton 1909; Saunders 1936;  Woods 1940), P i t e l k a  (1942) was the f i r s t  i n d e t a i l why hummingbirds defend  author  territories.  On  the b a s i s of h i s own o b s e r v a t i o n s of breeding male Ruby-throated Hummingbirds  (Archilochus  colubris),  accounts  f o r other  species  during  seasons,  Pitelka  concluded  that  territorial  behavior  around a food In  Subsequent  and  " a l l evidence  non-breeding suggests  that  supply".  that  concerned with 1942), i n  breeding  of male North American hummingbirds c e n t e r s  h i s review  concluded  and p r e v i o u s l y p u b l i s h e d  most  of  avian  "while  territoriality,  territorial  Hinde  behavior  (1956)  i s primarily  food i n a few s p e c i e s (e.g. hummingbirds, P i t e l k a cases  the  observations  food  have  resources  i s common among other  sunbirds  (Gill  and  Wolf  value  indicated  i s not that  nectarivorous  1975),  significant".  defense of food birds  including  honeycreepers (Carpenter and  41  MacMillen  1976;  is  observed  also  Kamil in  wagtails  (Davies and  appears  that  it  supply p r i m a r i l y  Economics  1978), and honeyeaters some  1984).  Nevertheless,  food (Welty  of non-breeding  will  defend  defense outweigh  it  a  predators.  that  territoriality  territory  the c o s t s .  grounds,  still  1982).  predicts  that  an  only i f the b e n e f i t s of that  T h e o r e t i c a l l y , these b e n e f i t s c o u l d  i n v o l v e access to a v a r i e t y of resources such as food, courtship  and  rare f o r b i r d s to defend t e r r i t o r i e s  Brown's (1964) economic t h r e s h o l d model animal  1979),  other b i r d s such as sandpipers and  Houston  is  (Paton  n e s t i n g / o v i p o s i t ion  However, d i r e c t comparison  shelter,  s i t e s , or refuges from  of b e n e f i t s and c o s t s  is  most f e a s i b l e i f both can be expressed i n a s i n g l e c u r r e n c y , and the  most  easily  the  economic  q u a n t i f i e d c u r r e n c y i s energy.  threshold  model  applies  Consequently,  particularly  well  to  economics  of  hummingbirds and other b i r d s that defend energy sources. So  far,  rigorous  territoriality non-breeding  empirical  work  the  i n n e c t a r i v o r o u s b i r d s has been c o n f i n e d season,  (Ewald  to  the  at which time i t i s thought the c o s t s and  b e n e f i t s of t e r r i t o r i a l i t y can be indexed energy  on  1980;  Hixon  1980).  solely Gill  and  in  terms  Wolf  of  (1975)  hypothesized that the b e n e f i t to sunbirds of e x c l u d i n g i n t r u d e r s from  their  territories  availability,  and  be  enhancement  and found nectar a v a i l a b i l i t y  higher on t e r r i t o r i e s Carpenter  would  than  MacMillen  on (1976)  neighboring predicted,  to be  of  significantly  undefended on  nectar  the  areas. b a s i s of  42  Brown's  (1964)  territories  f o r the  convincingly  Carpenter  that  honeycreepers  (1978)  supported and  availability  defend more than  e n e r g e t i c c o s t of that defense, and t h e i r this  Ewald  hypothesis.  as  Ewald  (1980) a l s o found that  Anna's Hummingbirds responded t o experimental energy  would  only i f enhancement of nectar a v a i l a b i l i t y  compensated data  model,  predicted  by  the  and  territorial  m a n i p u l a t i o n s of economic  threshold  model. S i z e s of t e r r i t o r i e s birds  held  by  non-breeding  can a l s o be accounted f o r by energy-based economic  (Schoener  1971, 1983; D i l l  1983).  In  1978;  particular,  the  Hixon  sizes  1980;  of  m i g r a t i n g Rufous Hummingbirds i n mountain c o r r e l a t e d with flower d e n s i t y Kodric-Brown  and  Brown  and  total daily  1 976;  held  by  Gass  1979;  F u r t h e r , the amount of energy c l o s e l y matches  1981; Montgomerie and Gass 1981).  b i r d s a d j u s t the s i z e s of t h e i r  territories  (Gass  Individual  i n response t o  both  (Gass 1979) and e x p e r i m e n t a l l y induced (Kodric-Brown and  Brown 1978; Hixon et a l .  Breeding t e r r i t o r i e s During territoriality (Hixon  et a l .  energy requirements of the r e s i d e n t b i r d s  Montgomerie  natural  models  meadows are n e g a t i v e l y  (Gass e_t §_1.  1978).  Hixon  territories  produced per day by flowers i n these t e r r i t o r i e s the  nectarivorous  the  1983) v a r i a t i o n s i n flower d e n s i t y .  of male North American  breeding  season,  an  i s improvement i n immediate  1980).  hold t e r r i t o r i e s  Among  North  American  hummingbirds  important  benefit  reproductive  of  success  hummingbirds, only males  during the breeding season ( P i t e l k a  1942, 1951;  43  Legg and  Pitelka  females  1956;  Williamson  sometimes  territories habitat  (Stiles  nest  nectar  i n open  producing  Stiles  1970).  often  observed  meadows, flowers  on  few  a  an important  (Pitelka  1951;  fairly  clear  aggregations  of  Legg and P i t e l k a females the few  are  most  copulations  i s o l a t e d o b s e r v a t i o n s , there are no  obvious  is  no  there  1956;  1982).  there  occurred  parental  of nectar a v a i l a b i l i t y  non-breeding more  energy  care  by  on t e r r i t o r i e s  season.  energetically  may  be  breeding  males  could  expensive  males'  l e a s t three d i f f e r e n t ways. to  perform  (Tamm 1985). to  copulate with availability  energetically  Second, high territories resident on  First,  activities  important  at that time of  energy  territories  expensive  c h a r a c t e r i s t i c which females use as q u a l i t y of males (Selander  1965).  their  act an  could  1970).  allow  Third,  of  attract  subsequently  as a secondary index  i n at  courtship displays  availability  (Stiles could  on  could  to feed, where they might males  (Stiles  r e p r o d u c t i v e success extra  nectar  as  Because breeding males are  particularly  increase  c o u l d be  season  In a d d i t i o n , enhancement of nectar a v a i l a b i l i t y  territories  females  around  b e n e f i t of defense d u r i n g the  the  involved i n  males  males'  1983).  Enhancement  year.  a  of  (Stiles  p a i r bonds formed, and  as  Although  i n wooded areas and males hold  males' t e r r i t o r i e s , and  Except  (Johnsgard  generally  usually  have  1970).  boundaries  I n t e r a c t i o n s between males and  observed  for  Stiles  the  1970), there i s  that have been  1971),  within  s e p a r a t i o n ; females nest  territories  well  1956;  the  nectar sexual genetic  44  Because  a l l the above b e n e f i t s c o u l d r e s u l t  nectar a v a i l a b i l i t y , benefits  could  However, other  be d e s c r i b e d  The  secure  best  those flowers.  their  territories  e x c l u s i v e access  the highest  males,  females may or  concentration  of  owners  may  nectar  T h i r d , some  and a t t r a c t  them.  not be producing  c o u l d allow males  than nectar a v a i l a b i l i t y  secondary sexual c h a r a c t e r i s t i c , this  these  t o any females that entered  females without i n t e r r u p t i o n . other  only  considerations.  from other  Second, e x c l u s i v e use of t e r r i t o r i e s  males' t e r r i t o r i e s  In  involved  by an energy-based economic model.  l o c a t i o n s f o r encountering with  to court  that  b e n e f i t s c o u l d outweigh e n e r g e t i c  F i r s t , by defending could  territoriality  from enhanced  quality  of  c o u l d a c t as a  females.  study, I ask whether breeding  t e r r i t o r i a l i t y of a  group of male C a l l i o p e Hummingbirds can be accounted f o r  solely  by an energy-based model.  Study Area  L o c a t i o n and t i m i n g of t e r r i t o r i a l i t y I  studied  a group of C a l l i o p e Hummingbirds i n the Ashnola  P r o v i n c i a l F o r e s t about 25 km (119°  47'  breeding within valley  of  Penticton,  B.C.,  Canada  W, 49° 18' N, e l e v a t i o n about 800 m) during the 1985  season. a  SW  strip  immediately  Hummingbirds  The of  males meadow  W  of  sometimes  meadow, the only  longer  I  about  Twin  hold term  studied  a l l held  territories  100 m wide ( F i g .  Lakes.  Although  territories territories  3.1) i n a  male  Rufous  f o r a few days i n the are held  by  male  45  DOUGLAS FIR COVER > 2 m DECIDUOUS COVER > 2 m OPEN MEADOW  Figure 3.1. 1 through 6 territories m o s t o f May conifers, immediately for defi boundaries.  P o s i t i o n s of territories on 1 June 1985. These were h e l d by m a l e s d u r i n g Females nest i n and June, Douglas f i r , primarily N o f t h e meadow. See t e x t nition of territory  46  Calliopes. in  Both Rufous and C a l l i o p e  meadows  on  the  males a l s o h o l d t e r r i t o r i e s  S slope of the v a l l e y .  Other s t u d i e s were  conducted i n t h i s area i n 1983 and 1984 (Tamm 1985; Tamm et a l . in  prep). Calliope  males a r r i v e  i n the  valley  in  defend t e r r i t o r i e s throughout May and June arrive  in  in  Seven C a l l i o p e  April,  and  ( F i g . 3.2). Females  e a r l y May, and begin i n t e r a c t i n g  t e r r i t o r i e s soon a f t e r .  late  with males on t h e i r  nests found i n t h i s area  1983 and 1984 were a l l w i t h i n the Douglas  f i r (Pseudotsuga  menziesi i ) growth N of the meadow ( F i g . 3.1). The average date of  egg  laying  of  these 7 nests appeared to be e a r l y  However, i t i s d i f f i c u l t clutches  were  to e s t i m a t e when  the  first  i n June. and  i n i t i a t e d , and t h e r e f o r e when the f i r s t  last  and l a s t  matings o c c u r r e d . I studied  the  same  6  territories  throughout  breeding season ( F i g . 3.1). These were the f i r s t in  the  meadow, and were a l l e s t a b l i s h e d  throughout  the  season.  defended s e q u e n t i a l l y mid-May,  territory  sometimes  3 males.  Territories  6 territories  by 6 May.  1,3, and 6 were each defended by a s i n g l e 2,  Territories  clearly 4,  the 1985  marked  and  5 were each  by at l e a s t 2 d i f f e r e n t males, and 5  was  divided  between  at  male  least  during 2 and  47  5  .  1  5  M A Y  25  5  15  25  J U N E  F i g u r e 3.2. Phenology of t e r r i t o r i a l i t y i n the meadow shown in F i g . 3.1, and approximate timing of reproductive activities. The i n f l u x of a d d i t i o n a l males d u r i n g mid-May i s c o r r e l a t e d with the peak of the Ribes f l o w e r i n g season.  48  D e f i n i t i o n of t e r r i t o r i e s If t e r r i t o r i e s are c o n s i d e r e d t o be 'defended areas' 1 939),  t e r r i t o r y boundaries  owners would expel or studies  on  (Noble  should i n c l u d e a l l p o i n t s from.which  attempt  to  expel  hummingbird t e r r i t o r i a l i t y  intruders.  (Gass et a l .  Previous 1976; Gass  1979;  Kodric-Brown and Brown 1978; Norton et a l .  1982; Hixon et  al.  1983)  boundaries  consisting  have of  primarily  the  defined  outermost  i n t e r a c t i o n s were observed.  will  around  p r e s e n t s two problems.  increase  locations  with  that  locations  at  which  increases  aggressive  intruders  perimeters,  this  F i r s t , estimated t e r r i t o r y  sizes  in  territory  observation  d i d not  time.  Second,  e n t e r , but would have been  e x p e l l e d from i f they had, w i l l not be i n c l u d e d .  In t h i s  I d e f i n e d each male's t e r r i t o r y on a given day i n terms perches 242  he  as  However, u n l e s s i n t r u s i o n r a t e s are  high, and w e l l d i s t r i b u t e d method  territory  study, of the  used while surveying h i s t e r r i t o r y on that day. Of  chases of i n t r u d i n g hummingbirds by t e r r i t o r i a l males d u r i n g  the 1985 season, within  10  m  228 (94%) were i n i t i a t e d when the i n t r u d e r  of the perimeter d e s c r i b e d by the perches  the owner on that day. criterion  Throughout  to d e f i n e t e r r i t o r y  this  study,  I  was  used by  used  this  boundaries.  Sources of nectar Males a r r i v e about a week before the f i r s t plants seen  begin to  to  consume  Neuroptera)  bloom. is  nectar  producing  At t h i s time, the only food males are  small  insects  (Diptera,  that they hawk from t h e i r perches.  Lepidoptera,  Bushes of Squaw  49  Currant  (Ribes cereum) begin  blooming i n e a r l y  May.  Ribes  is  the only source of nectar a v a i l a b l e t o b i r d s throughout May, and is  found w i t h i n a l l t e r r i t o r i e s  when the number of t e r r i t o r i e s almost  a l l bushes  dense aggregations areas  both  i n the meadow. i s at  are w i t h i n  During mid-May,  i t s peak  (Fig.  or near t e r r i t o r i e s .  of Ribes bushes are a l s o found on  within  and  3.2),  However, undefended  around the Douglas f i r growth N of the  meadow, and on the slope S of the meadow. S e v e r a l other the t e r r i t o r i e s Lemonweed repens),  nectar producing  a f t e r Ribes d r i e s up i n l a t e May.  (Lithospermum Larkspur  (Cynoglossum  I  species. on  these  in  June). One  is  ruderale),  (Delphinium  officinale),  o r e o p h i l u s and S. a l b u s ) . 1984,  saw  territorial  Mahonia  and  During  Snowberry preliminary  males  visit  flowers was very small  p l a n t that  (Grant  and  (Symphoricarpos observations  flowers  of  I studied,  Castilleja 1968)  s e c r e t e copious n e c t a r , and have a them  throughout  foraging  is a  typical  i n that long,  June,  i s Indian  in  a l l these  (< 1% of 15 h o b s e r v a t i o n  i s common i n the area  Grant  (Berberis  Hound's-tongue  However, the amount of time I observed b i r d s  ( C a s t i l l e j a miniata).  makes  Creeping  around  These i n c l u d e  nuttallianum),  not found on the t e r r i t o r i e s  plant  p l a n t s bloom i n and  time  but  Paintbrush  ornithophilous  i t s flowers are r e d , tubular  shape  which  a c c e s s i b l e t o hummingbirds, but not to most i n s e c t s  (Perkins  1977).  In c o n t r a s t , a l l flowers on the t e r r i t o r i e s a r e  shallow,  and a r e a c c e s s i b l e t o i n s e c t s as w e l l as  (Bombus  spp., Apis  sp., Emphoropsis  birds.  Bees  spp., Anthophora spp.,  50  Andrena spp.) and Hawk Moths seen  feeding  territories are  on  the  territories.  diffinis) Castilleja  are  commonly  occurs w i t h i n  l o c a t e d on the slope S of the v a l l e y , i n areas  defended  during  dense C a s t i l l e j a  both May and June.  growth that  g e n e r a l l y undefended. the  (Hemaris  are c l o s e s t  that  However, the areas of to  the meadow are  These are l o c a t e d on the steep p o r t i o n of  slope about 50-100 m S of and 50-100 m above the meadow.  51  PART I .  PROFITABILITY AND ENERGY PRODUCTION OF TERRITORIES  During  both May and June, I r e p e a t e d l y sampled nectar both  on t e r r i t o r i e s nectar  and on  producing  nearby  flowers.  undefended I  d i d this  F i r s t , c o u l d males o b t a i n energy f a s t e r territories  than  areas  with  abundant  t o answer 2 q u e s t i o n s . by  foraging  on  by f o r a g i n g on the undefended areas?  c o u l d males o b t a i n enough energy from nectar produced territories  t o meet t h e i r  their Second,  on  their  metabolic requirements?  Methods  Sampling d e s i g n Between  8  on 4 t e r r i t o r i e s undefended  and 21 May, I sampled nectar from Ribes flowers (territories  areas.  1,2,4, and 5 i n F i g .  Because most Ribes bushes  3.1)  i n the meadow were  e i t h e r w i t h i n or near the boundaries of a t e r r i t o r y , undefended areas c l o s e t o but o u t s i d e the meadow. a s e r i e s of bushes on the slope about  and 2  I  sampled  The f i r s t was  100 m S of and 100 m above  the  meadow.  the  Douglas f i r growth, a l l w i t h i n 20-50 m N of the meadow.  density  of  The second was a s e r i e s of bushes w i t h i n and around  Ribes  bushes  on  both of the undefended areas was  approximately equal t o that on the t e r r i t o r i e s . the  two  The  During each  of  weeks of Ribes sampling, I sampled each of the 6 areas  once. During each of 4 weeks  between  27  May  and  25  June,  I  sampled nectar from flowers of a l l p l a n t s p e c i e s that bloomed on  52  the  territories  after  Ribes.  Because  the  d e n s i t y of these  flowers was low, I had t o t r e a t a l l 6 t e r r i t o r i e s defended  area.  During  each  C a s t i l l e j a on a s i n g l e 700 m  2  as  a  of those same 4 weeks, I sampled undefended  area about  80  m  and above the meadow.  Although there were many i s o l a t e d  elsewhere,  the  this  was  C a s t i l l e j a growth. inflorescences,  On 3 and  largest  June,  this  single  area  of  i t contained  density  remained  S of patches  fairly  contiguous  1615  Castilleja  fairly  constant  throughout most of June. On and before 10 June, I conducted 4 sampling s e s s i o n s each day s t a r t i n g at 06:00, 10:00, 14:00, and 18:00. lengthening  photoperiod,  after  Because of the  10 June I conducted  s e s s i o n s s t a r t i n g at 05:00, 09:00, 13:00, 17:00, and divided  each  area I sampled  into  all  The f i r s t foragers.  covered with a session, time. with  and  21:00.  I  10 s i t e s , and measured n e c t a r  volumes of 3 flowers from each of 3 d i f f e r e n t site.  5 sampling  treatments at each  treatment was an uncovered branch a v a i l a b l e  to  The second treatment was a branch that had been plastic  screen  bag  since  the  last  sampling  thus had been u n a v a i l a b l e t o f o r a g e r s d u r i n g that  The t h i r d treatment was a branch covered with a wire  bag  3.2 cm by 2.5 cm hexagonal h o l e s , which allowed i n s e c t s to  enter, but not b i r d s .  53  Sampling  techniques  I measured nectar volumes in flowers by with  glass  microcapillary  concentrations  using  a  refTactometer.  Because  a  conditions),  I  random  sampling  appearance and  obtained  to  nectar  calibrated  sugar  (1-4  needed get  by choosing position.  measured  minimum volume i s r e q u i r e d before a  often  s e v e r a l flowers i n order  nectar  and  temperature  c o n c e n t r a t i o n reading can be light  tubes,  extracting  a  ul,  to accumulate nectar reading.  I  flowers haphazardly I  did  depending  not  on from  approximated  with respect to  include  old,  withered  flowers in samples.  E s t i m a t i o n of I  profitability  d e f i n e the p r o f i t a b i l i t y of an area as the r a t e at which  a b i r d c o u l d o b t a i n energy while given  time p e r i o d .  f o r a g i n g on that  In t h i s a n a l y s i s , I estimate  area  over  r a t e s at which  b i r d s c o u l d o b t a i n energy while f o r a g i n g f o r 10% of a 16 h T h i s r e p r e s e n t s the amount of time b i r d s were observed on  Ribes  on  their  territories  f l o w e r i n g season i n mid-May ( F i g . Instantaneous a  product  visit and  of  3  r a t e of nectar factors:  during  the peak of the  Ribes  3.9). intake (Watts) at any  time  (J/ul),  the number of flowers b i r d s v i s i t per time ( f l o w e r s / s ) . s e s s i o n on  the  is  the nectar volumes of flowers b i r d s  territories,  volumes of unbagged flowers at a l l 10 s i t e s , by  day.  to forage  ( u l / f l o w e r ) , the sugar c o n c e n t r a t i o n of the nectar  each sampling  a  I  averaged  For  nectar  and m u l t i p l i e d  this  the average of a l l c o n c e n t r a t i o n readings to o b t a i n a s i n g l e  54  estimate of the mean energy  content per f l o w e r .  For C a s t i l l e j a ,  I obtained at l e a s t one c o n c e n t r a t i o n reading f o r every site.  Therefore,  f o r each  separate estimates of estimate  consisting  energy  sampling  session,  content  per  estimate the r a t e at which b i r d s c o u l d measured r a t e s a t which b i r d s v i s i t e d timing  their  flower,  obtain  with  each  In order to  this  energy,  I  flowers of each s p e c i e s by  f o r a g i n g with a stopwatch  them at c l o s e  obtained 5  I  for 2 sites.  of data averaged  second  whenever I c o u l d  observe  range.  For each t e r r i t o r y or undefended a r e a , I used the r e s u l t i n g estimates of acquire  the  energy  instantaneous  to  was  less  at  which  birds  c a l c u l a t e the amount of energy  o b t a i n while f o r a g i n g f o r 10% of amount  rates  the  day.  As  a b i r d would  long  than the estimated t o t a l energy  could  as  this  p r o d u c t i o n on  the area, I assumed that the instantaneous r a t e was a reasonable estimate of p r o f i t a b i l i t y . the  Ribes  so  In  i n June,  flowers  of  some  sparse that b i r d s c o u l d have obtained energy  at the estimated day.  throughout  season, and f o r the C a s t i l l e j a area throughout  However, on the t e r r i t o r i e s were  T h i s was always the case  on  from them  the  best  a l l o c a t e t h e i r remaining  each  p r o f i t a b i l i t y of the t e r r i t o r i e s at that  time, I assumed that b i r d s would spend as much time as foraging  species  instantaneous r a t e f o r only a few minutes  calculating  June.  possible  s p e c i e s on t h e i r t e r r i t o r i e s , and would time to f o r a g i n g  s p e c i e s , t h i r d best s p e c i e s , and so on.  on  the  second  best  55  E s t i m a t i o n of t e r r i t o r y The  difference  flowers covered  energy  between  production the  average  with screen bags and  the same s i t e at the l a s t sampling of  the  The  estimate  the  average  product  The of  s e s s i o n p r o v i d e s an  energy p r o d u c t i o n per flower over  of  production  for  all  energy produced each day the  average d a i l y  time  on  of  a  estimate  that p e r i o d .  i s the sum  periods  I estimated  flowers of each s p e c i e s in each t e r r i t o r y  of  between  territory  energy p r o d u c t i o n per  the number of flowers on the t e r r i t o r y . of  content  that of unbagged flowers at  of 24 h energy p r o d u c t i o n per flower  estimates  samples.  energy  is  the  flower the  at l e a s t  and  number  once each  week. For  species  sufficiently  low  each t e r r i t o r y . count  other  than  Ribes,  that I c o u l d i n d i v i d u a l l y Because Ribes  individually,  I  volume  estimated  of the bush and  bushes are roughly  of  was  count a l l flowers  on to  the number of flowers on each  on the b a s i s  of  two  i t s phenological state.  hemi-ellipsoidal  measured the longest and  density  flowers were f a r too numerous  bush i n bloom w i t h i n a t e r r i t o r y the  flower  in  shape.  indices: Most  Ribes  Therefore,  s h o r t e s t h o r i z o n t a l a x i s and  I  the height  each bush, and c a l c u l a t e d the volume of a h e m i - e l l i p s o i d with  these  dimensions.  by counting branches  the phenology of each Ribes  the number of flowers i n bloom on  twice  each week.  the flower count f o r counted  I recorded  each  the  same  bush  sample  At the end of the season, I d i v i d e d date  by  the  maximum  number  ever  on that branch, and averaged the values of a l l branches  on each bush.  Throughout May,  I a l s o counted flowers on s e c t o r s  56  of  51 randomly s e l e c t e d bushes, with each s e c t o r making up  of  the  bush  volume.  The number of flowers ( f ) counted  bush s e c t o r s throughout 0.4v of  1/16  May  was  predicted ( r  = 0.60)  2  on 51  by  =  +2.3p, where v and p are the volume and p h e n o l o g i c a l s t a t e  the bush r e s p e c t i v e l y .  all  Ribes  bushes  on  By summing the p r e d i c t e d  each  territory,  values  for  I used t h i s equation to  estimate the t o t a l number of f l o w e r s .  E s t i m a t i o n of p r o p o r t i o n of nectar removed by The  insects  t o t a l amount of nectar removed by a l l f o r a g e r s  sampling  sessions  is  estimated  by  the d i f f e r e n c e i n average  n e c t a r volumes between flowers  covered  those  amount  left  uncovered.  The  with  screen  removed  by  estimated by the d i f f e r e n c e i n n e c t a r  volumes  covered with wire bags and those l e f t  uncovered.  nectar  harvesting  the r e l a t i v e  of  the  two  to  the  rates  branches were bagged. 72  assumption  that  wire  I f the r a t e of  observed  immediately  provides  an  insects.  to  reduce  bagged  before or a f t e r  Throughout the day of 16 May,  I  Ribes these  observed  branches f o r 30 minutes each both while bagged and unbagged.  ratio  that  is  flowers  bags d i d not  rates  Based on logarithms of v i s i t a t i o n r a t e s , I  1.04  insects  differences  i n s e c t f o r a g i n g by comparing v i s i t a t i o n branches  and  by i n s e c t s i s not a f f e c t e d by the wire bags,  magnitude  tested  bags  between  estimate of the p r o p o r t i o n of n e c t a r consumed by I  between  of  visitation  rates  estimated  that  t o bagged and unbagged flowers  (95% c o n f i d e n c e i n t e r v a l = 0.65  -  1.67),  the was  which  indicates  the bags d i d not have a s t r o n g e f f e c t on i n s e c t  visitation  57  rates.  Results  Comparative p r o f i t a b i l i t y of defended and undefended During both weeks I sampled Ribes d u r i n g territories two  contained  undefended areas  fairly  similar  I estimated  foraging  on  on  l e s s nectar on average than those on  the  3.3).  defended  Using these data, and 3.1),  flowers  (Fig.  on  areas  Nectar  May,  concentrations  and undefended areas  flower v i s i t a t i o n  rates for  (Fig. Ribes  these  areas.  These estimates suggest  areas than by f o r a g i n g on t h e i r flowers  that  territories  bloomed  contained very l i t t l e n e c t a r , and the  undefended  Delphinium  Castilleja these  Castilleja  and  approximately  the  flowers  territories volumes  of  3.3,  on  3.4).  the  (Fig.  that b i r d s undefended  3.5).  territories  f a r l e s s than (Figs.  the  that 3.3).  i n June found  in  Flowers  of  territories  However,  on the t e r r i t o r i e s  contained  there  (Table 3.2)  were  so  few  of  that owners c o u l d  24 h nectar p r o d u c t i o n i n no more than 3 to  foraging.  i n June ( F i g . of  (Table  same amount of energy on average as those of  have harvested t h e i r 4 minutes  on  flowers  S. albus  (Fig.  3.4).  r a t e s at which b i r d s c o u l d o b t a i n energy while  c o u l d have obtained energy f a s t e r by f o r a g i n g on the  Most  were  Estimates 3.5)  Lithospermum,  of  profitability  primarily reflect Berber i s ,  on  the low  Cynoglossum,  the  nectar and  S. o r e o p h i l u s . In order to o b t a i n those e s t i m a t e s , I needed to  make  some  58  10  20 MAY  5  15  25  JUNE  Figure 3.3. Nectar volumes of. flowers blooming on (•§• ) and o f f (-&) t e r r i t o r i e s over the course of the breeding season. Data f o r May are a l l f o r R i b e s . Date f o r June are f o r Castilleja off territories and Lithospermum ( L ) , Berber i s (B), Delphinium (D), Cynoglossum (C), S. o r e o p h i l u s (So), and S. a l b u s (Sa) on t e r r i t o r i e s . Each p o i n t i n d i c a t e s the mean n e c t a r volume of flowers at 10 sampling s i t e s , averaged over 4 or 5 times of day. Vertical bars indicate 95% confidence i n t e r v a l s .  Figure 3.4. C o n c e n t r a t i o n of nectar i n flowers blooming on ( # ) and o f f (•&) t e r r i t o r i e s . P o i n t s shown were obtained by calculating mean nectar c o n c e n t r a t i o n s at each time of day and averaging these. Vertical bars indicate 95% confidence i n t e r v a l s .  /  60  Table 3.1. Flower visitation Hummingbirds on 4 p l a n t species. r a t e s ± 95% c o n f i d e n c e i n t e r v a l s .  No. Bouts Observed  rates by male Calliope Values shown a r e mean  Flowers per second  Ribes  9  1.0  ±  0.1  Lithospermum  8  0.8  ±  0.05  Delphinium  10  0.7  ± 0.1  Castilleja  31  0.6  ±  0.05  61  Table 3.2. T o t a l number of flowers of 6 p l a n t s p e c i e s censused on t e r r i t o r i e s each week d u r i n g June. Underlined numbers i n d i c a t e that there were i n s u f f i c i e n t flowers of that s p e c i e s to conduct nectar sampling during that week.  Date  May 27  June 4  June 11  June 18  Lithospermum  1 669  1 502  357  38  Berberi s  1 068  580  0  0  Delphinium  • 0  253  187  22  Cynoqlossum  249  2180  6051  3853  S. o r e o p h i l u s  0  489  5570  698  S. albus  0  0  0  243  62  (a) MAY  (b)  JUNE  Figure 3.5 Comparative p r o f i t a b i l i t y of flowers blooming on (•) and o f f ( O ) t e r r i t o r i e s at s e v e r a l times of day over the course of the breeding season. P r o f i t a b i l i t y r e f e r s t o the gross r a t e a t which a b i r d could obtain energy while f o r a g i n g i n a given area, assuming that i t spends 10% of the day foraging. (a) compares p r o f i t a b i l i t y of Ribes on each of 4 t e r r i t o r i e s i n May with that on 2 undefended areas N and S of the meadow. (b) compares p r o f i t a b i l i t y of s e v e r a l p l a n t s p e c i e s on the t e r r i t o r i e s i n June (see F i g . 3.3) with that of C a s t i l l e j a on the slope S of the meadow. The top of the shaded region i s an approximation of the c o s t of hovering, based on Montgomerie's (1979) allometric equations. I f b i r d s ' r a t e s of energy intake were below t h i s v a l u e , they would l o s e energy even while f o r a g i n g .  63  assumptions.  Cynoglossum  nectar  c o u l d o b t a i n no c o n c e n t r a t i o n assumption obtained one  (see F i g .  sampling s e s s i o n . were  readings.  readings For other  negligible,  territories  in  Because  visitation the  Berber i s  May,  visitation average  the  assumed  rates  fairly  that  the  Of the 6 s p e c i e s that I  observed  of  similar  r a t e s were s i m i l a r .  birds  therefore  f o r the  morphologies  are  made  not  species  S. o r e o p h i l u s ,  that nectar volumes and c o n c e n t r a t i o n s preceding  or  succeeding  on  measure  (Table 3.1). and  I assumed that  Cynoglossum,  r a t e observed f o r Lithospermum.  average  S. a l b u s ,  to that of Ribes, For  nectar  f o r a g i n g on only  could  other  I  only  bloomed  I  used  the  For weeks d u r i n g which  there were too few flowers of a s p e c i e s to sample i t , I  closest  liberal  sampling s e s s i o n s , when  Lithospermum and Delphinium, and flower  I  so low that I  f o r S. o r e o p h i l u s during  I  c o n c e n t r a t i o n was unchanged. the  were  3.4) that i t s nectar was 40% sugar w/w.  concentration  volumes  volumes  were equal  assumed  to that on the  week on which I c o u l d  (Table  3.2). The  r e s u l t a n t estimates  for each week i n obtained of it  suggest  while  foraging (Fig.  i s not s u r p r i s i n g that I these  species.  Castilleja  obtained  that  birds  could  never  enough energy even to compensate f o r the metabolic  hovering  visiting for  June  of p r o f i t a b i l i t y on the t e r r i t o r i e s  on  Throughout  June,  observed  few  instances  In c o n t r a s t , estimates  the  energy f a i r l y  3.5). Given these  slope  indicate  r a p i d l y from t h i s  have cost  estimates, of  birds  of p r o f i t a b i l i t y  that b i r d s c o u l d have species  the b i l l s of a l l 6 t e r r i t o r y  ( F i g . 3.5).  r e s i d e n t s i n the  64  meadow  were  usually  coated  with  characteristic  of  Castilleja,  used C a s t i l l e j a  somewhere o f f t h e i r  the  bright  indicating  orange  pollen  that they must have  territories.  T e r r i t o r y energy p r o d u c t i o n and metabolic requirements of b i r d s I compared estimates of t e r r i t o r y estimates  of  equations.  ( 1974)  and  Montgomerie' s  3.6).  (Fig.  their  energy  Although i n s e c t s removed about  3.7), b i r d s may  from  with  ( 1979)  Estimates of energy p r o d u c t i o n by Ribes  in mid-May are g r e a t e r than b i r d s ' minimum (Fig.  production  b i r d s ' minimum d a i l y energy e x p e n d i t u r e s , which I  c a l c u l a t e d u s i n g both King's allometric  energy  have been able to  territories  at  that  requirements  60% of t h i s  obtain  enough  energy energy  time to meet t h e i r metabolic  requirements. In c o n t r a s t , estimates of t e r r i t o r y  energy  June were f a r below b i r d s ' minimum requirements insects that  consumed  most of t h i s energy  (Fig.  3.7).  3.6),  on the t e r r i t o r i e s  t h i s o u t l i e r was  caused by one  nectar  volume  Castilieja produced  out  of  360  in and  I estimated  i n s e c t s removed more than 75% of the nectar produced  s p e c i e s blooming  3.7).  (Fig.  production  by a l l  i n June except B e r b e r i s ,  and  flower with an e x c e p t i o n a l l y h i g h sampled.  Conversely,  data  for  on the slope confirmed that a l l or n e a r l y a l l nectar  by t h i s  species  was  removed  by  hummingbirds  (Fig.  65  J U N E  Figure 3.6. Estimated daily energy production for 4 territories during May, and f o r 6 t e r r i t o r i e s d u r i n g J u n e . V i r t u a l l y a l l e n e r g y p r o d u c t i o n t h r o u g h o u t May i s by Ribes. Minimum daily e n e r g y e x p e n d i t u r e i s b a s e d on M o n t g o m e r i e ' s (1979) e q u a t i o n s , and the assumptions that birds remain perched for 16 h a t 20°C and s l e e p f o r 8 h a t 5°C. King's (1974) allometric e q u a t i o n , which does not incorporate t e m p e r a t u r e , p r e d i c t s a s l i g h t l y l o w e r v a l u e o f 20.6 k J ,  % O F TOTAL H A R V E S T R E M O V E D B Y I N S E C T S n O fD i-i CO • Ch IQ O  c\ts> n  a>  -  n > in Z  3 o fD o r t TJ 0)  O <  > —  3  < n> o I-I i- 3 1  c 3  Cr fD <D  rr  0)  c  cn fD  nog  I—' € C fD Qj t-l (5 (fl  a  D 0) rt  O  C T I 0) S 3 CO a  t—  1  o  cn  a sl-l m o  fD  CT I-I cr cu  * « fD  <r> c ^  fD fD Qi CO  in  M- n 3  r t r t Cu O in r t fD  n 3 cn —* I-I fD • o fD cn  O  i-i T J I-I TJ O  CU fD 1 I-I 3 fD (-"O l-h fD Cu O (-• 3 O n a 3 i-l •< r-h 3 o >-t r t rt fD C O Co 3 r t 3 CT c r C O iQ 3 cr t->. cu 3 cr in O 0) o p> u a I-I 03 c r r t T J 3 i-l m a OJ cn cu CU fD M o in cn O 03 M- O cn (D 3 rt 03 3 P'Hl 3" a w • fD fD fD fD fD O i-t X 3- a OJ in Cu rt CU O C 3 ' I-' 3 l-h IT M3 O O rt H H i-l Co co 3 rt C 3 i O •< l-h rt 3 O a H- o (D O 3 O n rt H 3 fD 3 S in 3 rt 3">< fD QJ03 fD fD >-•• fD 3 O W O O — CT i-l 03 r t 01 cn OJ r t • fD 0» O" fD n cn fD Ch >-**' in t r I-I o 03 in O in o rtTJ 3 rt rt M. O 3 3* 3 3" n O i-h 3 0) r t Cu C a fD 3 rt rt o 3 O — W o fD 3 C fD O 0) O O t-> in in i-l 3 cr fD 3 fD fD fD O cr w o fD TJ in cn i-h >< ^ 3 ' CO fD O >< 3 V l-h  cn  •s  2  >-<  •-O  D  n °" ^  o DJ i O i—• cn cn O* cn cn c fD fD  o <  3 fD r t o fD 3 rt i-i in l-l f-l fD  3 <  o  c i IT]  a 3  s  3  o  0)  TJ 0J o i-l l-h H- l-h  LQ — fD 03  Co O iQ  in o •  M-  l-h « I-I  < O  X  H* O  ••  CT  in  O  > KJ O  a -  ••  A  >  67  Discussion Throughout  the  breeding  season,  f l o w e r s blooming on the  t e r r i t o r i e s appeared to be l e s s p r o f i t a b l e than those on undefended based  areas.  model  cannot  males i n the exercised  This r e s u l t  s t r o n g l y suggests that an energy-  account f o r the t e r r i t o r i a l i t y of C a l l i o p e  meadow.  Nevertheless,  some  caution  factor  not  considered  in  such  an  t e r r i t o r y owner may  know which p o r t i o n s  most  and  productive,  recently. from  which  portions  Consequently, a b i r d may  be  in  their  territories.  analysis  of  its  it  that a  territory  has  reality  is  visited obtain  are most  nectar  i t s t e r r i t o r y somewhat f a s t e r than i f i t foraged at random  ( G i l l and Wolf  1977; Chapter 2 ) .  In June, t e r r i t o r i e s were c l e a r l y not an of  should  i n using data from random samples of flowers to i n f e r  r a t e s at which b i r d s c o u l d o b t a i n energy on One  nearby  energy.  Given  that  produced on t e r r i t o r i e s ,  insects i t is  removed  extremely  important most  source  of the nectar  unlikely  that  birds  c o u l d have o b t a i n e d energy f a s t e r than p r e d i c t e d on the b a s i s of flower  sampling.  F u r t h e r , e s t i m a t e s of d a i l y t e r r i t o r y  p r o d u c t i o n at that time of year were f a r  below  birds'  energy minimum  d a i l y energy e x p e n d i t u r e s . For Ribes was on  the  May,  the  results  were  not as c l e a r .  the only nectar source i n the a r e a , and territories.  I  estimated  p r o f i t a b l e than nearby undefended  At that time, was  abundant  that t e r r i t o r i e s were l e s s  areas.  However,  given  that  b i r d s removed a s i g n i f i c a n t p o r t i o n of the Ribes nectar produced on  their  territories,  i t i s p o s s i b l e that t h e i r t r u e r a t e s of  68  e n e r g y a c q u i s i t i o n were h i g h e r t h a n my territory minimum  nectar energy  territories at on  mid-May  and  Therefore,  b u s h e s may  be an  important  account source  P a r t I I of t h i s paper r e p o r t s males  to  experimental  territories several  i n May.  Ribes  occur.  responses Ribes  of  Calliope  bushes  on  i s energy-based,  If t e r r i t o r i a l i t y  their  territories  occur.  Birds could increase off  their  nectar resources. by  expand  should them  i s not e n e r g y - b a s e d , b i r d s  o f e n e r g y t o b i r d s , one  their one  leave  behavior.  o f two  territories  perching in  in  i s not  should  of their  remain  on  energy-based,  but  a r e n e v e r t h e l e s s an  the  Otherwise,  reducing p a r t i c i p a t i o n courtship  birds.  territories.  source  expenditure  of e n e r g y t o t h e  bushes, or c h a l l e n g e owners of a d j a c e n t  territories.  foraging  bushes defense,  Birds  or  Ribes  incorporate  b u s h e s on  shift  closely  to  territoriality  their  should  entirely,  peripheral If  If t e r r i t o r i a l i t y  responses  territories  of  the  for territory  the  exclosure  on  energy requirements of  of  calculated  production  even i f the presence  cannot i n i t s e l f  Estimates  exceeded  nectar  have m a t c h e d b i r d s '  territories  these  in  expenditures,  may  that time.  production  estimates.  they less  important  types of responses  amount  of  time  they  should spend  t o compensate f o r the l o s s could  reduce  exposed  energetically  their  locations,  costly  of  energy or  by  agonistic  or  69  PART I I .  RESPONSES OF MALES TO EXPERIMENTAL EXCLOSURE OF TERRITORY NECTAR SOURCES  Methods On or b e f o r e 6 May, I color-marked r e s i d e n t s of t e r r i t o r i e s 1  through  6  by  catching  them  in  p a i n t i n g t h e i r b r e a s t f e a t h e r s by hand. territories  immediately  after  birds  a f e e d e r - b a i t e d t r a p , and I removed feeders  were trapped so that the  feeders would not i n f l u e n c e t h e i r t e r r i t o r i a l b e h a v i o r . for  from  Except  p e r i o d s of a few hours when I p l a c e d feeders on t e r r i t o r y 5  on 30 May and t e r r i t o r y 6 on 2 and  16  June  (see Chapter  4),  there were no feeders on t e r r i t o r i e s a f t e r 7 May. On  9,  12,  and  14 May, I observed b i r d s on t e r r i t o r i e s 1  through 6 f o r 90 minutes each. into  3  separate  sessions  These o b s e r v a t i o n s were  s t a r t i n g at 6:30, 11:00, and 15:30.  During each s e s s i o n , I s t a r t e d with t e r r i t o r y westward  until  each 30 minute  divided  1,  and  proceeded  I had observed, each b i r d f o r 30 minutes. o b s e r v a t i o n s e s s i o n , I recorded the  During  identity  of  the r e s i d e n t b i r d , a l l perches used, and d i v i d e d the b i r d ' s time budget  into  5  component  activities:  perching,  foraging for  n e c t a r , a g o n i s t i c or c o u r t s h i p behavior, o f f the t e r r i t o r y , unaccounted  for.  If  and  I observed a b i r d both f l y away from the  t e r r i t o r y and r e t u r n to the t e r r i t o r y , , I c o n s i d e r e d a l l time the  interim  to  have  been  spent  o f f the  territory.  unaccounted  f o r o c c u r r e d most f r e q u e n t l y when b i r d s flew  vegetation  or  perched i n inconspicuous l o c a t i o n s .  on the p o s i t i o n s of perches  used  by  each  bird  in Time  behind  I used data each  day  to  70  c a l c u l a t e the s i z e s and l o c a t i o n s of the On all  the  evening of 14 May  and throughout  15 May,  Ribes bushes on or w i t h i n 5 m of t e r r i t o r i e s  transparent staples.  plastic  sheeting  and  secured  on 18, 20, and 22 May,  the  exclosure. conducted  I  used  the  other  and  and  observed i n  15  June,  6  with  s h e e t i n g with  4  before and  territories  on  all  after  6  Ribes  as c o n t r o l s .  3 f u r t h e r days of o b s e r v a t i o n a f t e r the  9,  I covered  and compared time budgets of  b i r d s on the 2 experimental t e r r i t o r i e s  1,  3  I repeated the above o b s e r v a t i o n procedure  territories  on  territories.  Ribes  I  season  and compared time budgets with those  May.  Results Both of the b i r d s on experimental t e r r i t o r i e s defend remained  them  their  Ribes  on those t e r r i t o r i e s  original  owners  throughout Throughout bird  while  the  of  the  season,  control  their  territories subsequent not  3  in  June.  territories,  were  Ribes  one  replaced  i n May  expand ( F i g .  territories  bushes.  they defended day  Of  the  4  remained  during was  May.  h e l d by a  May.  B i r d s d i d not s h i f t t h e i r covered  late  most of June, each of the 6 t e r r i t o r i e s  e s t a b l i s h e d by 30  to  bushes were covered, and they  until  and  continued  The  on 9 May  per  never  for either bird,  3.8).  substantially after I cent o v e r l a p with the  f e l l below 50%  and t h e i r  covered, the owner of t e r r i t o r y  owner.  While  any  territories did  In a d d i t i o n , n e i t h e r b i r d  to c h a l l e n g e any other t e r r i t o r y  on  was  his  observed  Ribes  3 f r e q u e n t l y entered t e r r i t o r y  was 2  71  EXPERIMENTAL TERRITORIES  £ 2  CONTROL TERRITORIES  rsi  J 3000LU M  i/> 20004 >-  cr  o  Q : UJ  1000-  exclosure  MAY 10  MAY 20  MAY 10  MAY 20  Figure 3.8. Changes i n p o s i t i o n s and s i z e s of t e r r i t o r i e s throughout May. Males c o n t i n u e d t o defend t h e i r t e r r i t o r i e s a f t e r I covered a l l Ribes bushes on them.  72  to  feed, but was The  only  significantly amounts  of  the  off  d e t e c t e d by i t s owner.  components  of  i n response time  spent o f f t h e i r time  rarely  they  to  birds' Ribes  3.9).  t e r r i t o r i e s , and t h i s  l o s s of time f o r a g i n g on t h e i r  d u r i n g . the  same  time  period,  i n c r e a s e d t h e i r amount of time amount  of  time, b i r d s spent visiting  in an  May  average  on  and  to  Ribes was  of  only  In  more  matched  contrast,  control t e r r i t o r i e s  territories. similar  the  t e r r i t o r i e s and  territories.  while  were  increase c l o s e l y  foraging,  budgets of a l l 6 b i r d s were very birds  were  Both b i r d s spent  birds  time spent o f f t h e i r  experimental  exclosure  spent f o r a g i n g on t h e i r  territories (Fig.  their  time budgets that changed  0.06%  decreased  their  In June, the time those  of  covered. of  the At  their  2  this time  f l o w e r s on t h e i r t e r r i t o r i e s .  Di s c u s s i o n The  responses  of males to e x c l o s u r e of t h e i r Ribes bushes  support the c o n c l u s i o n s I reached by sampling nectar on and territories  (Part  I).  The  off  i n c r e a s e d time o f f the t e r r i t o r i e s  i n d i c a t e s that these bushes were an important food source to the males, and i t territories territory  could  accounted that  possible have  defense  opportunities. territories  is  that  caused  or  this  caused  for solely  non-breeding  that  their  them  off  to  lose  their  courtship  that the males r e t a i n e d territoriality  i n e n e r g e t i c terms.  Anna's  time  them some a d d i t i o n a l e f f o r t i n  However, the f a c t  indicates  extra  Hummingbirds  cannot  their be  Ewald (1980) observed defended  territories  73  EXPERIMENTA L TERRITORIES  15-  to UJ  o o  Ribes exclosure  6-~_ off "^-<> territory  10-  Z> m  ui  5-  foraging Q  m 15-  IDS'  CONTROL  TERRITORIES  i  foragi  ,  MAY 9-U  off  territory  /  MAY 18-22  JUNE 1 -15  F i g u r e 3.9. Changes i n time budgets of 2 males a f t e r I covered a l l Ribes bushes on t h e i r t e r r i t o r i e s with p l a s t i c s h e e t i n g on 14 and 15 May. I observed s i m i l a r changes i n time budgets of 4 c o n t r o l b i r d s i n June a f t e r Ribes f i n i s h e d blooming. This r e f l e c t s the f a c t that there were no p r o f i t a b l e flowers on the t e r r i t o r i e s throughout June. Each p o i n t shown i s an average f o r 2 or 4 b i r d s on 3 different days. V e r t i c a l bars i n d i c a t e 95% c o n f i d e n c e i n t e r v a l s based on a r c s i n vx" transformed d a t a .  74  around  artificial  feeders a f t e r those feeders were removed, but  t h i s p e r i o d of defense never exceeded 2 days, even i f b i r d s previously  held their t e r r i t o r i e s  the b i r d s i n t h i s study defended while  their  Ribes  bushes  a d d i t i o n a l days i n June  f o r 10-30 days.  while  In c o n t r a s t ,  their territories  were  covered,  their  and  had  f o r 15  days  f o r about  25  provided  no  territories  p r o f i t a b l e nectar sources. This  study  hummingbirds profitable  has  sometimes energy  documented defend  sources,  that  male  breeding  and  may  North  territories  possible'  that  this  t e r r i t o r i e s I studied meadow,  is a  were  fairly  the  May,  territories  however,  as  many  first  as  i n the meadow ( F i g .  6  5  The 6  established  i n the  the breeding  season.  additional  males  held  3.2), and t h i s i n f l u x of males  occurred a t the peak of the Ribes f l o w e r i n g season.  Defense  Ribes  role  bushes  may  have  t e r r i t o r i a l i t y of these observed  Calliope  additional  more  important  males.  As  aggregations of C a s t i l l e j a  although i t may not be t y p i c a l  to h o l d t e r r i t o r i e s that  do  indicate  are  that  territoriality  a  well,  of  i n the I  have  males i n surrounding areas whose t e r r i t o r i e s  were c e n t e r e d around Nevertheless,  played  no  However, i t  r a r e phenomenon.  and these were a l l h e l d throughout  During  with  do so even i f undefended  s i t e s with abundant flowers are a v a i l a b l e nearby. is  American  there  not  provide  other  inflorescenses.  f o r C a l l i o p e males  energy,  important  my  results  benefits  of  that sometimes outweigh e n e r g e t i c c o n s i d e r a t i o n s .  Territories  without  particularly  good  profitable  system  flowers  may  provide  a  f o r i n v e s t i g a t i n g the nature of these  75  benefits. One may  p o s s i b i l i t y not yet mentioned  i s that  territoriality  insure males access to other types of food.  In p a r t i c u l a r ,  i n s e c t s are an important constitute  their  entire  i n s e c t s c o u l d account insectivorous  component  feed  particularly  concentrated,  intuitively addition,  on  f l y by unlikely  in  cases  (Tamm 1985; Chapter  insects  Redsell  1982),  and  locations  perches. insect  i n which  they  may  Defense of toward  4 ) . However,  where  they  are  but i n s t e a d seem merely to hawk any  their that  in  Therefore,  prey  other  are  have  not  it  seems  defendable.  hummingbirds  observed to feed p r i m a r i l y on arthropods (Wolf and  diets  f o r the frequent d i s p l a y s of males  passerine birds  that  birds'  d i e t before Ribes blooms.  males do not  insects  of  have  In been  1970; Montgomerie  been t e r r i t o r i a l a t those  times. The  more  plausible  i n c r e a s e s males' not  related  territories.  to  hypothesis  the p r o f i t a b i l i t y of flowers blooming  During the males'  Ribes  remained  undetected by males or evaded During  territories.  season,  on  observation  I  have  on t h e i r  seen  females  However, these females them  sessions,  separate c o u r t s h i p encounters between thus  territoriality  r e p r o d u c t i v e success, and that t h i s i n c r e a s e i s  feeding  detection.  i s that  males  either  immediately I  after  recorded only 1 8  and  females,  i t i s impossible to q u a n t i t a t i v e l y compare encounter  at times when  territories  Nevertheless,  these  d i d and  encounters  d i d not  provide  and rates  nectar.  were spaced f a i r l y evenly over  the season, and 2 of the 18 o c c u r r e d on experimental  territories  76  while Ribes was covered. observation  that  These  observations,  along  with  the  males d i d not p r e f e r e n t i a l l y h o l d t e r r i t o r i e s  in areas with the  richest  flowers,  suggest  that  flowers  on  t e r r i t o r i e s do not have much e f f e c t on r e p r o d u c t i v e success. If  the primary b e n e f i t of t e r r i t o r y defense by males i s to  maintain areas without  where  interference,  they  can  encounter  and  court  females  there are s e v e r a l p l a u s i b l e e x p l a n a t i o n s  for t h e i r c h o i c e of t e r r i t o r y  sites.  First,  d i r e c t l y adjacent to females' n e s t i n g areas females have easy access t o them.  the t e r r i t o r i e s are (Fig.  3.1), so that  Second, t h i s meadow i s f a i r l y  devoid of v e g e t a t i o n except f o r grasses,.Ribes bushes and a prominent  perches.  From  i n t r u d e r s or females, and females. reduce observed observed  these, at  males  the  same  can time  easily  few  observe  are v i s i b l e  to  T h i r d , by choosing s i t e s without C a s t i l l e j a , males may intrusion  pressure  from  other  males.  i n t e r f e r e n c e by i n t r u d i n g males  during  i n t e r a c t i o n s with females,  would be important  Given 3  that I  of  the  18  i t seems reasonable that i t  f o r a male t o maintain e x c l u s i v e  use  of an  area. If t e r r i t o r i e s p r i m a r i l y p r o v i d e a c o u r t s h i p a r e a , then the meadow can roughly be d e s c r i b e d as a l e k k i n g ground. "classical" 1981).  lek consists  Lekking  (Phaethorninae) S t i l e s and Wolf Broad-tailed 1972).  Males  of a communal d i s p l a y area  i s common (Nicholson  among 1931;  the Snow  hermit  of  these  (Selasphorus species  (Bradbury  hummingbird^  1968; Snow 1974, 1977;  1979) and may a l s o occur i n the Hummingbirds  However, a  North  platycercus)  frequently  vocalize  American (Barash and/or  77  display  i n c l e a r view of one another and sometimes i n synchrony.  Barash (1972) noted that the 3 l e k k i n g B r o a d - t a i l e d Hummingbirds he observed were each separated during  between  There  these  was  generally  little  or  no  males, and thus t h i s t e r r i t o r i a l i t y  d e f i n e d as c l a s s i c a l that  centers  t e r r i t o r i e s h e l d by C a l l i o p e males i n the meadow was  about 130 m.  occur  In c o n t r a s t ,  June i n t h i s study, the average d i s t a n c e between  of adjacent  noted  by only about 7 m.  i n many  l e k behavior. species,  However,  interaction c o u l d not be  Bradbury  (1981)  "exploded l e k s " or " q u a s i - l e k s "  i n which males are b a r e l y w i t h i n s i g h t of each other.  territoriality category.  of these C a l l i o p e males f a l l s  best  within  The this  78  CHAPTER 4 WHAT DETERMINES THE  SIZES OF BREEDING TERRITORIES  HELD BY MALE CALLIOPE HUMMINGBIRDS?  There are s e v e r a l e x i s t i n g models of o p t i m a l t e r r i t o r y (Schoener 1983;  size  1983 and r e f e r e n c e s t h e r e i n ; Ford 1983; Hixon et a l .  Lima 1984,  specifically  1986;  Jones  with  and  Krummel  energy-based  1985)  feeding  which  deal  territories.  Consequently, most e m p i r i c a l s t u d i e s on t e r r i t o r y s i z e have been i n t e n t i o n a l l y r e s t r i c t e d t o the non-breeding time  season,  at  which  i t i s thought the c o s t s and b e n e f i t s of t e r r i t o r i a l i t y  be expressed s o l e l y 1981;  i n terms of energy  Myers et a l .  1981;  1983; Davies and Houston among  birds  during  the  Carpenter et a l .  1 984; Houston  particular,  breeding  aggressively (Welty  in  1979; D i l l  and  et a_l.  territories  the  et  al.  1983; Hixon et a l . 1985).  territoriality  season,  defended  (Pyke  can  However,  i s most p r e v a l e n t  most  conspicuous  and  are o f t e n h e l d at that time  1982).  It  is  likely  that  is  affected  hummingbirds territories.  the by  reproductive the  sizes  success of  of  their  Davies and O'Donald (1976) and Harper  breeding  (1985) found  that male a r c t i c  skuas  ( C a t h a r a c t a skua) and  rubecula)  held  l a r g e t e r r i t o r i e s at the beginning of the  that  breeding season were more l i k e l y holding  smaller  territories.  to  obtain  robins  male  mates  In promiscuous  (Erithacus  than  males  breeders such as  79  hummingbirds, the e f f e c t of breeding abilities  to  territory  size  i n the r e p r o d u c t i v e  males.  address  this  chapter,  I  the  determines the s i z e s of t e r r i t o r i e s defended Hummingbirds at my I  propose  breeding  study  a general model of optimal  This  territoriality  model  all is  (Schoener 1983,  its  the and  relevant  currency  that the optimal  reproductive  similar and  to  how  this  size.  Calliope  from  outside  its  those of energy-based  in  time  and  energy  I t d i f f e r s i n that I assume that  i s access  optimal  to females r a t h e r than energy,  Hummingbird's  size  I performed  the  requirements.  The  model  c o u l d be a f f e c t e d by s e v e r a l an  experiment  on  a  male  t e r r i t o r y to t e s t the p r e d i c t i o n that a  male's t e r r i t o r y s i z e w i l l while  by  male  what  of a male, s u b j e c t to the c o n s t r a i n t that  environmental f a c t o r s . Calliope  of  r e f e r e n c e s t h e r e i n ) in that I  he s a t i s f y h i s maintenance e n e r g e t i c predicts  question  t e r r i t o r y s i z e i s that which maximizes  success  of  t e r r i t o r y size for a  energy  assume that the cost of t e r r i t o r y defense i n c r e a s e s with t e r r i t o r y  success  to  site.  male that o b t a i n s  territory.  males'  o b t a i n mates c o u l d be even more pronounced due  the p o t e n t i a l l y great v a r i a t i o n In  on  i n c r e a s e i f h i s rate of energy  f o r a g i n g away from h i s t e r r i t o r y  increases.  intake  80  The Model  Access to females If a male's t e r r i t o r y least day  remaining  time  his  can  baseline  potentially  his territory,  first  size  spend  at  access  saturation  requirements.  His  to  all  females  I  that  and that he c o u r t s a l l females he d e t e c t s .  f a c t o r that c o u l d l i m i t  is  energy  each  be spent c o u r t i n g females.  assume that a male has s o l e  enter The  must  some " minimum • time f o r a g i n g away from h i s t e r r i t o r y  i n order to meet  will  p r o v i d e s no food, he  with  a  male's  optimal  a v a i l a b l e females.  territory  Each time a male  encounters a female, he spends both  time  that female.  To recover energy l o s t  i n c o u r t s h i p , the male must  spend  time f o r a g i n g away from h i s t e r r i t o r y .  extra  and  energy  The  these two time components r e p r e s e n t s the time d u r i n g male  is  unavailable  territory. females, territory female,  to  court  other  I f T = the t o t a l time a v a i l a b l e F =  the  number  of  approximated  the  number  by  t  on  6  c  of  females T  «  T,  the  day  to  court  the  time male  spent  per  can court i s  p c  t h i s value approaches  F (see Appendix  for  In 81 h of o b s e r v a t i o n s of t e r r i t o r i a l C a l l i o p e males  territories  in  1985  (Chapter  3),  encounters with females, or one encounter f o r average.  which  that enter h i s  per  = the t o t a l  c  T + Ft However, i f F t  sum of  d e t e c t a b l e females e n t e r i n g the  d u r i n g that time, and then  details).  females  courting  I  observed only 18 every  4.5  h  on  T h e r e f o r e , i t i s u n l i k e l y that s a t u r a t i o n with females  81  is  an  important  realistic  factor  t o assume t h a t  f e m a l e s he d e t e c t s The day, be  number  an  of  females  that  and i t i s  courtship  with a l l  function  function  enter  a male's t e r r i t o r y per  by him i f he i s p r e s e n t , of  i s not n e c e s s a r i l y  allometric  size,  on h i s t e r r i t o r y .  increasing  relationship  territory  a male c a n a t t e m p t  and c a n be d e t e c t e d an  limiting  territory  is  area.  linear, I will  likely  to  Because  assume  i t  this  to  be  of the form z  f  F = f k, A where and his  f i s density  kf  and  from  be an i n c r e a s i n g  Cost  of d e f e n s e energy  through extra intruders the  total  allometric  and t h e number  be l e s s t h a n  also  Because  and  area,  i f t h e owner must  defend  c a n be e x p r e s s e d temporal  cost  in units of  of t e r r i t o r y  i = the density  Z-, a r e c o n s t a n t s .  entire  area  must be  cost  of time.  expelling  actually likely  area.  intruders  the  he  of defense w i l l  of t e r r i t o r y  chasing  spend a l l h i s time  of females  The c o s t  foraging,  time where  F.  function  spent  time  function  However,  is territory  i n t r u d i n g m a l e s , he c a n n o t  his territory,  encounters w i l l  A  i n the area,  Zf a r e c o n s t a n t s .  territory  surveying  of females  of  I will  intruders  recovered expelling  assume  that  i s a l s o an  o f t h e form  lost = i k ; A '  of p o t e n t i a l  2  intruders  I f T i s t h e amount  of  i n the area, time  and k-,  potentially  82  available foraging of  for courting requirement  females  territory  the  females,  taking  i n t o account  the baseline  f o r meeting maintenance c o s t s ,  male  encounters  per  day  as  the  a  number  function of  size i s  f (A) = (T - i k; A ) f k A Z  Z<  (  As  long  as  function  a l l these  parameters a r e greater  than  zero,  this  i s m a x i m i z e d when  A  I" ^ ] ' Z  -  iki(z 4-z[) T  Therefore,  optimal  decreases with The  i,  above  influence  territory k;,  that  suggests  s i z e s of breeding  territory  (Dill  size  1978; H i x o n  will  decrease  The c o s t  a l s o vary  to habitat.  were  difficult  (high  k; ) and a c c e l e r a t e  the  center  territory male's  testing  size.  ability  territory One  of  to  the  some  territories  intruder pressure. according  T and  with  Z j , and  a n d Z; .  equation  energy-based models  size increases  detect,  factors  1980), t h i s  model  predicts  this cost ( Z; >>  could 1)  intruders  i n which be h i g h  with  resulting  in  females  (z  f  <<  could  intruders on a v e r a g e  distance a small  S i m i l a r l y , a sharp decrease with distance to detect  Like  i n response t o increases i n  In a h a b i t a t  territory,  could  d e f e n d e d by m a l e s .  of e x p e l l i n g those  sharply  that  1) w o u l d l i m i t  from optimal in  a  optimal  size. prediction is  that  a  response t o increases  particularly male's  amenable  territory  size  to will  experimental increase i n  i n t h e r a t e a t w h i c h he c a n o b t a i n  energy  83  while  f o r a g i n g away f r o m  energy  at  a  faster  rate,  time  required  to  obtain  time  required  to  recover  (decreased  k; ) .  territory  size  prediction  by u s i n g a  territorial foraging  If  it  energy  energy  lost  a male c o u l d  his  intruders  s i g n i f i c a n t , optimal  the  could  baseline  expelling  increase.  increase  Hummingbird  his  obtain  ( i n c r e a s e d T) a n d  were  consequently to  both  while  changes  feeder  his  If  should decrease  maintenance  should  from  territory.  these  Calliope  away  his  I  rate  tested at  obtain  this  which  energy  a  while  territory.  Methods The  male  throughout primarily of  the  metal  most used  perch  For  June, a  by w h i c h  replaced similar  increments  it  I  the  the  with  an  owner  about  5  a  had  m  begun  until  At  location,  the  owner,  was n e v e r  but  the  he  S end  3 m high  exclusively  perch  conspicuously  periodically  about  territory  at  almost  s o l u t i o n on t h e  territory this  30 m SW o f  experiment,  placed a  N on t h i s  to  inconspicuous clear I  I  3.1)  over  which  4.1).  placed  solution. of  used  distance  (Fig.  30% w/w s u c r o s e time  he  (Fig.  one a n o t h e r  experiment,  which  6  Before the  10 m o f  this  area,  territory  1985.  within  and measured  containing  a  3 perches  in that  2  defended  o f May a n d J u n e  expelled intruders On  h,  studied  territory.  thereafter, he  I  the  it  metal  feeder  was  colored  territory. use  the  glass  bottle  moved  this  After  feeder,  in a grove  visited  feeder  of  frequently  d i s c o v e r e d by a n y  other  1 I  containing  was l o c a t e d w e l l  perch  feeder  off  by the  trees. by  bird.  the  84  INCONSPICUOUS  METAL PERCH  0  20 m DOUGLAS FIR COVER > 2 m DECIDUOUS COVER > 2 m  x  DECOY FEEDER  gravel road  F i g u r e 4.1. Design of experiment to examine the e f f e c t of energy a v a i l a b i l i t y on the s i z e of t e r r i t o r y defended by a male C a l l i o p e Hummingbird. The male perched p r i m a r i l y on a metal perch p r o v i d e d , and defended the area N of that perch. Intruders e n t e r e d the t e r r i t o r y to sample conspicuous decoy feeders c o n t a i n i n g e i t h e r water or a r t i f i c i a l sweetener. On 5 of the 10 mornings on which the experiment was conducted, the male had access to an inconspicuous feeder o u t s i d e h i s t e r r i t o r y c o n t a i n i n g a 30% w/w sucrose s o l u t i o n .  85  On 2 June, I a l s o p l a c e d 6 feeders sucrose the  solution  territory  used  on  containing  a  20%  w/w  the o p p o s i t e s i d e of the g r a v e l road N of  ( F i g . 4.1).  These feeders  were  discovered  and  by s e v e r a l male and female C a l l i o p e Hummingbirds and a few  Rufous Hummingbirds. decoy  feeders,  artificial  On 7 June, I  2  containing  sweetener  equivalent  sweetness  replaced  pure  (Sucaryl; to  the  water Abbott  sugar  4 and  of  these 2  with  containing  Laboratories)  solution.  of  I subsequently  s h u f f l e d the l o c a t i o n s of these feeders every few hours i n order to  t r a i n these b i r d s to c o n t i n u a l l y sample  the  artificial  decoy  feeders  sweetener the  birds  the decoys.  I  used  simply to c r e a t e another category of needed  to  sample;  birds  easily  d i s t i n g u i s h e d i t from the r e a l sucrose s o l u t i o n . I  conducted  mornings by  from 11-14  the  experiment  and 17-22  June.  sampling  I measured  to  09:00 on  territory  10  size  the  proportion  of  distance  on  I compared the p r o b a b i l i t i e s of response at  s i z e when he had access to h i s feeder.  s e l e c t e d day of each p a i r . he  began  randomly was  i t at about 05:00, and t h e r e f o r e  had been f e e d i n g from i t f o r about began.  territory  On mornings on which h i s feeder  visiting  was  The experiment c o n s i s t e d  p a i r s of days, with t h i s feeder a v a i l a b l e on one  available,  each  mornings when the inconspicuous feeder was and  a v a i l a b l e to determine i f the owner i n c r e a s e d h i s  5  intruders  each of these feeders that were chased or d i s p l a y e d to  by the owner.  of  06:30  p l a c i n g decoy feeders 5, 15, 25, 35, and 45 m N of the metal  perch ( F i g . 4.1), and o b s e r v i n g  not  from  1 1/2 h b e f o r e the experiment  On other mornings, the feeder was gone when he awoke.  86  At 09:00 each day, feeder  if  i t had  I  replaced  the  owner's  inconspicuous  been removed the night before the experiment.  I a l s o moved a l l decoy feeders N of the g r a v e l road, and two  of  them  with  r e p l a c e d these decoys  in  sucrose  solution.  A f t e r dark each n i g h t , I  sucrose c o n t a i n i n g feeders with decoys, p l a c e d  the p o s i t i o n s shown i n F i g .  them with opaque p l a s t i c bags u n t i l 16  filled  4.1,  and covered  c o n t a i n i n g feeders and 08:30. in  T h i s was  those  3 decoys on the t e r r i t o r y  to prevent  positions  from  contained  c o n t a i n i n g feeders were p l a c e d on the  sucrose.  territory  06:30  to  feeders  No at  On  sucrose  i n t r u d e r s from l e a r n i n g that  never  a l l of  06:30 the next morning.  June, when I d i d n ' t conduct o b s e r v a t i o n s , I p l a c e d 2  6  sucrose  any  other  time d u r i n g the experiment.  Results The  presence  of  the  owner's  n o t i c e a b l e e f f e c t on the s i z e of 4.2).  The  data  difference in male  available  (Fig.  owner's  but  the  probability  4.2a).  For 8 of 481  reasonable  were  not  of  results  were  too  estimates  few  not  have  defended  responding  not much to  and was  noticeably  sex  a  (Fig.  v i s i t s to decoy feeders  i d e n t i f y the s p e c i e s and/or  i n c l u s i o n or e x c l u s i o n of these There  he  at any d i s t a n c e when h i s feeder was  i n t r u d e r s , I c o u l d not bird,  territory  did  i n d i c a t e f a i r l y c l e a r l y that there was  the  intruding  feeder  of  a f f e c t e d by  an not by the the  data.  intrusions  by  females  to  obtain  of the owner's p r o b a b i l i t y of responding  to  87  5  15  25  35  45  DISTANCE OF INTRUDER FROM PERCH (m)  F i g u r e 4.2. C o m p a r i s o n o f t e r r i t o r y d e f e n s e when t h e m a l e d i d and d i d n o t have a c c e s s t o t h e a r t i f i c i a l f e e d e r o u t s i d e his territory. Vertical bars represent 95% c o n f i d e n c e intervals for probabilities of response. These data illustrate that availability of the feeder d i d not significantly affect territory size. Responses t o females probably constituted attempted courtship rather than t e r r i t o r y defense.  88  them at each d i s t a n c e .  I t i s p o s s i b l e that h i s p r o b a b i l i t i e s of  responding to males and  females were q u i t e s i m i l a r  The  natures  of  the  (Fig.  responses, however, were q u i t e d i f f e r e n t .  The owner chased males i n 83 of 84 encounters, but females  4.2b).  i n 7 of 27 encounters.  only  chased  He d i v e - d i s p l a y e d at females i n  21 of those 27 encounters, but never d i v e - d i s p l a y e d at males all.  He  began d i v e ascents 5 to 10m  before reaching females,  and continued to d i v e at them and perform other they  perched  territory.  in  at  displays  until  inconspicuous l o c a t i o n s or flew away from the  T h i s type of behavior probably c o n s t i t u t e d  attempted  c o u r t s h i p r a t h e r than a g g r e s s i o n . Although the presence of the inconspicuous feeder noticeably  affect  the  owner's  territory  s i g n i f i c a n t l y a f f e c t other aspects of h i s inconspicuous  feeder was  d i s p l a y s toward Mann-Whitney  b i r d s not v i s i t i n g decoy  U  test;  d i r e c t e d at p a s s e r i n e sp.),  Nashville  Fig. birds  Warblers  4.3a).  This  (p  did  When  the  =  including  flycatchers  (Empidonax Rufous-sided  Robins  and  l e a s t some aspects of h i s * b e h a v i o r were normally energy addition,  when  the  feeder was  4.3b).  that  at  limited.  a v a i l a b l e , there were  chases of C a l l i o p e males not v i s i t i n g decoy feeders (p = Mann-Whitney U t e s t ; F i g .  (Turdus  i n d i c a t e s that the presence of the  feeder d i d i n f l u e n c e the owner's e n e r g e t i c s t a t e ,  In  0.008;  Most of these d i v e s were  (Vermivora r u f i c a p i l l a ) ,  result  it  many more d i v e  feeders  Towhees ( P i p i l o erythrophthalmus), and American migratorius).  not  size,  behavior.  a v a i l a b l e , he performed  did  fewer 0.008;  89  F i g u r e 4.3. D i f f e r e n c e s i n the number o f (a) d i v e displays and (b) c h a s e s by t h e t e r r i t o r y owner when he h a d a c c e s s to a feeder. Most d i v e d i s p l a y s were directed at passerine birds rather than hummingbirds. Chases noted here are those of intruding males not v i s i t i n g decoy feeders. These i n t r u s i o n s , w h i c h may have a l l been by a single male, appeared to constitute challenges for the territory.  90  Di s c u s s i o n Given  that  the  experiment  was  t e r r i t o r y , e x t r a p o l a t i o n of the caution. nature that  results  improved access  territory  on  should  N e v e r t h e l e s s , the r e s u l t s do suggest of t e r r i t o r i a l i t y  only  be  energy  did  not  one  done  with  i n s i g h t s i n t o the  of at l e a s t that one male.  to  affect  The the  result owner's  s i z e , but d i d i n f l u e n c e other aspects of h i s behavior  r a i s e s two q u e s t i o n s . after  performed  energy  success?  First,  manipulation  could have  the  changes  influenced  Second, what f a c t o r s d i d determine  in  his the  behavior  reproductive size  of  his  territory?  Does energy Tamm site  e f f e c t males' r e p r o d u c t i v e success? (1985)  a l s o observed  increased their  access  to  a  that C a l l i o p e males at the same  frequency  feeder.  of  However,  dive  displays  given  when  given  that most of the d i v e s  observed  i n both s t u d i e s were at p a s s e r i n e s , i t i s not c l e a r  or  males  how  benefit  from  passerines provide s t i m u l i intruding  hummingbirds,  t h i s energy  expenditure.  s i m i l a r to but weaker than so  that  responses  t e r r i t o r y owners' e n e r g e t i c c o n s t r a i n t s are The decreased visiting  decoy  number  feeders  r e p r o d u c t i v e success. intruder him.  may  chases  of  intruding  his  those  of  only  if  males  be more i n d i c a t i v e of an e f f e c t occurred  flew w i t h i n 1 m of the owner and appeared  when  Perhaps  reduced.  Most of these chases  The owner appeared  vigorously  of  occur  to expel  intruders  inconspicuous  if  more  feeder  was  after  not on an  to c h a l l e n g e quickly  and  available.  91  I n t r u d e r s may days,  resulting  territorial dominate hold a  in  the  male's  i t may  territory.  of  affects  on  those  chases.  a  ability  to  a f f e c t h i s a b i l i t y to e s t a b l i s h  and  although  the  his  If  results  of  the  that energy a c c e s s i b i l i t y does not normally  territory size,  it  could  nevertheless  be  an  important  of a male's r e p r o d u c t i v e success.  What does determine Other  persistent  frequency state  Therefore,  suggest  determinant  lower  energetic  intruders,  experiment limit  consequently have been l e s s  than  territory  size?  access to energy, p o s s i b l e f a c t o r s  influencing  t e r r i t o r y s i z e suggested by the model were i n t r u d e r p r e s s u r e and intruder d e t e c t a b i l i t y . the  possibility  The  for  the  to  upper  territories  detect limit  defended  intruders  on by  ability.  may  territory  hypothesis  responding  generally  size.  Given  impose that  an the  C a l l i o p e males at t h i s study s i t e are f o r hummingbirds  (Fig.  3.8),  it  i f t h e i r s i z e s are l i m i t e d only  Time and energy c o n s t r a i n t s may  important f o r s p e c i e s such as  the  hermit  h o l d small t e r r i t o r i e s on " c l a s s i c a l " male  This  s i m i l a r p r o b a b i l i t i e s of h i s  should perhaps not be s u r p r i s i n g detection  l i m i t e d only  decrease i n h i s responsiveness with  probably l a r g e r than t y p i c a l  by  intruders.  suggest  females.  Ability absolute  detect  gradual  d i s t a n c e and the f a i r l y to males and  experiment  that the s i z e of the t e r r i t o r y was  by the owner's a b i l i t y to accounts  r e s u l t s of the  leks.  hermits g e n e r a l l y h o l d t e r r i t o r i e s  hummingbirds,  be more which  However, given that  i n densely wooded areas  92  where v i s i b i l i t y  i s poor  (Skutch  1964), i t may  them to defend anything but very small because  territories.  As  well,  male hermits are t i g h t l y packed on l e k s , t e r r i t o r y s i z e  may  be p r i m a r i l y  of  males  experiments used  be impossible f o r  on  determined  each  lek  by  i n t e r - l e k spacing and the  (Bradbury  s i m i l a r to that  1981).  reported in t h i s  to d i s t i n g u i s h between these  number  Energy manipulation chapter  possibilities.  could  be  93  Appendix  Female  saturation  T  = the length  o f t i m e a v a i l a b l e p e r day f o r p e r c h i n g  F  = t h e number  of d e t e c t a b l e  during C t  that  females e n t e r i n g  territory  time  = t h e number o f f e m a l e s t h a t c  the  = t h e amount o f p e r c h i n g  t h e male  courts  t i m e t h e male l o s e s p e r f e m a l e he  courts  Because  the  females,  the proportion  etc  male  Assuming  T  territory female,  t  spends  that  units  c  of h i s time that the  i s independent  probability  o f whether  t h e number o f l o s t  =  c  =  FT _ , Ft 1 - — c  As  F  approaches  approach  0,  c  co  ,  approaches  c  Fct  each of c  courting  is  of a female e n t e r i n g h i s courting  opportunities  will  another be  c  —-  TF T + Ft  approaches F.  he s p e n d s  he i s busy  courtship  F - c Therefore  of time c o u r t i n g  c  JL .  As  either  t  c  or F  94  Intruder  pressure  A = territory = density  f  fk A f  Z <  area  of  = the  females  number of  females  t h e male e n c o u n t e r s  per  available  time T = potential i  = density  ik;A  Z |  =  p e r c h i n g time per  of  intruding  time  spent  energy  The  amount of  males  chasing  lost  day  intruders  while chasing  time a v a i l a b l e  or  f o r a g i n g to  recover  intruders  to c o u r t  females  is  T- ik,A Therefore, territory  the  number of  size  females  encountered  as  a  function  of  is  f (A) =  *m • A  =  (T- ik,A  ) f k«A  Z f f k ^ A ^ ^ z ^ ^ i k . - f k f A  2  ^  2  ' -  1  1 if  Therefore, as  l o n g as  d  f  ( > dA A  there z  ;  *  0.  -  0  ^en  A  =  iki(z +-z ) T  is a single  ;  i n f l e c t i o n p o i n t which i s d e f i n e d  95  d f(A) _ = fk A " Z f  ^ z 1 ^z T - ^ z + z - i ^ z , + Z f jikj A  2  f  r  f  2  f  +  dA Substituting  the value d  Because value  * dA f  A  is  always point  f  i s the optimal  inflection  point  yields  f  parameters  negative. i s always  at the  = -z,z k fTA  )  a l l t h e component  inflection point  Az,  of  are always  Therefore, a maximum,  territory  area.  the value  and  positive, of  the value  f(A) of  this at i t s at  that  96  CHAPTER 5 CONCLUDING REMARKS  The on  r e s e a r c h presented  territorial  behavior  i n t h i s t h e s i s extends e a r l i e r of  North American hummingbirds.  r e s u l t s i n t u r n suggest  further questions  of  particular,  hummingbirds  in  f o r a g i n g behavior,  and  chapter,  review  I briefly  work  mating  about  and  systems  the primary  territoriality  about in  The  territoriality,  general.  In  this  r e s u l t s of each study, and  discuss p r o f i t a b l e d i r e c t i o n s for further research. The  simulations  in  2 suggest  Chapter  that a hummingbird  f o r a g i n g on an energy-based f e e d i n g t e r r i t o r y c o u l d avoiding  flowers  visited  benefit  r e c e n t l y d u r i n g a f o r a g i n g bout, but  would not b e n e f i t s i g n i f i c a n t l y by a v o i d i n g patches previous  bouts.  This  often  than  apparent  pattern  foraging  bouts  on  systematic  concentrate The  would to  while be  the  foraging  hummingbirds  expected locations  (Sutherland, of  that  t o a b i r d , but would allow  spatial  distribution  exploit  easily  of  l a t e r on.  which  on  nectar  flowers  there i s no  they  initiate  F u r t h e r e m p i r i c a l work  hummingbirds  suggest  revisit  chance,  at  in prep.).  flowers would not only allow a higher intake  by  on o b s e r v a t i o n s of within-bout simulations  visited  r e s u l t provides a p l a u s i b l e explanation  f o r the o b s e r v a t i o n t h a t , less  by  should  therefore  f o r a g i n g movements.  short-term  avoidance  immediate r a t e  of  of  energy  i t to c r e a t e a c o a r s e - g r a i n e d availability  Although  that  i t could  i t has been acknowledged i n  the f o r a g i n g theory l i t e r a t u r e that c o a r s e - g r a i n e d  distributions  97  of food a v a i l a b i l i t y can be e x p l o i t e d 1984),  I  know  possibility  more  efficiently  of no p r e v i o u s s t u d i e s that have d e a l t with the  that  creation  of  such  distributions  important component of f o r a g i n g s t r a t e g i e s . developments closely  be  an  might p r o f i t a b l y address t h i s i s s u e , as w e l l as the  related  issue  of whether the f o r a g i n g s t r a t e g i e s that r a t e of energy  s t r a t e g i e s over a longer time  scale.  The r e s u l t s of Chapter Hixon  may  Further t h e o r e t i c a l  p r o v i d e the h i g h e s t immediate  1980;  (Green  1980)  3  confirm  intake are the best  the  suggestion  (Ewald  that p u r e l y energy-based models do not account  f o r the behavior of t e r r i t o r i a l hummingbirds d u r i n g the breeding season.  Although  this  study i n d i c a t e s that other b e n e f i t s of  t e r r i t o r i a l i t y must sometimes outweigh e n e r g e t i c  considerations,  it  of  only s u p e r f i c i a l l y  benefits  are,  and  deals what  prominent  the  therefore  l o c a t i o n s of t e r r i t o r i e s . around  with  Because  determines  what  the  territories  were  those  preferred situated  perches adjacent to females' n e s t i n g areas, i t  i s p l a u s i b l e that l o c a t i o n s with these particularly  issues  good  for  characteristics  may  a t t r a c t i n g and c o u r t i n g females.  be  Given  that males r e a d i l y use a r t i f i c i a l perches, t h i s h y p o t h e s i s might be addressed by attempting to territories  and  to  l o c a t i o n s by s t r a t e g i c would  create  manipulate territories  in  of  existing  previously  l o c a t i o n of such perches.  This  unused  approach  be analogous to m a n i p u l a t i n g q u a l i t y of energy-based  breeding t e r r i t o r i e s u s i n g feeders (Ewald Ewald  quality  1980, Chapter  and  Carpenter  non1978;  1985). 4  suggests p o s s i b l e f a c t o r s determining the s i z e s  98  rather  than  experiment territory detect  the  locations  performed s i z e might  intruders.  needed to  on  one  territories. male's  Results  territory  suggest  experiments  on  Although  of t h i s experiment  the  repetition  generality  of  the  abilities  conclusion  I t would a l s o be u s e f u l to perform male  Snow 1968;  Snow 1974;  conducted  in  determining  order  preferred  territories.  It  comparative  energy  S t i l e s and Wolf  during  the  sources  1979).  to  understand  sites  for  the  further  I  feel  season  and  may  research  proximate  territories  factors  sizes  of  should be p o s s i b l e to o b t a i n t h i s i n f o r m a t i o n  of t h e i r environments,  Understanding of  breeding  At p r e s e n t , more  by o b s e r v i n g the b e h a v i o r a l responses of manipulations  that  hummingbird t e r r i t o r i a l behavior i s probably more  research  needed  is  hermit hummingbirds, some of which defend  u l t i m a t e l y prove the most rewarding. is  to  r e q u i r e d to  complex d u r i n g the breeding season than at other times, that  that  However,  much s m a l l e r breeding t e r r i t o r i e s c o n t a i n i n g no (Skutch 1964;  an  males'  s i z e i s not l i m i t e d by the time and energy  expel i n t r u d e r s .  of  simply be l i m i t e d by  determine  territory  of  these  questions.  proximate  First,  what  territorial  to  as I have d e s c r i b e d above.  factors  will  determines  lead  to  two  which males h o l d  t e r r i t o r i e s , and  which  males  sites?  is  male's r e p r o d u c t i v e success a f f e c t e d by  Second,  a  whether or not he holds a  hold  males  territory  territories  and/or  on  by  the  preferred  size  and  l o c a t i o n of h i s t e r r i t o r y ? Given  that  observed  encounters  between  hummingbirds are i n f r e q u e n t , measurements  of  male and the  female  reproductive  99  s u c c e s s o f m a l e s w i l l p r o b a b l y be d i f f i c u l t such  measurements  will  allow  direct  to obtain.  investigation  possibility  that natural s e l e c t i o n takes place  among  the  of  This  basis  their  territorial  behavior.  e x c i t i n g a v e n u e o f i n v e s t i g a t i o n , a n d one n o t s t u d i e s of non-breeding  territoriality.  However, of  males  the on  w o u l d be an  clearly  open  to  100  LITERATURE CITED Aldrich,  Altum,  E.C. 1939. N a t u r a l h i s t o r y of the A l l e n Hummingbird (Selasphorus a l l e n i ) . M.A. Thesis. U n i v e r s i t y of C a l i f o r n i a , Berkely. J.T.B. 1868. Munster.  Anderson,  Der  vogel  D.J. 1983. Optimal salesman. Theor. Pop.  und  sein  leben.  Germany,  f o r a g i n g and the traveling Biol. 24: 145-159.  Armitage, K. 1955. 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