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Factors affecting reproduction in great blue herons (Ardea herodias) Simpson, Keith 1984

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FACTORS AFFECTING REPRODUCTION IN GREAT BLUE HERONS (Ardea herodlas) By KEITH SIMPSON B.Sc, Simon Fraser University, 1 9 7 ^ A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s thesis as conforming to the required standard /7 THE UNIVERSITY OF BRITISH COLUMBIA September 1 9 8 ^ © Keith Simpson, 1 9 8 ^ In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head of my department or by h i s or her representatives. I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of 2-QO t - 0 6 ^  The University of B r i t i s h Columbia 1956 Main Ma l l Vancouver, Canada V6T 1Y3 i i ABSTRACT Reproductive success and size of 15 heronries were monitored from 1977 to 1980 i n south coastal B r i t i s h Columbia. My main objectives were to inventory e x i s t i n g colonies, assess changes i n colony status from h i s t o r i c a l information, and document factors important to reproduction. I co l l e c t e d data on banded herons at one colony to describe the movement of herons between and within heronries, and to i d e n t i f y the c h a r a c t e r i s t i c s of individuals that related to reproductive success. Many heronries formerly i d e n t i f i e d were no longer present while others were at new locations or much larger i n s i z e . Heronry movements followed destruction of the nest trees or reproductive losses i n several cases. Relocation normally occurred i n the f i r s t or second year following heavy losses of young or adults. Disturbances by people sometimes forced herons to leave t h e i r nests and increased losses of eggs and young to predators. Severe predation continued a f t e r human disturbance had stopped. The number of young raised per successful nest was not a useful measure of reproductive success, since i t varied l i t t l e among colonies. The percentage of nests that succeeded, or numbers of young raised per breed-ing p a i r , provided better measures of reproductive success. Marked herons at one colony were not attached to s p e c i f i c nests or mates, and many adults probably switched colonies each year. Unsuccessful pairs did not renest i n the same colony during the same breeding season. Although herons i n central nests were more successful than those near the edge of the i i i colony, central nests were not occupied by birds which were dominant on feeding areas. Herons are probably attracted to colonies to f i n d new mates each year, and to reduce the v u l -n e r a b i l i t y of t h e i r young to predators. Although 78 percent of the herons i n one colony fed i n the nearest feeding areas, many chose to travel further to feed. These distant feeders suffered higher nest losses to predators, probably because they l e f t t h e i r nests unattended more often than other l o c a l l y feeding bir d s . Some evidence suggested that males tr a v e l l e d further than females, and were les s attentive at the nest. Males may play a dominant r o l e i n i n i t i a t i n g colony relocations. The lack of attachment of herons to nest s i t e s or mates helps to explain the changes i n size and frequent movements of heronries i n coastal B.C. i v Table of Contents PAGE Abstract i i Table of Contents i v L i s t of Tables v i L i s t of Figures v i i i Acknowledgements i x General Introduction 1 CHAPTER I — L o c a t i o n , Size, History and Reproductive Success of Heronries i n South Coastal B.C. 3 Introduction 4 Methods 5 Results Heronry locations, sizes and general descriptions 8 H i s t o r i c a l and present colony data 11 Reactions to human disturbance 2 5 Reproductive success 2 5 Discussion Colony sizes 30 Reactions to human disturbance 3 ^ Predation 3 5 Reproductive success 3 8 Conclusions 42 CHAPTER II—Movements, Behavior and Breeding Success of Banded Herons at Pender Harbour 4 4 Introduction 4 5 Methods 4 7 Results Capture and banding 5 2 Resightings and movements of banded herons 52 Feeding areas and nesting status 53 Nest s i t e s , mates and feeding areas 5 8 V PAGE Results (Cont.) Feeding on t i d a l areas 61 Feeding at the b a i t tanks 6 k Aggressive Interactions 64 Sex and reproductive success 6 7 Discussion Colony formation 68 Predation, reproductive success and adult survival 71 Colonial nesting and information exchange 73 Sex and reproductive success 75 Conclusions 7$ General Discussion and Recommendations 79 Literature Cited 81 Appendix 86 v i L i s t of Tables PAGE 1-1 Number of successful nests i n heron colonies surveyed i n south coastal B r i t i s h Columbia. 10 1-2 Comparison of past and present sizes of four colonies In the lower mainland. 11 1-3 Descriptions of the study colonies. 12 1-4 Size and locations of the Stanley Park colony from 1921 to 1 9 8 0 . 2 2 1-5 Number of nests abandoned and newly-occupied between survey dates at eight colonies i n 1 9 7 8 and two i n 1 9 7 9 . 28 1-6 Mean numbers of young fledged per suc-c e s s f u l nest and per breeding p a i r at eight colonies i n 1978 and two i n 1 9 7 9 . 2 9 1- 7 Number and % of nest platforms occupied during the breeding season at eight heronries. 3 1 2 - 1 Number of sightings of banded herons at Pender Harbour from 1 9 7 8 to I 9 8 O categorized by frequency of observation, sighting l o c a t i o n and presence i n the colony. 5 5 2 - 2 Comparison of the reproductive success of l o c a l feeding (LF) and distant feeding (DF) herons at Pender Harbour i n I 9 7 8 and 1 9 7 9 . 56 2 - 3 Comparison of the number of sightings of distant feeding (DF) herons to determine i f unsuccessful birds were seen less often than successful birds a f t e r t h e i r young were l o s t . 5 7 2 - 4 Mean distances (m) of successful and unsuccessful nests from the center of the colony i n 1 9 7 8 and 1 9 7 9 at Pender Harbour. 5 9 v i i PAGE 2 - 5 Total number of banded herons from each sector of the Pender Harbour colony seen feeding together on four d i f f e r e n t days i n 1 9 7 9 . 59 2 - 6 The number of sightings for each member of 11 banded pairs at f i v e feeding l o c a -tions i n Pender Harbour i n 1 9 7 9 . 60 2 - 7 Factors a f f e c t i n g the weight (gm) of prey caught by herons on t i d a l feeding areas. 62 2 - 8 Comparison of the weight of prey caught by herons which successfully raised young and those that f a i l e d to ra i s e young i n 1 9 7 9 . 64 2 - 9 Comparison of the dominance of banded herons with d i f f e r e n t reproductive success and feeding areas at Pender Harbour i n 1 9 7 9 . 6 6 2 - 1 0 Comparison of the t o t a l number of aggressive interactions f o r banded herons i n r e l a t i o n to reproductive success and feeding areas at Pender Harbour i n 1 9 7 9 . 6 6 2 - 1 1 Changes i n reproductive success for eight males and six females from 1 9 7 8 to 1 9 7 9 . 6 7 v i i i L i s t of Figures PAGE 1-1 Heron colony l o c a t i o n s i n the lower mainland and Sunshine Coast areas of B r i t i s h Columbia. 9 1-2 Mean numbers of young fledged per s u c c e s s f u l nest a t 10 h e r o n r i e s from 1 9 7 7 to 1 9 8 0 . 2 7 1-3 The r e l a t i o n s h i p between the percentage of nest platforms occupied a t f l e d g i n g and the percentage of unsuccessful p a i r s at e i g h t c o l o n i e s i n 1 9 7 8 and two i n 1 9 7 9 . 32 1- 4 The r e l a t i o n s h i p between the percentage of nest platforms occupied at f l e d g i n g and the percentage of unsuccessful p a i r s at c o l o n i e s i n Quebec and _ Montana. 4.1 2 - 1 Locations of t r a p s , o b s e r v a t i o n b l i n d s and s e i n i n g s i t e s a t Pender Harbour. 48 2 - 2 Frequency d i s t r i b u t i o n of the number of s i g h t i n g s of 41 l o c a l and 2 5 d i s t a n t feeding banded herons at Pender Harbour. 54 2 - 3 C o r r e l a t i o n between the mean weight o f prey caught by herons (N = 9 0 7 ) and the mean weight caught by s e i n i n g (N = 244) f o r f i v e months i n 1 9 7 9 . 6 3 i x ACKNOWLEDGEMENTS The Canadian W i l d l i f e S e r v i c e provided f i n a n c i a l support f o r t h i s study and I e s p e c i a l l y wish to thank Dr. John K e l s a l l , Dr. Don Flook, Rick McKelvey and Pam Whitehead, a l l of C.W.S., f o r a s s i s t a n c e and advice during t h i s study. Rhonda Markel, Margrlet Wyborne, Scott Forbes, B i l l Harper and Tony Wideski a s s i s t e d w i t h summer fi e l d w o r k and c h e e r f u l l y t o l e r a t e d some long and unusual working hours. Many i n d i v i d u a l s a s s i s t e d i n l o c a t i n g c o l o n i e s or allowed access to t h e i r property where c o l o n i e s were l o c a t e d . S p e c i a l thanks a l s o go to the Cameron's at Pender Harbour, p a r t i c u l a r l y B i l l , Don and Jim, who allowed us access to t h e i r b a i t ponds at a l l hours of the day and nigh t to capture and observe herons. F i n a l l y ^ I wish to thank my advisor s at U.B.C., p a r t i c u l a r l y Dr. Jamie Smith, f o r advice and d i r e c t i o n i n w r i t i n g t h i s t h e s i s . I am very g r a t e f u l to a l l o f the above f o r t h e i r a s s i s t a n c e . 1 GENERAL INTRODUCTION G r e a t b l u e herons (Ardea h e r o d l a s ) a r e a l a r g e , c o n s p i c u -ous b i r d d i s t r i b u t e d a c r o s s N o r t h A m e r i c a . Herons a r e p r e d a t o r s and f e e d on a wide v a r i e t y o f f i s h e s , a m p h i b i a n s , r e p t i l e s , mammals and b i r d s . T h e i r h a b i t o f f e e d i n g i n marshes, open f i e l d s and t i d a l a r e a s makes them e a s i l y o b s e r v a b l e , and th e y a r e a f a m i l i a r s i g h t i n a r e a s where t h e y o c c u r . G r e a t b l u e herons n o r m a l l y n e s t i n groups and, once l o c a t e d , heron c o l o n i e s can be e a s i l y o b s e r v e d . Because o f t h e i r wide d i s t r i b u t i o n , easy o b s e r v a b i l i t y , and t h e i r p o s i t i o n n e a r the top o f the f o o d c h a i n , herons were c o n s i d e r e d a p o t e n t i a l l y good i n d i c a t o r s p e c i e s f o r m o n i t o r i n g e n v i r o n m e n t a l c o n t a m i n a t i o n (Canadian W i l d l i f e S e r v i c e 1 9 7 1 ) . T h i s s t u d y was i n i t i a t e d by the Canadian W i l d l i f e S e r v i c e i n 1977 p r i m a r i l y t o i d e n t i f y and enumerate heron c o l o n i e s i n the l o w e r m a i n l a n d o f B r i t i s h Columbia (Simpson and K e l s a l l 1 9 7 8 ). My s u r v e y s were l a t e r i n c o r p o r a t e d i n t o a n a t i o n - w i d e program t o m o n i t o r g r e a t b l u e heron p o p u l a t i o n s (Des Granges 1 9 8 0 ) and to d e v e l o p a d i s t r i b u -t i o n and abundance a t l a s f o r B.C. (Forbes e t a l . 1 9 8 3 ) . There have been many s t u d i e s o f h e r o n s , most r e l a t i n g t o t h e i r n e s t i n g o r f e e d i n g h a b i t s ^ i n t h i s and o t h e r a r e a s . I n C h a p t e r I , d a t a c o l l e c t e d a t 15 h e r o n r i e s i n so u t h c o a s t a l B.C. i s compared to h i s t o r i c a l d a t a f o r the a r e a and to r e s u l t s o f s t u d i e s i n o t h e r a r e a s . The e f f e c t s o f c o l o n y movements and changes i n s i z e , p r e d a t i o n and human d i s t u r b a n c e a r e a s s e s s e d by comparing the r e p r o d u c t i v e performance o f c o l o n i e s s u b j e c t 2 to varying influences from 1 9 7 7 to I 9 8 O . In Chapter I I , I document the behavior, movements and reproductive success of marked herons within one colony i n 1 9 7 8 and 1979* Some unexpected c h a r a c t e r i s t i c s of Individuals within this colony are related to res u l t s from Chapter I and a i d i n explaining some of the size fluctuations and movements of other heron colonies. Several factors considered important to repro-ductive success were assessed and other unforeseen factors were i d e n t i f i e d . CHAPTER I L o c a t i o n , S i z e , H i s t o r y and Reproductive Success of Heronries i n South C o a s t a l B.C. 4 INTRODUCTION Great blue herons (Ardea herodlas) have been s t u d i e d i n many areas of North America. Many authors r e p o r t the l o c a t i o n s , numbers of nests and reproductive success i n heronries w i t h i n a geographical area (Des Granges 1 9 8 1 , Werschkul e_t a l . 1 9 7 7 , Vermeer 1 9 7 3• Benning I 9 6 9 ) . Heron nests have been found i n a v a r i e t y of tre e species, on man-made s t r u c t u r e s , i n shrubs and even on the ground (Blus et a l . 1 9 8 0 , Des Granges 1 9 7 9 t Vermeer 1 9 7 0 ). Comparison of h i s t o r i c a l and annually c o l l e c t e d data has shown t h a t , although there are many long-standing h e r o n r i e s , c o l o n i e s f l u c t u a t e d r a m a t i c a l l y i n s i z e , are abandoned or r e l o -cate f r e q u e n t l y . The suspected reasons f o r t h i s i n s t a b i l i t y have i n c l u d e d h a b i t a t d e s t r u c t i o n , disturbance from nearby human a c t i v i t y or avian predators n e s t i n g i n or near the h e r o n r i e s . Egg and n e s t l i n g l o s s e s to predators and a d u l t I n t e r a c t i o n s with a v i a n predators have been reported i n many areas (Hjertaas 1 9 8 2 , Fry 1 9 8 0 , Koonz I 9 8 O, Bayer 1 9 7 9 , Werschkul 1 9 7 9 , Taylor and Michael 1 9 7 1 , Temple 1 9 6 9 , Dusi and Dusi 1 9 6 8 , Santy 1 9 6 4 ) . The e f f e c t s of predation have v a r i e d from minor l o s s e s of a d u l t s or broods to complete d e s t r u c t i o n and abandonment o f c o l o n i e s . The response o f herons to predators has g e n e r a l l y been incon-s i s t e n t and un p r e d i c t a b l e . The r o l e of predators i n a f f e c t i n g heronry movements and reproduction has, t h e r e f o r e , been d i f f i c u l t to determine.' A l l researchers r e p o r t the number of young r a i s e d per s u c c e s s f u l p a i r and some incl u d e an estimate o f the percentage of s u c c e s s f u l p a i r s (see reviews by Parker 1980, Quinney and 5 Smith 1979, McAloney 1973). Most heronries produced a t l e a s t 1.9 young per breeding p a i r per year, the number b e l i e v e d s u f f i c i e n t to maintain a s t a b l e heron p o p u l a t i o n i n the north-ern U.S.A. (Henny 1972). Although there i s some geographic v a r i a t i o n i n the numbers o f young fledged per s u c c e s s f u l nest, few d i f f e r e n c e s have been found between c o l o n i e s i n the same area, between years or between d i s t u r b e d and undisturbed c o l o n i e s . In f a c t , the number of young r a i s e d per s u c c e s s f u l p a i r i s a s u r p r i s i n g l y s t a b l e s t a t i s t i c w i t h " s u r p l u s " young reported f o r the m a j o r i t y of c o l o n i e s . The l a c k of s e n s i t i v i t y of t h i s s t a t i s t i c to v a r y i n g c o n d i t i o n s i n heronries makes i t a poor choice f o r assessing the reproductive h e a l t h of a heron p o p u l a t i o n . Using data c o l l e c t e d at 15 c o l o n i e s from 1977 to 1980, I have i n v e s t i g a t e d some of the f a c t o r s that have been suggested to cause heronry r e l o c a t i o n s and f l u c t u a t i o n s i n s i z e . I a l s o examined the r e l a t i o n s h i p between predators and h e r o n r i e s , and I propose a b e t t e r method of assessing heron reproductive success. METHODS I s t u d i e d 15 c o l o n i e s , 11 of which were shown to me by n a t u r a l i s t clubs o r i n d i v i d u a l s , and four which were found by ground or a e r i a l searches. I v i s i t e d the 15 study c o l o n i e s 162 times from 1977 to 1979. 31 times {2.2%) p r i o r to the a d u l t herons' a r r i v a l , 66 times (41$) during egg l a y i n g and incuba-t i o n , and 64 times (40$) during the p r e - f l e d g i n g p e r i o d when 6 adults were seldom present. In 1980, three persons who had assisted me previously did the colony surveys. V i s i t s were timed to minimize the disturbance to the colony while c o l l e c t -ing data. I questioned residents and landowners i n the area of each colony regarding i t s h i s t o r y , periods of abandonment, l o c a l movements, incidences of predation or human harassment, and changes i n s i z e . I counted the t o t a l number of nest platforms (unoccupied nest sites) and the number of occupied nests i n each colony i n A p r i l p r i o r to the development of dense deciduous f o l i a g e . I Judged platforms to be occupied i f there were Incubating adults present or i f there were obvious signs of a c t i v i t y on or under the nest ( i . e . droppings, hair p e l l e t s , food items, broken branches, eggshells). I considered nests to be success-f u l i f there were one or more young present on the nest i n l a t e June or early July, just p r i o r to fledging. At that time young birds were about two-thirds of adult size and were r e a d i l y v i s i b l e from the ground. The number of young birds was recorded for each nest where they could be accurately counted. I assumed that the numbers of young counted on these more v i s i b l e nests were representative of a l l nests i n each colony. I calculated the mean number of young fledged per successful nest (MYSN) based on t h i s sample. In two colonies, located i n cottonwoods (Populus trlchocarpa), many nests were no longer v i s i b l e at fledging, so the number of occupied nests i n A p r i l was used as an estimate of the number of successful nests each year (see "Results"). The Haney colony 7 was surrounded by dense coniferous f o r e s t so only a minimum nest count was p o s s i b l e and no f l e d g l i n g counts were made. I mapped e i g h t c o l o n i e s i n I978 and 1979 to o b t a i n repro-ductive Information on i n d i v i d u a l n e s ts. I n a i l e d numbered aluminum tags to the trunk o f each t r e e c o n t a i n i n g one or more nest p l a t f o r m s , whether these were occupied or not. The l o c a -t i o n o f each t r e e was p l o t t e d on graph paper by t a k i n g compass bearings and pacing the distance between t r e e s . I recorded whether each pl a t f o r m was s t i l l present, and i f i t was vacant or occupied on each survey. Newly constructed nests were a l s o l a b e l l e d and mapped. I n the l a r g e P o i n t Roberts colony, the number of occupied nests was recorded i n A p r i l 1978 and 1979 and a sample of 40 trees was l a b e l l e d , mapped and checked a t f l e d g i n g . The change i n numbers of occupied nests i n those 40 trees was used to estimate the change i n the e n t i r e colony between A p r i l and June. A s i m i l a r estimate was made i n the Coquitlam colony using 17 of the 35 nest trees In 1979. The number and c o n d i t i o n o f dead young found on the ground i n each colony was recorded and some specimens were c o l l e c t e d . Evidence of predators or scavengers w i t h i n the c o l o n i e s such as s c a t s , t r a c k s , owl p e l l e t s , broken eggs and remains of herons was a l s o recorded. Signs of human a c t i v i t y such as t r a i l s , t r a c k s and f a l l e n t r e e s were a l s o noted. 8 RESULTS Heronry L o c a t i o n s , Sizes and General D e s c r i p t i o n s Figure 1-1 shows the l o c a t i o n s of the 15 study c o l o n i e s . I searched f o r nine a d d i t i o n a l c o l o n i e s of u n c e r t a i n s t a t u s which were reported by Mark (1976). I found no evidence of an a c t i v e colony at those s i t e s i n 1977, despite extensive ground and a e r i a l searches and conversations w i t h i n t e r e s t e d n a t u r a l -i s t s and persons who had reported the c o l o n i e s . I t i s p o s s i b l e that some may have moved to d i s t a n t , unreported l o c a t i o n s . Three of the c o l o n i e s ( M c G l l l i v r a y , Gibsons and Chehalis) were abandoned the year I l o c a t e d them. The c o l o n i e s I l o c a t e d on the Sunshine Coast represent the f i r s t * ; w r i t t e n records of heronries i n that area although many l o c a l r e s i d e n t s were aware of them. I b e l i e v e that the study c o l o n i e s represented a l l of the l a r g e r h e r o n r i e s w i t h i n the populated p o r t i o n o f the lower mainland. Smaller c o l o n i e s , and those i n remote areas, may w e l l have been overlooked. Colonies v a r i e d from 10 to 240 s u c c e s s f u l nests i n the lower mainland (Table 1-1) compared to a range of three to 130 f o r c o l o n i e s reported by Mark (1976) i n the same areas. The average s i z e o f the four c o l o n i e s reported p r e v i o u s l y by Mark (1976) was 62$ l a r g e r during t h i s study (Table 1 -2) . Herons nested i n stands of a l d e r (Alnus r u b r a ) , broadleaf maple (Acer macrophyllum), cottonwood (Populus t r l c h o c a r p a ) , Douglas f i r (Pseudotsuga m e n z l e s i l ) and S i t k a spruce ( P l c e a  s i t c h e n s l s ) . Some nests were a l s o found i n western hemlock (Tsuga h e t e r o p h y l l a ) , lodgepole pine (Plnus contorta) and FIGURE l - l . Heron colony locations in the lower mainland and Sunshine Coast areas of Brit ish Columbia. 10 Table 1-1. Number of successful nests i n heron colonies surveyed i n south coastal B r i t i s h Columbia. Colony name Number successful of nests 1977 1978 1979 1980 Coquitlam 169 162 31 26 Crescent 37 46 42 22 Edgewater 16 31 38 30 Haney 10* 10* 10* 10* Mclvor 8 8 5 6 Pender Harbour 25 33 0 Point Roberts 216 240 236 222 Powell River 6 " 19 Salwein 96 101 109 91 Sechelt 28 . 36 35 Stanley Park 19* 43 38 33 U.B.C. 82 103 118 130 * Minimum count due to v i s i b i l i t y i o r other l i m i t a t i o n s — see text. 11 Table 1-2. Comparison of past and present sizes of four colonies In the lower mainland. Colony name Before 1976 1977- 1980 Mean % increase mean no. successful N nests mean no. successful N nests Coquitlam 63 2 97 4 54 McGillivray 27 1 46* 1 70 Point Roberts 120 4 228 4 90 U.B.C.. 80 3 108 4 35 Mean 62 * Refers to occupied nests l a t e r abandoned, 1977. cedar (Thuja p l l c a t a ) . In 11 colonies^herons nested i n only one tree species (Table 1-3), although there were other trees of similar height i n the same area. In a l l colonies, most successful nests were i n the same tree species (Table 1-3) and i n two cases, Coquitlam and Stanley Park, l i m i t e d s i t e s i n the primary species may have forced herons to use other trees. I have used t h i s c h a r a c t e r i s t i c of herons as i n d i r e c t evidence that heronries at d i f f e r e n t locations, but i n the same tree species, represent l o c a l movements of one population. H i s t o r i c a l and Present Colony Data To assess long-term trends i n the heron population, I compared past and present data. Mark (1974, 1976) summarized h i s t o r i c a l information on heron colony locations and s i z e s . 12 Table 1-3. Descriptions of the study colonies • Colony name Year No. of successful nests* Tree species No. of nest trees Chehalis 1979 47 31 5 1 Broadleaf maple Cottonwood Douglas f i r Cedar 18 7 2 1 Coquitlam 1978 151 7 4 Sitka spruce Lodgepole pine Western hemlock 27 5 3 Crescent 1977 37 Douglas f i r 5 Edgewater 1977 16 Cottonwood 3 Gibsons 1978 !;42 12 Broadleaf maple Red alder 14 11 Haney ' 1977 10 Douglas f i r 2 McGillivray 1977 46 Cottonwood 10 Mclvor 1977 8 Sitka spruce 1 Pender Harbour 1978 25 Red alder 23 Point Roberts 1977 216 Red alder 192 Powell River 1978 6 Douglas f i r 2 Salwein 1977 96 Cottonwood 19 Sechelt 1978 28 Red alder 28 Stanley Park 1978 32 9 2 Douglas f i r Cedar Western hemlock 3 5 2 U.B.C. 1977 82 • Red alder 61 * Refers to occupied nests for colonies abandoned p r i o r to fledging (Chehalis, Gibsons, M c G i l l i v r a y ) . 13 I used t h i s and a d d i t i o n a l data from other p u b l i c a t i o n s and v e r b a l r e p o r t s to assess the h i s t o r y of the study c o l o n i e s . I assume that v a r i e d l o c a t i o n s of a colony w i t h i n 10 k i l o -meters i n d i f f e r e n t years represent l o c a l movements o f one po p u l a t i o n . Chehalis Colony This colony was a c t i v e from 1957 to i960 (Mark 1976). The colony had been abandoned s h o r t l y before my f i r s t v i s i t on A p r i l 19» 1979• Eggshells found on the ground had been broken by predators or scavengers and no a d u l t herons were observed. I found feathers of one dead a d u l t b i r d . Residents confirmed that the colony was a c t i v e i n 1978 and that the herons had a r r i v e d , as u s u a l , i n March 1979. B a l d eagles (Hallaeetus leucocephalus) which congregate i n the area to feed on salmon (Oncorhynchus sp.) carcasses normally r o o s t i n the f o r e s t e d area where the herons nest and leave s h o r t l y before the herons a r r i v e . Residents speculated that the herons abandoned t h e i r nests.due to harassment by the eagles which l e f t l a t e i n 1979. My observations of abandoned eggs and one dead a d u l t support that c o n c l u s i o n . Coquitlam Colony Two c o l o n i e s were recorded i n Coquitlam. The Newberry Road colony had 78 nests i n spruce trees i n 1971 but was abandoned, probably i n 1972, e i t h e r because of an adjacent s u b d i v i s i o n development (Mark 1976), or because of a j u v e n i l e banding pro-gram c a r r i e d out by a heron research group working at the 14 University of B.C. In 1971. 94 of 190 nestlings were banded (Campbell et a l . 1973). The other colony, on the Essondale Indian Reserve (Fig. 1-1) , was f i r s t recorded i n 1973 when i t contained at l e a s t 48 nests (Jerema 1973)• Evidence indicated that there had been herons at that l o c a t i o n for many years (Mark 1976) . Judging by the size of the Coquitlam colony (over 160 nests i n 1977 and 1978, Table 1-1), and apparent overcrowding, i t i s l i k e l y that the Newberry Road colony joined an e x i s t i n g colony at t h i s l o c a t i o n i n 1972. In 1979 and 1980 the e a r l i e r nesting s i t e was abandoned. Bald eagles were nesting i n the center of the vacated colony. Thirty-one occupied nests were located about one kilometer north of the old l o c a t i o n . Reproduction i n those nests appeared to be normal, but most young had fledged p r i o r to the census i n 1979. The status of the remaining 130 pairs that nested i n 1978 i s unknown. Crescent Colony The Crescent colony was referred to by Urhahn (1968), but no size of exact l o c a t i o n was given. According to l o c a l residents, the Crescent colony was o r i g i n a l l y located i n large f i r s i n Crescent Park but was forced to move when the trees were f e l l e d . The birds occupied two more s i t e s on private property, where they were unwelcome and the trees were f e l l e d , before they found sanctuary at th e i r present location about 1970. The nest trees are a l l within 20 meters of a private residence. This l o c a t i o n was abandoned for two years about 15 1972-73 when a pair of great horned owls (Bubo vlrglnianus) occupied one of the nests (R. Nitsch, pers. comm.). The Nitsch's observed eagles capturing young herons on three occasions. One eagle landed on a heron nest i n 1978 and ca r r i e d two young away. The eagle dropped one young heron i n the Nitsch's back yard as i t flew away. The Nitsch's deterred eagle attacks by f i r i n g a starter p i s t o l when eagles approached. Edgewater Colony Mark (I976) recorded t h i s colony as present i n 197^. Mrs. M. Pastrlck (pers. comm.) indicated that the colony was occupied from 1975 to 1976 and was v i s i t e d r e g u l a r l y by the Langley N a t u r a l i s t s Club. Many crows (Corvus caurlnus) were present i n t h i s colony at each v i s i t . The sh e l l s of several eggs, apparently eaten by crows, were found i n A p r i l of both 1977 and 1978. Gibson's Colony The colony was situated about 200 meters from a proposed sawmill s i t e . The m i l l s i t e was cleared to the outermost nest and extensive excavating began i n 1978 and continued i n 1979. The colony was occupied i n A p r i l 1978 and again i n May 1979 but was abandoned i n both years. Many crows were seen i n t h i s colony. Eggs l a i d i n 1978 were eaten on the nest platforms probably a f t e r the adults l e f t , judging by the sh e l l s found on the nests. A possible previous nest s i t e nearer the Gibson's town s i t e was not reoccupied during t h i s time. Two nests were 16 occupied and successful i n 1980, i n d i c a t i n g that more herons may return to thi s s i t e i n the future. Haney Colony Mark (1976) reported that t h i s colony was f i r s t formed i n 1974. It was also active from 1975 to 1976, according to U.B.C. Research Forest s t a f f . Accurate nest and f l e d g l i n g counts i n t h i s colony are impossible due to the dense conif-erous foliage surrounding the nests. A r e d - t a i l e d hawk (Buteo jamaicensis) buzzed the colony i n A p r i l 1977. and v i s i t o r s reported that eagles harassed the colony. I found^a decapitated young heron under the nests i n June 1977. confirming predation by eagles or owls. McGillivray Colony The McGillivray colony was present since at least 1974 (Mark 1976). Local n a t u r a l i s t s indicated that i t was active u n t i l 1977. although no systematic records were kept. On March 10, 1977. 46 nests were occupied, although I described the b i r d s ' behavior as "very spooky and e a s i l y disturbed." Many nests were abandoned by May 6 and most (43) by May 18, 1977. An occupied eagles' nest was found 200 meters east of the colony i n A p r i l 1978. The colony was not reoccupied i n 1978 or 1979. Mclvor Colony The Mclvor colony could be one of several referred to by Mark (1976) i n the P i t t Meadows area. Residents said i t had been at t h i s l o c a t i o n since 1970 and averaged 10 nests a year. 17 Eagles frequently harassed the nests but no young were seen taken. At lea s t f i v e young f e l l from the nests i n 1977. The nest tree was three meters from a private residence b u i l t i n 1976. Pender Harbour Colony The Pender Harbour heronry was found on a property being subdivided for r e s i d e n t i a l l o t s 200 meters south of Gunboat Bay. The colony had been at that l o c a t i o n since at l e a s t 1963 when i t was estimated to contain 75 nests (A. Joss, pers. comm.). I t contained 43 occupied nests i n 1978 and 45 occu-pied nests i n 1979. The colony was bounded on the east and north by subdivision l o t s and a new access road respectively, while the southernmost nests were exposed to view from Highway 101 by removal of the trees. The subdivision work began in the summer of 1977 and continued through the spring and summer of 1978. Extensive excavating and some bl a s t i n g were required to i n s t a l l water mains in A p r i l and May and two water wells were d r i l l e d i n June 1978. The work was completed i n the spring of 1979 and there was no further road work or house construction a f t e r A p r i l of 1979. During the construction work i n 1978, adult herons were frequently frightened from t h e i r nests. I observed eagles taking young herons from t h e i r nests with adults present on two occasions i n 1978 and three other eagle attacks were reported (M. Wise, pers. comm.). Ravens (Corvus corax) were commonly present i n the colony and I 18 observed them t a k i n g s i x young from unprotected nests i n 1978 and 1 9 7 9 . Ravens p u l l e d young herons from t h e i r n e s t s , i n the absence of a d u l t s , and ate them on the ground. I found 14 e v i s c e r a t e d young on the ground i n 1 9 7 8 . I a l s o found evidence of three a d u l t k i l l s (feathers) i n c l u d i n g one banded a d u l t (A83) i n 1 9 7 8 . Eagles n e s t i n g i n a l a r g e f i r o v e r l o o k i n g the colony appeared to use the herons as a convenient food supply. I found heron bones and feathers under the e a g l e s 1 nest i n 1 9 7 8 . Eagles d i d not nest there i n 1979 but returned i n I98O when the heronry was abandoned. In 1979 a t o t a l of 12 p a i r s , which attempted n e s t i n g , f a i l e d to fledge any young. Three nests were occupied by a d u l t s and abandoned p r i o r to egg l a y i n g . Four other nests contained small young, judging by s h e l l s found on the ground, but the young were l o s t and the nests abandoned s h o r t l y a f t e r h a t c h i n g . In such cases i t i s impossible to t e l l i f l o s s of the young i s the cause or the r e s u l t of the abandonment. Two n e s t s , each c o n t a i n i n g three l a r g e young, were robbed, probably by raccoons (Procyon l o t o r ) , between June 17 and 24. The nests were covered with feathers and chewed bones and raccoon t r a c k s were found on the ground under the n e s t s . I found four dead young under two other empty nests from which ravens had attempted to p u l l young on e a r l i e r dates. In one other nest three l a r g e young disappeared between June 20 and 24 p r i o r to f i r s t f l e d g i n g . I suspected eagles but no eagle a t t a c k s were observed i n 1 9 7 9 » 19 Adult herons In the Pender Harbour area were banded i n 1978 and 1979 at feeding areas. Many of the banded birds were observed i n the colony (see Chapter I I ) . Point Roberts Colony A colony of 165 nests (1948) and I85 nests (1949) was o r i g i n a l l y located near R a l t t Road i n Delta Municipality and was displaced a short distance by power l i n e c l earing (E. Taylor, pers. comm.). In 1959 a 100-nest colony in the same area was destroyed by clearing of the cottonwood and alder nest trees for a r e s i d e n t i a l development (Mark 1974). Another colony of unknown size i n south Point Roberts was recorded as destroyed by development about 1970 (Mark 1974). In 1973 a colony of 30 nests was located o f f C h u r c h i l l Road just south of the Canada-U.S.A., boundary (Mark 1974). The Point Roberts colony containing over 200 nests (Table 1-1) was at t h i s l o c a t i o n from 1977 to I98O. I f the nest counts from previous years are correct, i t i s l i k e l y that the colony s p l i t up from 1958 to 1973 and has recently re-congregated at the C h u r c h i l l Road l o c a t i o n . I believe that a l l the herons nesting i n South Delta-Point Roberts are now at the one large colony. Judging from tracks, domestic dogs and cats frequently v i s i t e d the heronry. Some trees near the edge of the colony were felled"<by woodcutters and two nest trees had been chopped but not f e l l e d , probably by children playing with an axe. 20 Powell River Colony The Powell River colony was located i n a dense forest of immature Douglas f i r and western hemlock behind Abbotsford Street Elementary School i n 1978 and contained six nests (Table 1-1) . Three former nesting s i t e s were located, includ-ing one which contained 16 nests i n 1974 (C. Burton, pers. comm.). Dense second growth forest i n the Powell River area and l i m i t e d access prevented l o c a t i o n of other active heron-r i e s . The colony contained 19 active nests i n 1980 (Table 1-1). Salwein Colony The Salwein colony was located in large cottonwoods about 100 meters from the Canadian Forces wet bridging practice area (F i g . 1-1). The area i s a s i t e of frequent mock combat man-euvers. The colony was f i r s t recorded i n the B.C. Nest Record Scheme in 1976 and had an estimated 50 nests. The increased size i n 1977 to 96 nests (Table 1-1) coincided with the abandonment of the McGilllvray colony about eight kilometers away. I believe that many of the McGilllvray herons sh i f t e d to the Salwein colony i n the spring of 1977. A p a i r of great horned owls nested i n the colony i n 1979. A r e d - t a i l e d hawk harassed the herons i n A p r i l and May 1977. Five dead and one l i v e young were found on the ground on May 30, 1978. Sechelt Colony I located the Sechelt colony i n March 1978 about 1.5 kilometers north-west of Porpoise Bay. The hi s t o r y of the 21 colony i s unknown; but other reported locations include Four Mile Point on the north shore of Porpoise Bay, and Sechelt Marsh at the head of the bay. Dense undergrowth i n the logging slash through which the colony i s reached makes access d i f f i -c u l t so i t i s u n l i k e l y that the colony suffers any d i r e c t disturbance from people. I found six dead young on the ground and saw three others hanging from nests i n 1 9 7 8 . A l l of the dead birds were i n t a c t . No crows were observed i n the colony but I saw a r e d - t a i l e d hawk harassing adults on two of the four v i s i t s i n 1 9 7 8 . In 1979 I recorded two nest f a i l u r e s at Sechelt. No dead young were found under those nests although, under other trees, s i x dead young were found, of which one had been eaten. Stanley Park Colony There has been a heron colony i n Stanley Park since at least 1921 when 39 nests were recorded at Brockton Point. Table 1-4 shows the locations and numbers of nests i n the colony from 1921 to I 9 8 O . The exact date of the move from Brockton Point to the zoo area could not be determined. A newspaper a r t i c l e i n 1928 stated that the dead spruce tree at Brockton Point, where the birds nested, was to be removed. A photograph of the tree showed 2 7 nests and 81 young during the " l a s t " nesting season. I t i s probable that the colony has moved at least twice since 1 9 2 1 . The nest count i n 1 9 7 7 was probably an underestimate since I did not check other trees i n the v i c i n i t y of the two large " f i r s used that year. 22 Table 1-4. Size and locations of the Stanley Park colony from 1921 to 1980. Date Successful nests Location Source 1921 39 Brockton Point Mark (1976) 1923 23 Brockton Point Mark (1976) 1928 27 Brockton Point Vancouver Sunday Province June 17, 1928 1959 25 Brockton Point Mark (1976) 1961 25+ Exact l o c a t i o n not s p e c i f i e d Mark (1976) 1966 28 Exact l o c a t i o n not s p e c i f i e d Mark (1976) 1967 40 Exact l o c a t i o n not s p e c i f i e d Mark (1976) 1968 25 Exact l o c a t i o n not s p e c i f i e d Mark (1976) 1969 Active Exact l o c a t i o n not s p e c i f i e d Mark (1976) 1970 40 Exact l o c a t i o n not s p e c i f i e d Mark (1976) 1971 30 Zoo area Paine (1976) 1974 21 Zoo area Mark (1976) 1977 19 Zoo area Simpson and K e l s a l l (1977) 1978 43* Zoo area 1979 38 Zoo area 1980 33 Zoo area * Twenty-nine nests i n the two Douglas f i r s and 14 nests located i n hemlock and cedar around the aquarium. 23 This colony i s unique i n that i t i s i n the center of a very high use recreation area and the nests are r e a d i l y v i s i b l e from the ground. The herons are apparently undisturbed by human a c t i v i t i e s on the ground. No avian predators or scaven-gers were observed at t h i s colony although eagles, ravens and crows are present i n the park. U.B.C. Colony A colony of 40 nests was reported on the north shore of Sea Island i n 19^1 and remained active u n t i l 19^9. I t was destroyed by a i r p o r t expansion about 1950 (Mark 1976). The birds of that colony probably moved to the U.B.C. colony (Fig. 1-1). The U.B.C. colony was f i r s t enumerated i n 1970 when i t contained 125 nests. In December 1970 a b l i n d was b u i l t on the periphery of the colony for use i n the summer of 1971 (Paine 1972). Occupancy of that b l i n d resulted i n the immediate abandonment of the three closest nests. The main group of birds returned to the colony one month l a t e . Incubation, which began i n early May, and the f i r s t n e s t l i n g , observed on May 20, were equally l a t e . During 1971 the colony contained about 55 nests, with another 20 located 200 meters north of the main colony. The " s a t e l l i t e " contained poor qua l i t y nests, and was an addi-t i o n a l three weeks l a t e r than the main colony. The nest count was approximately 75» 50 less than lh:1970 (Campbell et a l . 1973)• In 1972 the main colony was abandoned and the " s a t e l l i t e " contained 22 nests. No other nests were found and the decline was thought to be caused by "...losses to the 24 breeding p o p u l a t i o n due to severe w i n t e r s , " ( J . Krebs In: Campbell et a l . 1974, p. 22). In 1971, 106 of 176 young were banded (Campbell et a l . 1973). In 1972, 57 of 62 young were banded. Those e f f o r t s would have i n v o l v e d climbing almost every tree i n the colony and r e q u i r e d s e v e r a l days of continuous disturbance each year. In 1973 and 1974 the research a c t i v i t y stopped, and the colony was recorded as being a c t i v e with no nest counts (Mark 1974). In 1977 there were 70 nests i n the s a t e l l i t e colony while the main colony remained abandoned. A check on l a t e n e s t i n g on J u l y 19 revealed 12 a d d i t i o n a l nests approximately 100 meters northwest of the colony (Area A ) . The presence of l o u d l y - c a l l i n g unfledged young a t t r a c t e d a t t e n t i o n to that l o c a t i o n , and i n d i c a t e d that the group bred about three weeks l a t e r than the s a t e l l i t e colony, which had few unfledged young. In A p r i l of 1978 the s a t e l l i t e colony had increased by 4 l nests over the 1977 count. There were 19 I n a c t i v e nest s i t e s i n Area A and another separate group (Area B) c o n t a i n i n g 24 nests,, a l s o i n a c t i v e . I f the m a j o r i t y of those 43 a l t e r n a t e nests had been a c t i v e i n 1977, then the observed increase i n nests i n 1978 can be explained simply by a s h i f t of the p e r i -p h e r a l b i r d s back to one l a r g e colony. I t seems l i k e l y that the observed decreases i n 1971 and 1972 were r e l a t e d to herons abandoning d i s t u r b e d s i t e s and r e l o c a t i n g elsewhere i n undiscovered s i t e s . I n 1979 a l l o l d l o c a t i o n s of t h i s colony were completely abandoned and a new and l a r g e r colony of 118 nests was formed about three 25 kilometers to the east. A l l the newly constructed nests were occupied i n June 1979• Four dead young were found on the ground and six dead young were seen on nests i n 1978. I saw a raccoon eating a young heron on the ground i n 1978 and raccoon tracks were common under the heronry. An eagle flew over the colony dur-ing one v i s i t , butv.did not cause adults to leave t h e i r nests. Reactions to Human Disturbance In two colonies located within 20 meters of private r e s i -dences (Crescent and Mclvor), and one located i n a highly developed park (Stanley Park), the herons were completely unconcerned during observational v i s i t s . In six other lower mainland colonies, which were surrounded by forest but close to populated areas, herons reacted to researchers by c a l l i n g , r a i s i n g feathers and standing but remained on or near t h e i r nests. At both Pender Harbour ( A p r i l 1978) and McGillivray (March 1977) most birds f l e d as the colonies were approached. When nest trees were climbed, birds flew from that tree and also from nearby trees. Most birds c i r c l e d the colony giving continuous, loud c a l l s when disturbed. However, on one occasion an adult b i r d a c t i v e l y defended i t s nest against the climber by ph y s i c a l l y blocking his path, s t r i k i n g at him and f l a r i n g i t s wings. Reproductive Success I measured reproductive success at most colonies from 1977 to 1980 by ca l c u l a t i n g the mean number of young fledged per 26 successful nest (MYSN) (Fig. 1-2). I compared MYSN between colonies and between years for the f i v e lower mainland colonies, for which I had fledging estimates each year, by analysis of variance. There were no s i g n i f i c a n t differences between years or between colonies. I compared MYSN between a l l colonies i n 1978 and 1979 by analysis of variance. There were no d i f f e r -ences between colonies i n 1979 and few differences i n 1978. The Pender Harbour and Mclvor colonies had lower MYSN than the U.B.C. and Crescent colonies i n 1978 (p < .05, Duncan's Multiple Range Test). I followed the fate of in d i v i d u a l nest platforms i n eight colonies i n 1978 and two in 1979. Although the status of some platforms was undetermined during each survey, minimum numbers of nests abandoned and newly occupied or constructed betxvreen surveys were obtained (Table 1-5). At most colonies, the number of additions exceeded the number abandoned over the nesting season. At Pender Harbour and U.B.C. the number of nests abandoned exceeded the number newly occupied, a s i g n i f i -cant difference from the combined proportion of other colonies (Table 1-5, x 2 = 13.3. 3 df, p < .01). Both of these colonies were completely abandoned the next year. Using abandonment data I was able to determine the number of breeding p a i r s , both successful and unsuccessful, and calculated the mean number of young fledged per breeding pair (MYBP)' (Table 1-6). MYSN was s i g n i f i c a n t l y greater than MYBP (t = 3.88, 9 df, p < .01) by 15$ on average and by up to 50$ i n cases where nest abandonment was high. N O | -UJ CO C2> UJ Q Z mi u_ CD CO Z CO o o >- o o to H I i + 42 43 26 18 17 13 22 I I 12 10 8 6 5 6 19 32 39 39 33 53 3 4 43 14 13 28 12 22 18 19 23 30 21 51 1977 78 8 0 7 7 78 79 8 0 77 7 9 8 0 7 7 78 7 9 8 0 78 7 9 7 7 78 7 9 80 7 7 8 0 78 7 9 8 0 7 7 78 7 9 8 0 7 7 78 79 8 0 PORT CRESCENT EDGE- M c l V O R PENDER POINT SALWEIN SECHELT STANLEY U B C COQUITLAM WATER HARBOUR ROBERTS PARK FIG. 1-2 Mean numbers of young f ledged per successful nest at 10 heronries f rom 1977 to 1980. Table 1-5. Number of nests abandoned and newly-occupied between survey dates at eight colonies i n 1978 and two i n 1979. Colony Time period No. occu-pied i n Mar/Apr No. abandoned or f a l l e n by Jun/Jly No. newly occupied or constructed Mar-Jly No. success-f u l nests Coquitlam 1 Apr 7-Jun 27 141 9 30 162 Crescent Apr 21-Jun 29 39 5 12 46 Mclvor Apr 12-Jun 27 9 1 0 8 Pender Harbour Apr 30-Jly 17, 78 33 14 6 25 Mar 31-Jly 13, 79 44 12 1 33 Point Roberts 2 Apr 21-Jun 29 221 21 40 240 Sechelt Apr 19-Jly 17. 78 24 2 6 28 Mar 27-Jly 14, 79 31 2 7 36 Stanley Park Apr 5-Jun 28 43 1 1 43 U.B.C. Apr 11-Jun 30 107 19 15 103 1. Estimated from 17-tree sample. 2. Estimated from 40-tree sample. Table 1-6. Mean numbers of young fledged per successful nest and per breeding p a i r at eight colonies i n 1978 and two i n 1979. Colony Mean young fledged/ successful nest (MYSN) Mean young fledged/ breeding pair (MYBP) % difference Coquitlam 2.3 2.2 4.5 Crescent 2.8 2.5 12.0 Mclvor 2.1 1.9 10.5 Pender Harbour 1978 2.1 1.4 50.0 1979 3.0 36.4 Point Roberts 2.5 2.3 8.7 Sechelt 1978 2.6 2.4 8.3 1979 2.8 2.6 7.7 Stanley Park 2.6 2.6 0.0 U.B.C. 2.8 2.4 16.7 Mean (unweighted) 2.56 2.25 15.5 30 At most colonies the number of occupied nests and the percent of the t o t a l platforms occupied did not fluctuate much over the breeding season (Table 1-7). To determine the number of nests abandoned, each nest must be l a b e l l e d and rechecked on subsequent v i s i t s . Counts of nests do not provide abandon-ment data because nest additions between surveys compensate for nest losses (Table 1-5). La b e l l i n g i n d i v i d u a l nests and repeated surveys are time consuming and may cause nesting disruptions and losses i n colonies unaccustomed to human intrusions. I found that the number of nests abandoned and, thereby, the t o t a l number of breeding pairs at each colony, could be estimated using the percentage of platform 5 occupied . at fledging ( F i g . 1-3)• As the percentage of nests abandoned increased, the proportion of platforms occupied at fledging decreased, despite the confounding e f f e c t of nest additions. DISCUSSION Colony Sizes The absence of nine previously reported colonies and the suspected amalgamation of the Newberry-Coqultlam and McGillivray-Salwein colonies suggest that the recent increase i n sizes of e x i s t i n g heronries (Table 1-2) has resulted from the amalgamation of smaller colonies. Frequent reference to clearing for developments and power l i n e s suggests that urban expansion and loss of forested nesting habitat i s l i k e l y res-ponsible f o r concentrating herons i n fewer and la r g e r breeding colonies. Table 1-7. Number and % of nest platforms occupied during the breeding season at eight heronries. Coquit- Pender Point Stanley m 4 4 j lam 1 Crescent Mclvor Harbour Roberts 2 Sechelt Park3 U.B.C. Time period N # N # N # N $ N . # N # N # N $ Incubating A p r i l 1978 68 91 9 90 39 42 47 98 24 73 30 100 111 85 Young present , May-June 1978 37 4 95 8 80 43 46 28 88 31 100 Fledging June-July 1978 78 94 46 87 8 80 25 27 51 91 28 88 30 97 103 77 July 1979 33 35 36 95 1. 17-tree sample. 2. 40-tree sample. 3. Nests i n two large Douglas f i r s only. 4. Sample of nests examined by a tree climber i n 1977. 32 FIG. 1-3 The relationship between the percentage of nest platforms occupied at f ledging and the percentage of unsuccessful pairs at eight colonies in 1978 and two In 1979. 33 Data for nine colonies i n the lower mainland from 1977 to 1979 showed that the smallest decreased i n size from eight nests to f i v e , one decreased due to abandonment of i t s o r i g i n a l s i t e , one remained s t a t i c and the other six increased i n size from nine to 138$. In 1980, most colonies showed s l i g h t declines, ranging from six to 17% (Table 1-1). These fl u c t u a -tions can best be understood by considering the circumstances and hi s t o r y of each colony. The Increase i n the U.B.C. colony i n 1978 was probably the r e s u l t of the return of adults from alternate nesting s i t e s to one central nesting area. In 1979 the entire colony r e l o -cated and contained 118 active nests. In I98O i t increased to 130 nests, f i v e more than the previous high count of 125 nests i n 1970. The same phenomenon may be responsible for changes at the Crescent colony. Complete abandonment i n 1972 and 1973 suggests that the colony has some undiscovered alternate nest-ing s i t e . Interchange between s i t e s could account for year-to-year v a r i a t i o n s i n numbers of occupied nests, independent of actual changes of population. The large increase i n size of the Stanley Park colony from 19 to 43 nests Indicates again the apparent mobility of great blue herons i n choosing nest s i t e s . Based on previous years' data (Table 1-4), 38 nests i s not unusually large for the Stanley Park colony. The low nest counts of 1971. 1974 and 1977 could well be the r e s u l t of f a i l u r e to locate other nests i n the general area. Colonies i n the Sunshine Coast area have shown si m i l a r f l u c t u a t i o n s . Successful nests at Pender Harbour and Sechelt 34 increased by almost one-third from 19?8 to 1979 (Table 1-1), although h i s t o r i c a l data indicated that the Pender Harbour colony may have been larger previous to the subdivision develop ment there. In 1980 the Pender Harbour colony was completely abandoned while the Sechelt colony remained stable and the small Powell River colony t r i p l e d i n size (Table 1-1). The Coquitlam colony dropped from 162 nests i n 1978 to 31 nests i n 1979. Such massive changes i n colony size from year to year cannot be explained i n terms of adult mortality or recruitment. The death of 260 adult herons i n Coquitlam could scarcely have gone unnoticed. The abandonment of the McGillivray colony and the con-current increase at the Salwein colony lends further support to the idea of the adult movement between colonies. I t may be inferr e d that, although herons prefer to nest i n groups, the presence of one group does not preclude the existence of others i n the same l o c a l i t y . In fa c t , the presence of one concentration i s often associated with one or more alternate s i t e s among which breeding adults may relocate from.year to year. Reactions to Human Disturbance Marked differences were noted i n the response of herons i n some colonies to the presence of people. These ranged from no reaction, to standing and c a l l i n g , to taking f l i g h t . Those varied reactions to the same stimulus imply that herons have d i f f e r e n t tolerance l e v e l s to humans i n d i f f e r e n t l o c a l e s . 35 In general, colonies located close to areas of human a c t i v i t y showed les s response than those i n more remote areas. Some individuals within colonies were also more tolerant or l e s s a f r a i d than others. I believe that many of the herons at Pender Harbour which successfully raised young i n 1978 did so because they remained on t h e i r nests despite the d i s t u r -bance from construction nearby. Nesting herons could benefit from a close association with people i f predators are less tolerant of humans. The deterrence of eagle attacks by landowners probably reduced predation at the Crescent colony. The presence of people and residences near other colonies may have i n h i b i t e d the a c t i v i t y of some predators. The s c a r c i t y of predators at Stanley Park may r e s u l t from intense human a c t i v i t y . Predation I observed avian or t e r r e s t r i a l predators i n every heron colony except Stanley Park. I recorded eagle or owl nests at f i v e colonies, eagle or hawk harassment at seven, crows or ravens at seven and mammalian carnivores at three. Predators are probably attracted to heronries because of the young and eggs i n the nests and by the food items and young f a l l i n g from the nests. I believe that predation was a s i g n i f i c a n t cause of nest f a i l u r e at some heron colonies, e s p e c i a l l y at Pender Harbour. Fourteen eviscerated young found at Pender Harbour i n 1978 were probably k i l l e d by ravens. Seven int a c t young found at 36 Sechelt probably f e l l a c c i d e n t a l l y from t h e i r nests since they were not eaten. My other observations implicated preda-tors i n f i v e of 12 nest f a i l u r e s at Pender Harbour i n 1979. Heavy losses to predation at Pender Harbour were probably predisposed by the frequent absence of adult herons from the nests which, i n turn, was caused by the construction work nearby. Predators apparently became accustomed to the r e a d i l y available food supply afforded by unprotected nests. The eagle attacks witnessed occurred with adults i n attendance at the nests, however, ind i c a t i n g that any i n h i b i t i o n that may be provided normally by adults, was not operating. The loss of three adults further reinforces that conclusion. Other studies have indicated that complete nest losses are probably caused by predators (Dusl and Dusi 1968, Pratt 1972). I concluded that predators were unusually successful i n the Pender Harbour colony and were persistent at vulnerable nests u n t i l a l l young were removed. Many observations have been made on the interactions of avian and other predators with herons and heron colonies. Occupancy of heron nests by predatory birds has been c i t e d as the cause of colony abandonments both i n t h i s study (Chehalis, Crescent, Coquitlam and McGillivray colonies) and others (Mark 1976, Vermeer 1973)• In contrast, great horned owls and bald eagles have been recorded as nesting i n or near 10 d i f f e r -ent heronries without causing abandonment (Vermeer 1972 and 1973, Bayer 1979, Koonz 1980) . The Salwein colony had a pair of great horned owls nesting i n one of the "heron" nests but 37 that colony had a successful nesting season i n 1979• Eagles nesting near the Pender Harbour colony i n 1978 did not cause herons to abandon nesting. The v a r i a t i o n i n the effects of predators on nesting success may be due to differences i n the relationships of the species i n d i f f e r e n t areas. Both owls and eagles have been reported to prey upon or harass adult and n e s t l i n g herons (Bayer 1979. Werschkul 1979. Krebs 197^). Crows, ravens and turkey vultures (Cathartes aura) also prey on young herons (Taylor and Michael 1971, Temple 1969, Dusi and Dusi 1968), but have never been suspected of causing colony abandonment. I t may be that the i n t e n s i t y of predator interactions deter-mines the response of a nesting colony. Presumably i f predation causes s i g n i f i c a n t losses i n reproduction or adult mortality i t would be to the advantage of the herons to relocate, provided that predation i s reduced at the new l o c a t i o n . The abandonment of heronries at U.B.C. i n 1979. and Pender Harbour i n I 9 8 O , suggests that herons move to a new s i t e i f nests l o s t at the o l d s i t e exceed 15% (Table 1-5) or i f the number leaving a colony exceeds the number entering during the breeding season. The reduction of nest losses i n the U.B.C. colony, following r e l o c a t i o n , may have r e l a t e d to lower predation at the new s i t e . Observations of eagle-heron interactions i n 1980 suggested that herons may show r a d i c a l l y d i f f e r e n t responses to eagles at d i f f e r e n t locations, possibly r e l a t e d to the l e v e l of pre-dation suffered by the herons. Feeding herons at Pender Harbour 38 r a r e l y allowed an eagle to approach within 100 meters without f l e e i n g , whereas those at Sechelt reacted only by assuming an a l e r t posture even upon the close approach of an eagle (Forbes I98O). Heron responses to eagle harassment si m i l a r to those at Pender Harbour are described by Bayer (1979). Eagle preda-t i o n on both adult and juvenile herons was observed at Pender Harbour but not at Sechelt. Those observations again suggest that herons may a l t e r t h e i r response to cert a i n predators as a r e s u l t of previous experience. Reproductive Success Reproductive success was assessed at a l l colonies using the mean number fledged per successful nest (MYSN). Although the number of young fledged per breeding p a i r (MYBP) i s a better estimate of heron productivity, MYSN has been the standard used i n many previous reports (see reviews by Parker I98O, Quinney and Smith 1979). Our fledging rates were s i m i l a r to the r e s u l t s of others and few differences were found among study colonies using MYSN. Nest losses were the most important parameter determining the reproductive status of a colony. The e f f e c t of the d i s -turbance at Pender Harbour was not r e f l e c t e d by MYSN. Successful pairs raised as many young as adults i n undisturbed colonies (Fig. 1-2). Reproductive losses were r e f l e c t e d by increasing the percentage of unsuccessful pairs (Table 1-5). Pender Harbour had proportionately more unsuccessful nests than s i x other colonies i n 1978 (Table 1-5. x 2 = 38.4., 1 df, p < .001). Pender Harbour also had proportionately more nest 39 f a i l u r e s than the Sechelt colony i n 1979 (Table 1-5, x 2 = 6.73, 1 df, p < .01) or the U.B.C. colony i n 1978 (x 2 = 7.^9. 1 df, p < .01). Those res u l t s show that some adults lose a l l t h e i r young under adverse conditions, while others are unaffected. At Pender Harbour i n 1978, tolerance of disturbance and tenacity at the nest were probably important factors determining the success or f a i l u r e of d i f f e r e n t nesting p a i r s . Determining nest abandonment requires repeat inspections of i n d i v i d u a l l y l a b e l l e d platforms at each colony. This i s a tedious and time-consuming job and may represent a considerable disturbance to nesting herons i n non-urban colonies. The pro-portion of platforms occupied i n each colony can be used to estimate the number of nests abandoned, p a r t i c u l a r l y just p r i o r to fledging when platform occupancy i s generally high (Table 1-7). The low platform occupancy at Pender Harbour i n 1978 {27%) and 1979 (36$) was probably related to disturbance from work i n the adjacent housing development. Nest abandonment decreased platform occupancy while construction of new nests, further from the disturbance than ex i s t i n g platforms, increased the number of apparently suitable nest s i t e s . Platform occupancy can be used to estimate the number of unsuccessful pairs at each colony and provide a more mean-in g f u l index of the population and productivity of herons than MYSN. For example, i f surveys had been undertaken for the f i r s t time i n I 9 8 O , low platform occupancy at Gibsons and Coquitlam would have indicated substantial nest abandonment. The rel a t i o n s h i p between platform occupancy and nests abandoned 40 (Fig. 1-3) can be used to estimate the number of nest f a i l u r e s at colonies where there i s a large proportion of vacant nests. For colony s i t e s which are completely abandoned (e.g. Pender Harbour and U.B.C.) I would expect to f i n d alternate nesting s i t e s . I tested this relationship using data c o l l e c t e d else-where by Des Granges and Laporte (1981, 1983) and Parker (I98O). I noted that 33 small (<20 nests) colonies frequently had no unsuccessful nests (15) or were completely abandoned ( 2 ) . I excluded data from these colonies. I approximated nest occupancy at fledging by subtracting unsuccessful nests from the number reported to be occupied i n May for each colony. Figure 1-4 shows the relationship obtained using that data and the combined regression l i n e . Analysis of covariance showed that the means of residuals from the combined regression l i n e were the same for a l l studies. This rel a t i o n s h i p may be useful i n estimating nest f a i l u r e s i n heronries i n other areas which have low platform occupancy at fledging. At Pender Harbour and U.B.C, where the numbers of nests abandoned exceeded those of platforms occupied during the breed-ing season i n 1978 or 1979 % the colonies were abandoned the following year. I did not c o l l e c t data on nest abandonments at U.B.C. i n 1979 hut the 100 percent platform occupancy at fledging suggests that there were few abandonments and MYBP increased at the new l o c a t i o n . MYSN did not change following colony s h i f t s at U.B.C, Coquitlam and McGillivray-Salwein (Fig. 1-2). Since the number of young raised per successful 41 QUEBEC, MONTANA 20 4 0 6 0 80 100 % PLATFORMS OCCUPIED AT FLEDGING FIG. 1-4 The relationship between the percentage of nest platforms occupied at fledging and the percentage of unsuccessful pairs at colonies in Quebec and Montana. Note: Data from DesGranges 1981 and 1983 and Parker 1980. 42 nest does not change when heronries are severely disturbed or relocate, i t i s probably a poor indicator of reproductive success. Numbers of successful and abandoned nests, which can be estimated using percent platform occupancy at fledging, more accurately represent the reproductive success of heronries. CONCLUSIONS Frequent heronry abandonments, changes'in si z e , and relocations have resulted i n concentrating herons into larger colonies at fewer locations than were h i s t o r i c a l l y present i n the lower mainland. Many of these changes may have been caused by urban development. Sudden large changes i n colony sizes between years probably r e s u l t from movement of adults between colonies. The frequent, and i n some -cases apparently unpro-voked, movements of heron colonies suggest that such occurrences may represent a normal part of a heron's l i f e cycle. Human disturbance at colonies unaccustomed to people increases predation, since adults are e a s i l y frightened from t h e i r nests. Heronries which have adapted to human a c t i v i t y suffer less predation since birds do not r e a d i l y flush from t h e i r nests, and predators may be i n h i b i t e d by human a c t i v i t i e s . Some nest predation occurs i n most heronries, but disturbances which frighten adults from t h e i r nests increase losses of eggs and young and reduce the number of successful nests. Heavy predation continued at one colony a f t e r a construction d i s -turbance stopped. Responses of herons to predators varied 4-3 i n r e l a t i o n to the severity of predation. When large numbers of young or adults are l o s t , heronries relocate. The number of young fledged per successful p a i r i s an i n s e n s i t i v e measure of reproductive success. I t did not vary between colonies even when a colony was severely disturbed. Disturbance affected reproduction by increasing the number of unsuccessful p a i r s . The number of unsuccessful pairs cannot be estimated by nest counts at two periods because nest addi-tions usually exceed nest losses within a colony. To avoid the need to l a b e l i n d i v i d u a l nests and do repeat counts, the percent of nests abandoned can be estimated from the percent of platforms occupied at fledging. This method i s applicable i n other areas, at least for larger heronries (>20 nests). Nest losses, which can be estimated using platform occupancy at fledging, are more important than the numbers of young fledged per successful p a i r i n assessing heron reproductive success and population status. 44 CHAPTER II Movements, Behavior and Breeding Succe of Banded Herons at Pender Harbour 45 INTRODUCTION Results i n Chapter I showed that c h a r a c t e r i s t i c s of breeding birds, such as nest tenacity, were important to reproductive success. Studies of another c o l o n i a l b i r d have shown that selecting the same colony, mate and nest s i t e each year i s associated with increased breeding success (Coulson and Thomas 1983). Other c h a r a c t e r i s t i c s of parents, such as feeding areas used, feeding success and s o c i a l dominance, might also influence reproduction, but no Information exists f o r herons because individuals have not previously been r e l i a b l y i d e n t i f i e d i n colonies or on feeding areas (Quinney and Smith 1979, Mock 1976, McAloney 1973. Pratt 1972 and 1970, Henny and Bethers 1971). Marked birds would also confirm the movements of herons between colonies, f o r which there i s con-siderable circumstantial evidence (Chapter I ) . One of the p r i n c i p a l theories attempting to explain c o l o n i a l nesting suggests that colonies act as information centers for food finding (Custer and Osborn 1978, Ward and Zahavi 1973)• This theory proposes that adjacent individuals within colonies follow each other to good feeding s i t e s . Krebs (1974) and Des Granges (1978) showed that a r r i v a l and departure frequencies and f l i g h t directions of herons at colonies were s i g n i f i c a n t l y clumped, suggesting that birds were t r a v e l l i n g together to and from feeding areas. More recent studies, based on the movements of adults to and from heronries, have shown that many herons have predetermined feed-ing areas or join aggregations of feeding birds adjacent to the 46 colony (Pratt 1 9 8 0 ) and that departure and a r r i v a l clumping i s r e l a t e d to the time of low tide (Bayer I 9 8 I ) . The lack of i d e n t i f i a b l e individuals within heronries has prevented the d i r e c t testing of the information exchange hypothesis. More general studies have rel a t e d feeding s i t e s to heron colony locations. Most larger heronries are located within 10 kilometers of productive marshland or t i d a l feeding areas, and most breeding herons have r e l a t i v e l y predictable feeding areas (Thompson 1 9 7 9 a , Werschkul et a l . 1 9 7 7 . Vermeer 1 9 7 3 ) . Some breeding herons, however, do not use the closest feeding areas and t r a v e l much further to feed (Parr i s and Grau 1979. Thompson 1 9 7 9 b ). Some researchers have observed that many great blue herons have feeding s i t e s which they frequent and sometimes defend (Pratt 1 9 8 0, P i e f e r 1 9 7 9 . Hedeen 1 9 6 7 ). Feeding t e r r i -tory defence i s most pronounced i n winter when herons have dispersed from the breeding colonies and exclusive t e r r i t o r i e s are large and obvious. In contrast, the large feeding fl o c k s , observed i n summer, suggest that herons are not t e r r i t o r i a l . Some reports suggest that communal feeding occurs at seasonal prey concentrations and t i d a l habitats which, because of t h e i r l i m i t e d a v a i l a b i l i t y , are not worth defending (Bayer 1 9 7 8 , Stingle 1 9 7 8 ) . Bayer ( 1 9 7 8 ) also suggested that the greater morta l i t y ' f o r young herons over winter (Henny 1972) may r e s u l t from t h e i r exclusion from winter feeding areas by more domi-nant t e r r i t o r i a l adults. Although feeding area information exchange could p o t e n t i a l l y be useful to c o l o n i a l b irds, the 47 need for such a mechanism among herons, many of which have s p e c i f i c feeding s i t e s or t e r r i t o r i e s , i s questionable. In order to obtain information on mate sel e c t i o n , feeding areas and movements, I banded 60 percent of the adult herons at the Pender Harbour colony (Simpson and K e l s a l l 1979). I compared the reproductive success of banded individuals to t h e i r feeding success; feeding, mate and nest s i t e selection; movements; and s o c i a l dominance. Using data from banded birds I was able to assess the r e s u l t s and conclusions of others who have used le s s d i r e c t methods to study great blue herons. METHODS Adult herons were captured i n net-covered box traps l o c a t -ed on top of the b a i t tanks at three s i t e s i n Pender Harbour (Simpson and K e l s a l l 1979) (Pig. 2-1). Each b i r d was marked with a number-coded red p l a s t i c leg band (Sladen 1978) and a conventional aluminum band on the l e f t ankle. The numbered leg band allowed p o s i t i v e i d e n t i f i c a t i o n of individuals at distances up to 200 meters. The sex of each banded heron was determined from morphological c h a r a c t e r i s t i c s (Appendix I ) . Banded birds were i d e n t i f i e d in the nesting colony, on two t i d a l feeding areas and at three trap s i t e s within Pender Harbour (Fig. 2-1) during the summers of 1978, 1979 and 1980 ( K e l s a l l and Simpson 1980). Sightings and recoveries of dead birds within Pender Harbour and from surrounding areas were also recorded. Most OD FIG. 2-1 Locations of traps, observation blinds and seining sites at Pender Harbour. 49 sightings were at the bait tanks and t i d a l feeding areas during May, June and July 1979. I defined a nest platform as an occupied nest or a struc-ture which had obviously been a nest i n the past. The locations of platforms i n the colony were plotted by measuring the d i s -tances and taking compass bearings between l i t t e r and dropping marks under each. A l l trees containing platforms were numbered and the band numbers of herons occupying each nest recorded i n 1978 and 1979. The geometric center of the colony was deter-mined by taking the mean X-coordinate and mean Y-coordinate of occupied platforms i n each year. The distance of each platform from the colony center was then calculated f o r 1978 and 1979 by simple algebra. In 1979 I v i s u a l l y subdivided the colony into three s e c t o r s — n o r t h , east and west. The north sector•_ contained 18 occupied nests and 16 banded birds; the east, 15 nests and 15 banded birds; and the west, 12 nests and 13 banded b i r d s . Each banded b i r d seen i n the colony was then related to a nest s i t e , a nest distance from center measure and a sector within which i t s nearest neighbors were located. Feeding rates of herons on two t i d a l feeding areas were determined using telescopes from blinds located on the shore from A p r i l to August 1979 (Fig. 2-1). Observers recorded the size and species of each prey item during manipulation by herons p r i o r to swallowing. The length of each item was e s t i -mated by comparison with the b i l l length of the heron (Krebs 1974). I established the weight-length r e l a t i o n s h i p for each 50 prey species by measuring f i s h captured using a beach seine i n Oyster Bay. The biomass caught by herons during each 10-minute observation period was estimated by converting each fish-length to a weight and summing a l l weights. Eleven r e p l i c a t e t r i a l s showed that biomass estimates did not d i f f e r s i g n i f i c a n t l y for three d i f f e r e n t observers concurrently recording information on the same heron (F2 t 30 = »0?6, p > .92). The average weight of prey caught over several t r i a l s was used for comparison of indivi d u a l s and groups of individ u a l s using analysis of variance. Other factors thought to a f f e c t feeding rate were also recorded for each 10-minute t r i a l , Including tide d i r e c t i o n and l e v e l , substrate, water depth and date. I also observed herons feeding at night on b a i t tanks from A p r i l to July 1979* Most b a i t tanks had overhead l i g h t i n g , and t h i s was supplemented by spotlights mounted at leg l e v e l to make band numbers readable. Size estimates of herring caught were unreliable because a l l herring were longer than a herons's b i l l . I estimated the t o t a l weight of f i s h caught i n 10 minutes by multiplying the number caught by the mean weight of samples of f i s h taken from the b a i t tanks i n A p r i l and Ju l y . I established nine seine s i t e s i n Oyster Bay i n March 1979 (Fig. 2-1). Six of those s i t e s were used consistently from March through September. Wooden stakes were used as markers and as anchors for the net during seining. Each s i t e was seined on the ebb and flood tide for at l e a s t three consecutive 51 days each month. The number and species of f i s h captured at each s i t e were recorded. Specimens were taken to confirm i d e n t i f i c a t i o n of each species and a sample of each species was weighed and measured to determine t h e i r mean weight and length each month. The change i n prey biomass per month was estimated by adding the t o t a l weight caught for each species (number caught x mean weight) for a l l seine s i t e s . Prey abundance was then expressed as a mean weight caught per set each month. The dominance and aggressiveness of some banded herons were determined by recording interactions between herons on feeding areas. The retreating b i r d was deemed the l o s e r . In cases where there was no clear winner the i n t e r a c t i o n was c a l l e d a t i e . Herons with the greatest proportion of wins were given the highest rank for dominance; those with the greatest number of interactions were given the highest rank for i n t e r a c t i o n s . For most banded herons the number of young fledged was determined by counts made i n the colony. I had reproductive information for both 1978 and 1979 for 14 banded herons. Reproductive data from 1979 was used for comparison with colony, sighting, feeding rate and s o c i a l i n t e r a c t i o n data, most of which was also c o l l e c t e d i n 1979. 52 RESULTS Capture and Banding We captured a t o t a l of 79 d i f f e r e n t herons—6 8 i n 1978 (Simpson and K e l s a l l 1979) and 11 i n 1979* During June 1979 an average of 14 birds were counted i n Oyster Bay and 26 i n Klein Bay each day (Fig. 2-1). The proportion of banded herons feeding i n those bays was 54 ± 3% (95% C I . ) i n 1978 and 53 ± 6% i n 1979. I estimated that about 125 birds used the Pender Harbour area each year, based on the r a t i o of banded to unhanded bir d s . In 1979» counts of the proportion of birds banded were also made i n the nesting colony (60%) and on the bait tanks (62 ± 11%). Resightings and Movements of Banded Herons A l l but three banded birds were resighted at lea s t once. Resightings averaged 21 times per b i r d with a maximum of 61 sightings. Five banded herons moved the 24 km between Sechelt and Pender Harbour from 1978 to 1 9 8 0 . One in d i v i d u a l (band number A84) moved from Pender to Sechelt and back within 13 days while another (A82) moved from Sechelt to Pender to Secret Cove, 11 km south, within 33 days. Two ind i v i d u a l s , each sighted 11 times at Pender i n June and July 1979 , were seen at Sechelt i n July 1979 or May I 9 8 O . One in d i v i d u a l (A43) was never again seen at Pender Harbour (banded i n June 1978) but was sighted twice at Sechelt i n both 1979 and I 9 8 O . Another heron (A48) was seen at Pender Harbour i n both 1978 and 1 9 8 0 , but not i n 1 9 7 9 . Forty-eight banded herons seen at the Pender Harbour colony i n 1978 or 1979 and a l i v e i n 1979 averaged 25 sightings each. Eighteen others, not seen at the colony, averaged 12 sightings each. Two of those herons (A94 and A57) 53 were seen 50 and 42 times respectively and, although they were never i d e n t i f i e d i n the colony, I suspect that they were two of four u n i d e n t i f i e d banded herons nesting at Pender Harbour i n 1979. Feeding Areas and Nesting Status of Banded Herons I c l a s s i f i e d a l l banded birds as l o c a l feeding or distant feeding, based on the number of sightings at Pender Harbour from 1 9 7 8 to I 9 8 O . Herons with 15 or fewer sightings at Pender Harbour were classed as distant feeding (DF) birds and those over 15 as l o c a l feeding (LF) ( F i g . 2 - 2 ) . I also classed 48 banded herons seen at the colony i n 1 9 7 8 or 1 9 7 9 as colony b i r d s , and 18 not seen, as non-colony b i r d s . Overall there were 36 colony LF birds, f i v e non-colony LF birds, 12 colony DF birds and 13 non-colony DF birds (Table 2 - 1 ) . In 1 9 7 8 , 75% (N = 2 1 ) of the colony birds were LF, and 81$ were (N = 44) i n 1979. For both years combined, LF birds had more successful nests than DF birds (Table 2 - 2 ) , although the difference was not s i g n i f i c a n t for either year alone. DF herons i n successful nests r a i s e d as many young as LF herons i n both years. I examined the sighting records of a l l unsuc-ce s s f u l DF herons to determine i f they were classed as DF birds because they l e f t Pender Harbour or fed less often a f t e r l o s i n g t h e i r young. Only one i n d i v i d u a l (A55, Table 2-3) had s i g n i f i -cantly fewer sightings a f t e r l o s i n g i t s nestlings compared to sightings for successful DF b i r d s . Most DF herons l o s t t h e i r young l a t e i n the nesting season so i t i s u n l i k e l y that they were m i s c l a s s i f l e d . 54 DF—I LF 0 5 10 15 20 25 30 35 4 0 45 5 0 55 NO. OF SIGHTINGS FIG. 2 -2 Frequency distribution of the number of sightings of 41 local feeding ( L F ) and 25 distant feeding (DF) banded herons at Pender Harbour. Table 2 -1 . Number of sightings of banded herons at Pender Harbour from 1978 to 1980 categorized by frequency of observation, sighting l o c a t i o n and presence i n the colony. Local (>15 s feeding ightings) Distant feeding (<15 sightings) Colony (seen at colony) Non-colony (not at colony) Colony Non-colony No. of banded birds 36 5 1 12 1 3 2 No. of sightings on tide f l a t s 797 137 37 29 No. of sightings on bait tanks 316 21 38 35 1. Includes A 94 and A57 suspected colony b i r d s — s e e • text. 2. Includes f i v e seen at Sechelt and one recovered May 1979 on Texada Island. Note: With one noted exception includes only birds known to be a l i v e i n June 1979 or l a t e r and for which feeding status was known. Table 2 - 2 . Comparison of the reproductive success of l o c a l feeding (LF) and distant feeding (DF) herons at Pender Harbour i n I 9 7 8 and 1 9 7 9 . No. success-f u l MYSN SD No. unsuc-cessf u l x 2 P 1 9 7 8 LF 1 3 1 . 8 . 7 3 2 DF 3 2 . 7 . 5 8 2 I . 6 7 > . 1 0 1 9 7 9 LF 2 7 3 . 0 . 7 6 7 DF 3 2 . 7 . 5 8 5 3 . 6 5 •y . 0 5 1 9 7 8 LF 40 9 and 1 9 7 9 DF 6 7 6 . 7 5 < . 0 1 Note: Three herons questionable for which LF or ( A 5 5 , Table 1 6 ) , DF status was not included. undetermined ( A 6 8 , A 7 6 ) or Table 2-3. Comparison of the number of sightings of distant feeding (DF) herons to determine i f unsuccessful birds were seen l e s s often than successful birds a f t e r t h e i r young were l o s t . B i r d Sightings before date young l o s t Date young l o s t Sightings a f t e r date young l o s t X2 P A21 SDF 0 17 June 19 3 14 1.95 < .1 A27 SDF 4 22 June 24 3 20 0.03 < .8 A38 SDF 0 5 May 20 0 37 1.89 < .1 A46 SDF 5 22 June 24 0 20 2.35 < .1 A 54 SDF 2 17 June 16 0 25 2.76 .05 A55 SDF 5 5 May 20 4 37 6.17 < .02 58 Nest Sit e s , Mates and Feeding Areas of Banded Herons Twenty-one banded birds were i d e n t i f i e d on nests i n the colony i n 1978 and 44 i n 1979 including, for both years, 18 banded p a i r s . In 1978, I I d e n t i f i e d f i v e pairs where both mates were banded. None of these pairs d e f i n i t e l y remained together i n 1979• Seven of the birds were i d e n t i f i e d with d i f f e r e n t mates i n 1979. two were not seen i n the colony and one was paired with an u n i d e n t i f i e d banded b i r d . Of the 21 herons i d e n t i f i e d on nests i n 1978, 13 were on d i f f e r e n t nests i n 1979. one on the same nest and seven were not seen i n the colony. I did not observe any adults moving between nests within one nesting season. Eight pairs l o s t a l l t h e i r eggs or young i n 1979• None of the 13 banded birds involved i n these pairs attempted to renest i n the Pender Harbour colony i n 1979. although I did observe two unsuccessful attempts at l a t e nesting by unbanded i n d i v i d u a l s . Successful nests were much closer to the center of the colony than unsuccessful nests i n 1978. In 1979. however, there was l i t t l e difference between these groups (Table 2-4). The mean distance of a l l occupied nests from the colony center was about 30 meters i n both years. I examined nest p o s i t i o n and feeding locations of banded herons to determine i f banded herons nesting close together also fed i n the same areas i n Pender Harbour. Herons that nested i n the same sector of the colony did not feed i n the same areas. In fact, herons from each sector of the colony were uniformly d i s t r i b u t e d among the four p r i n c i p a l feeding areas i n Pender Harbour (Table 2-5). 59 Table 2-4. Mean distances (m) of successful and unsuccessful nests from the center of the colony i n 1978 and 1979 at Pender Harbour. Successful Unsuccessful Median 1 I SE N X SE N test 1978 24.1 I.96 25 42.5 4.41 14 p <.001 1979 29 .4 I.71 33 31.6 3.36 12 p <.56 Table 2-5. Total number of banded herons from each sector of the Pender Harbour colony seen feeding together on four d i f f e r e n t days i n 1979. Feeding Number of herons from each sector l o c a t i o n North East West X K l e i n Bay 10 21 15 Oyster Bay 18 11 13 Trap 1 18 15 15 Trap 2 9 8 9 6.05 (P > .3) I also examined the feeding locations of banded pairs i n 1979 to test i f the male and female tended to use the same foraging zone. In eight of the 11 p a i r s , the male and female d i f f e r e d s i g n i f i c a n t l y i n t h e i r frequency of sightings at fi v e d i f f e r e n t feeding locations (Table 2-6). One member of each of the three pairs which did not d i f f e r s i g n i f i c a n t l y had few sightings, making a v a l i d comparison d i f f i c u l t . The members of banded p a i r s , which fed mostly within Pender Harbour, therefore used d i f f e r e n t feeding areas i n 1979. 6o Table 2-6. The number of sightings for each member of 11 banded pairs at f i v e feeding locations i n Pender Harbour i n 1979. Banded pair Feeding status Kl e i n Feeding Oyster locations T l T2 T3. X 2 df • P A26 LF 18 0 11 0 • 0 A37 LF 15 •5 3 0 0 6.1 2 . 0 5 A65 LF 16 0 0 0 0 A35 LF 3 21 1 0 0 26.4 1 < . 0 5 A33 LF 6 11 2 0 0 A21 ? 2 1 0 0 0 1.4 1 > . 0 5 A 54 DF 1 1 0 0 0 A93 LF 28 1 1 1 1 2.1 1 > . 0 5 A59 LF 24 13 13 6 0 A7l LF 39 8 0 0 2 1 9 . 5 3 < . 0 5 A63 LF 21 2 0 4 0 A90 DF 6 1 0 1 0 0.2 1 > . 0 5 A70 LF 18 5 7 0 1 A 64 LF 0 8 6 0 0 14 .6 2 < . 0 5 A78 LF 20 3 4 0 2 A77 LF 1 16 10 0; 1, 24.8 3 < . 0 5 A86 LF 11 11 2 0 1 A 79 LF 2 18 11 0 0 12.0 2 < . 0 5 A95 LF 4 22 15 1 0 A66 LF 26 2 10 1 0 3 3 . 7 2 < . 0 5 A99 LF 12 1 0 7 0 A56 LF 6 17 12 0 1 14 .6 3 < . 0 5 Note: Includes only pairs with 220 sightings i n 1979 and categories combined i f expecteds<l. 61 Feeding on T i d a l Areas We completed 907 10-minute feeding t r i a l s from A p r i l to August 1979. Several environmental factors influenced feeding rates of the herons (Table 2-7). Most of those factors r e f l e c t i n d i v i d u a l choice and could be considered inherent i n the feed-ing rates of d i f f e r e n t i n d i v i d u a l s . Herons captured more prey on ebbtides, and i n deeper water (Table 2-7). The biggest source of v a r i a b i l i t y i n feeding rates, however, was the change i n prey abundance with time. The mean weight of prey caught per feeding t r i a l and prey abundance, estimated from seine sampling, increased from A p r i l to June then decreased i n July and August (Fig. 2-3) . Catch rates did not d i f f e r between May, July and August. I compared the catch rates of banded herons which successfully raised young with those that f a i l e d i n 1979. Catch rates did not d i f f e r during June or during the combined May, July, August period (Table 2-8). I also compared the prey capture rates between successful herons, which raised from one to f i v e young, to determine i f birds which fledged more young had higher prey capture rates. There was no relationship between the feeding rates of the parents and the number of young raised ( F 3 , I+QJ = 1«5, P > .22). Other data also sug-gested that food a v a i l a b i l i t y and catch rates of herons did not l i m i t the reproductive success of herons at Pender Harbour. A single female successfully raised two young a f t e r her mate died on June 9, 1979. Despite the fact that the maximum food demand of young birds occurs i n la t e June, just p r i o r to fledging, t h i s single female was able to supply the needs of her young. 62 Table 2-7. Factors a f f e c t i n g the weight (gm) of -prey caught by herons on t i d a l feeding areas. Factor Level f o ^ i n ? ^ ! S D N F S i g n i f . Tide Ebb 13.86 10.5 556 9.4 .002 d i r e c t i o n Flood 11.48 12.3 336 Location K l e i n 13.09 11.7 539 3.1 .079 Oyster 11.71 9.4 309 Water Dry 5.19 3.4 9 depth Ankle 11.95 11.1 161 < knee 12.60 9.9 378 > knee 12.31 9.6 211 Feather 17.55 16.2 148 7.7 .000 Bottom Algae 12.58 5.8 20 substrate Eelgrass 14.26 11.6 717 Marsh 14.17 16.1 4 Mud 8.30 9.5 147 Oysters 10.33 11.9 16 7.6 Rock 2.19 2.1 3 .000 Grand mean 13.15 t o t a l 907 63 FIG. 2-3 Correlation between prey caught by herons ( N • 907) and prey caught by seining (N = 244) for five months in 1979. 64 Table 2-8. Comparison of the weight of. prey caught by herons which successfully raised young and those that f a i l e d to r a i s e young i n 1979. T i * e „ du?tri°ve * w t ; c a ^ f c / SD N F S i g n i f . P e r l o d status 1 0 - m i n - t r l a l June Successful 16.47 9.9 70 F a i l e d 13.01 5.9 24 2.6 .110 May, July, Successful 13.49 8.1 182 August Fa i l e d 14.98 10.0 38 1.0 .319 Feeding at the Bait Tanks Observations of herons feeding on the b a i t tanks i n A p r i l (N = 15) and June (N = 6) showed that catch rates were much higher (392 ± 64 and 186 ± 22 g, respectively) than on natural t i d a l areas (2.97 ± .33. N = 54 and 16.18 ± .79, N = 314, res-pectively) for 10-minute feeding t r i a l s . Herons at the b a i t tanks frequently captured two or three herring i n a single s t r i k e . I saw two herons capturing herring then releasing them apparently a f t e r t h e i r hunger was satiated. Feeding success of herons on the b a i t tanks vari e d greatly depending on the number and behavior of f i s h i n the tank. The b a i t tanks did provide an extraordinary and a t t r a c t i v e food source for herons at Pender Harbour. Aggressive Interactions Most aggressive interactions (82 percent) occurred on the b a i t tanks because of the crowding of birds competing for favorable feeding s i t e s . Forty-nine evening counts made from 65 mid-June to mid-July 1979 on two bait ponds showed a mean of 10.5 herons with a maximum of 2b and a minimum of six birds using each pond. Other interactions (18 percent) were recorded on t i d a l feeding areas during feeding t r i a l observations. I recorded 572 aggressive interactions involving 51 banded herons i n 1979. Although the average was 11.2 interactions per b i r d , the d i s t r i b u t i o n was skewed right,so that only 16 herons were involved i n more than 10 i n t e r a c t i o n s . I was also able to c l a s s i f y 38 of those 51 birds as successful or unsuccessful breeding and l o c a l (LF) or distant feeding (DF) i n 1979. Successful LF herons consistently won more aggressive interactions than f a i l i n g DF herons, but there were few f a l l -ing birds of known status and the difference was not s i g n i f i c a n t (Table 2-9). Successful and LF birds were involved i n more encounters than f a l l i n g or DF herons (Table 2-10) . Differences i n numbers of interactions for DF and LF birds probably r e l a t e to t h e i r d i f f e r i n g frequency of observation at s i t e s where interactions were recorded. For LF birds, the lower number of interactions for f a i l i n g herons may relate to t h e i r avoidance of confrontations with other b i r d s . I found no relat i o n s h i p between the distance of a nest from the colony center or the number of young raised and the dominance or i n t e r a c t i o n rank of the parents. 66 Table 2-9. Comparison of the dominance of banded herons with d i f f e r e n t reproductive success and feeding areas at Fender Harbour i n 1979. Reprod. status Local/ distant feeding N x % wins Kruskal-Wallis H. S i g n i f . — LF DF 38 13 45.6 30.2 2.13 0.14 Succ. F a i l . LF LF 27 6 47.2 23.8 2.59 0.11 Succ. F a i l . DF DF 3 2 18.6 50.0 0.08 0.77 Colony-Colony LF DF 33 5 42.9 31.1 0.67 0.41 Succ. F a i l . — 30 8 44.3 30.4 0.74 0.39 Table 2- 10. Comparison of the t o t a l number of aggressive interactions for banded herons i n J r e l a t i o n to reproductive success and feeding areas at Pender Harbour i n 1979. Reprod. status L o c a l / distant feeding N x no. i n t e r -actions Kruskal-Wallis H. S i g n i f . LF DF 38 13 13-2 5.5 5.76 0.02 Succ. F a i l . LF LF 27 6 15.8 4.2 6.47 0.01 Succ. F a i l . DF DF 3 2 7.3 1.0 1.33 0.25 Colony Colony LF DF 33 5 13.7 4.8 3.21 0.07 Succ. F a i l . — 30 8 15.0 3.4 9.15 0.01 67 Table 2-11. Changes i n reproductive success for eight males and six females from 1978 to 1979. Reproductive status 1978 1979 Males F a i l . Succ. 1 1 7 7 Females F a i l . Succ. 0 6 3 3 Sex and Reproductive Success A l l 79 captured herons were sexed using the discriminant function shown i n Appendix I. The banded group included 42 males and 37 females. For herons not nesting at Pender Harbour there were s i g n i f i c a n t l y more male DF birds (10) captured than female DF birds (three) (binomial, p < .05). I obtained reproductive information for both 1978 and 1979 for eight males and six females (Table 2-11). Their reproduc-tive performance was si m i l a r , but that of females was less predictable. Two females at Pender Harbour i n 1979 exhibited greater nest tenacity and defence than males. One female raised two young a f t e r losing her male partner. Another female defended her nest against scavenging ravens for two days a f t e r her young were l o s t to raccoons. 68 DISCUSSION Colony Formation Most herons known to nest at Pender Harbour foraged close to the colony s i t e . The food demand of young herons ensures that foraging parents return regularly to the i r nests and, i f the maximum time away from the nest i s to be used for gathering food, feeding s i t e s must be close to the colony. Werschkul et a l . (1977) found that heron colonies i n coastal Oregon are placed adjacent to good feeding grounds and colony size was rel a t e d to. the size of. the adjacent estuaries. Colonies are probably formed by groups of herons which congregate at the large estuaries to feed. Locally-feeding herons formed the majority of the Pender Harbour colony i n both 1978 and 1979 and probably determined i t s l o c a t i o n . Most herons nesting at Pender Harbour i n 1978 selected a new mate and nest s i t e i n 1979. This i s i n sharp contrast to other c o l o n i a l species, e s p e c i a l l y g u l l s , which show con-siderable colony, nest and mate f i d e l i t y between years (Southern and Southern 1982). It has been shown that kittiwakes (Rissa trl d a c t y l a ) have improved reproductive success i f they mate with the same Individual each year (Coulson and Thomas 1983). Factors which can cause mate switching, such as death of the former mate or unsuccessful* reproduction, did not cause herons to switch at Pender Har-bour. Eight of 10 individuals paired i n 1978 were s t i l l present i n the colony i n 1979 and a l l f i v e banded pairs were 69 successful i n 1978 yet none remained together i n 1979. This lack of nest s i t e and mate f i d e l i t y coincides with r e s u l t s i n Chapter I which suggested that breeding herons frequently move between colonies and establish new nests. Changes i n loca-tions and numbers of nests during the nesting season resulted from unsuccessful birds leaving and new birds entering the Pender Harbour colony rather than from movements within the colony. Those facts again suggest that herons show l i t t l e attachment to s p e c i f i c nesting s i t e s . Because of t h i s lack of p r e d i c t a b i l i t y of nests or mates, heronries are Important assembly areas for herons to f i n d new mates and nest s i t e s each year. Although herons nesting i n peripheral nests had poorer reproductive success (Table 2-4), central nests were not occu-pied by herons more dominant on the feeding areas. The nest p o s i t i o n of pairs i n a colony i s l a r g e l y dependent on the males, which select and defend nest s i t e s early i n the breeding season p r i o r to the formation of p a i r bonds (Rodgers 1978, Meyerriecks i 9 6 0 ) . Others have observed that older, established nests and those furthest from the ground are the f i r s t occupied by displaying males, probably to a i d i n a t t r a c t i n g females (Parker I 9 8 O, Rodgers 1978, Henny and Bethers 1971). Nest height may also be important i n colonies where average heights are low (<4.m) or i n tree species such as cottonwood and Douglas f i r where there i s a large difference between the lowest and highest nests and there are many nests per tree. Higher nests should be less vulnerable to tree-climbing predators since 70 lower nests would be encountered f i r s t . A lternately, higher nests may be more vulnerable to avian predators which come from above. At Pender Harbour there was generally one nest per tree (Table 1-3). nest heights varied l i t t l e (x = 26 m, SD = 1.51» n = 65) and there was no r e l a t i o n s h i p between nest occupancy or reproductive success and nest height. Parker ( I 9 8 O ) noted that returning herons occupied nests i n a l l parts of the colonies and did not group together. Later a r r i v i n g birds were then able to occupy s i t e s between established pairs, who had reduced the t e r r i t o r y defended to the area immediately surrounding t h e i r nests (Rodgers 1978). The f i n a l position of a nest i n a heronry i s dependent on where subsequent birds choose to nest. At Pender Harbour there was a large number of vacant nests. Early a r r i v i n g birds which chose nests near the center of the available s i t e s may s t i l l have ended up near the edge of the colony i f l a t e r a r r i v i n g birds nested mostly to one side. This suggests that a l l s i t e s are equally available to any i n d i v i d u a l . The possible advantages of joining a colony, which are derived mainly from the proximity of neigh-bors (Wittenberger I 9 8 I ) , would therefore be equally available to each of i t s members. Herons are attracted to colony s i t e s to f i n d mates, to locate t h e i r nests near good feeding s i t e s and to reduce the v u l n e r a b i l i t y of t h e i r young to predators by nesting i n groups. 71 Predation, Reproductive Success and Adult Survival Predation was probably the primary cause of nest f a i l u r e at Pender Harbour. Predation t y p i c a l l y r e s u l t s i n the loss of entire nests (Jennl 1969. Dusi and Dusi 1968), i n comparison to starvation or s i b l i n g r i v a l r y , which reduce the number fledged per successful nest (McAloney 1973. Pratt 1972, Owen i 9 6 0 ) . Higher losses among peripheral nests (Table 2-4) also implicate predators as the cause. Nests near the edge of a colony are the f i r s t encountered by predators and, because they have fewer close neighbors than central nests, the advan-tages of swamping are reduced (Wittenberger 1981). Predators have been considered important causes of nest f a i l u r e for great blue herons i n other areas, and for c o l o n i a l species i n general (Hjertaas 1982, Ward and Zahavi 1973. M i l s t e i n et a l . 1970). DF herons had more nest f a i l u r e s than residents but, i n successful nests, raised as many young as residents (Table 2-2). DF birds must have chosen areas outside Pender Harbour for feeding. Two of those individuals were seen only at the bait tanks while the others were seen so i r r e g u l a r l y at t i d a l areas that I doubt i f they could have supported nestlings without alternate feeding areas. Thompson (1979b), i n Montana, and P a r r l s and Grau (1979). on the Great Lakes, showed that some great blue herons t r a v e l l e d up to 20 or 30 km, respectively, to feeding areas from t h e i r nesting s i t e s . I f those distances apply to herons at Pender Harbour, some breeding birds may have been feeding as far away as Sechelt. Since short term movement 72 between Pender Harbour and Sechelt was seen, such long d i s -tance foraging by breeding adults seems possible. Yom-Tov (1974) experimentally manipulated the food supply of breeding crows and found that nest losses increased when food was placed further from the nest. He suggested that when food i s dispersed the nestlings are more vulnerable to preda-t i o n because the parents spend more time f a r away from the nest. DF herons may have more nest f a i l u r e s because they spend more time away from t h e i r nests while foraging for food. The reasons why DF herons fed i n areas f a r away from t h e i r nests, apparently at the r i s k of lo s i n g t h e i r young, are not clea r . Although LF herons were not c l e a r l y dominant over nesting DF birds, i t i s possible that DF herons were forced away from Pender Harbour by competition from LF b i r d s . Several LF herons excluded a l l other herons from selected parts of the bait tanks. Differences i n number of interactions between LF and DF herons probably r e l a t e to t h e i r d i f f e r e n t occurrence on feeding areas (Table 2-1). Fewer interactions for DF herons was probably a r e s u l t rather than a cause of t h e i r observed lower use of feeding s i t e s within Pender Harbour. Bayer (I978) found that some herons which maintain feeding t e r r i t o r i e s over winter had better s u r v i v a l than non-t e r r i t o r i a l herons. DF herons nesting at Pender Harbour may have t r a v e l l e d to distant feeding areas to maintain preferred t e r r i t o r i e s . Improved winter s u r v i v a l may have been more important to the individuals than decreased reproductive success. My data was i n s u f f i c i e n t to tes t i f DF birds had 73 above average over-winter survival and I d i d not distinguish between t e r r i t o r i a l and n o n - t e r r i t o r i a l herons within Pender Harbour during the winter. Colonial Nesting and Information Exchange Krebs (197*0 postulated that transfer of feeding informa-ti o n may be one of the p r i n c i p a l advantages to c o l o n i a l nesting i n herons. There was no evidence to suggest that herons at Pender Harbour used any type of feeding area information exchange. Neither paired herons nor neighbors tended to feed i n the same areas as would be expected i f herons followed each other to good feeding s i t e s (Krebs 1974). In fact, pairs tended to feed i n d i f f e r e n t areas (Table 2-6). Prey abundance and d i s t r i b u t i o n was predictable and feeding rates of herons did not vary greatly on l o c a l t i d a l areas (Table 2-7). Large tides, i n June, expose the maximum estuarlne habitat coincident with the peak prey abundance, peak adult feeding rates (Pig. 2-3) and maximum food demand of the young herons. Reproductive synchrony and timing at the Pender Harbour colony probably ensures that the food demand coincides with the seasonal changes i n supply. A single adult successfully r a i s i n g two young suggests that food i s not l i m i t -ing. I found no evidence of the reduced brood sizes normally associated with food l i m i t a t i o n and starvation (Des Granges 1979. Pratt 1972, Owen i960), nor was there any r e l a t i o n between feeding rates of parents and number of young fledged. Under those favorable circumstances there i s no need f o r herons to "share" feeding area information. 74 Although much of my Information indicates that many herons at Pender Harbour had s p e c i f i c feeding areas (Table 2-6), other data also shows t h e i r a b i l i t y to exploit extraordinary or unusually abundant food supplies. Sightings of four of the 13 DF non-colony birds (Table 2-1) suggest that they came from frequently used areas outside Pender Harbour. B i r d A43 was resighted only at Sechelt, suggesting that i t may have been a DF heron from the Sechelt colony when i t was captured i n 1978. The movements of A82 and A84, and the sighting data for A48, show that they frequented other areas and only occasionally v i s i t e d Pender Harbour. Those birds were probably attracted to Pender Harbour by the abundant food supply at the ba i t tanks. Some of the breeding DF herons at Pender Harbour may have had si m i l a r movement habits. I f those birds occasionally located extraordinary feeding s i t e s , that information could be trans-ferred to other neighboring adults i n the colony when they returned to feed t h e i r young. Local food shortages have occurred at heronries i n other areas (review by Des Granges 1979) possibly due to unpredict-able declines i n prey abundance or poor weather conditions. I f the l o c a l food supply at the Pender Harbour colony were to f a l l , DF herons i n the colony could lead other colony members to alternate feeding areas. A l o c a l food shortage would be required to adequately test the information exchange hypothesis using great blue herons at Pender Harbour. 75 Sex and Reproductive Success Great blue heron pairs share the r e s p o n s i b i l i t y of incubating and feeding the young. Adult herons tend the nest continuously during incubation and for the f i r s t three to four weeks a f t e r hatching (Pratt 1970). From June 1 u n t i l fledging both parents return regularly to feed the young, but do not maintain constant v i g i l a n c e at t h e i r nests. I have some information which suggests that d i f f e r i n g habits of males and females may a f f e c t reproduction. Sig-n i f i c a n t l y more males than females were captured i n the DF non-colony group at Pender Harbour. Although there i s no information on the sex r a t i o i n the great blue heron population, there are generally more females than males i n populations of large c o l o n i a l species (Coulson and Thomas 1983). Band recoveries at Pender Harbour suggest that adult m o r t a l i t i e s were divided equally between females (seven) and males ( s i x ) . I f we conservatively assume an equal male/female r a t i o i n surrounding areas, more DF males must have t r a v e l l e d to the b a i t tanks i n Pender Harbour to increase t h e i r capture f r e -quency. I t i s possible that male DF herons feed at greater distances from t h e i r nests than females and are, therefore, le s s attentive at the nest s i t e . I f i n d i v i d u a l herons maintain s i m i l a r reproductive e f f o r t from year to year, changes i n reproductive success probably r e l a t e to other factors such as the e f f o r t of the mate. Three of s i x females successful i n 1978, f a i l e d i n 1979 when paired with d i f f e r e n t males. Only one of seven males involved i n 76 i successful 1978 matings f a i l e d i n 1979 (Table 2-11). Although those observations are not s i g n i f i c a n t l y d i f f e r e n t , they suggest that female reproductive success can be affected by the behavior of t h e i r male partners. Other observations of female tenacity at the nest and a b i l i t y to rai s e young alone suggest that females maintain a higher and more consistent l e v e l of reproductive e f f o r t than males. The suggested differences i n the movements and nest tenacity of males and females may Indicate that males have greater r e s p o n s i b i l i t y f o r finding food, while females have greater r e s p o n s i b i l i t y for tending the young. CONCLUSIONS The Pender Harbour heron colony was composed of a core group of LF herons which fed p r i n c i p a l l y i n the two adjacent t i d a l estuaries during the day and at l i v e b a i t f i s h ponds at night. Herons with feeding areas outside Pender. Harbour (DF) constituted about 22 percent of the Pender Harbour colony. Most herons selected a new mate and nest s i t e from 1978 to 1979. The colony s h i f t s and size fluctuations noted i n Chapter I undoubtedly relate to the lack of attachment of breeding herons to nesting s i t e s or mates. Central nest s i t e s were most successful but differences i n i n d i v i d u a l dominance, determined at feeding s i t e s , were not re l a t e d to nest s i t e occupancy. Herons are probably attracted to colonies primarily to f i n d suitable mates, to locate t h e i r nests near good feeding 77 s i t e s and to reduce the v u l n e r a b i l i t y of t h e i r young to preda-tors by swamping. DF herons suffered more nest f a i l u r e s than LF birds, but fledged as many young as LF herons i n successful nests. The frequent loss of entire clutches, higher losses i n peripheral nests and d i r e c t observations of predators i n the colony confirmed predation as the primary cause of nest losses. DF herons probably spent more time away from t h e i r nests while foraging for food, and thereby exposed t h e i r young to heavier predation. DF herons nesting at Pender Harbour may have been le s s dominant at feeding s i t e s than LF birds, so i t i s possible that they were forced to use more distant feeding s i t e s by i n t r a s p e c i f i c competition. Other research indicates that herons which maintain feeding t e r r i t o r i e s have better over-winter s u r v i v a l . DF herons nesting at Pender Harbour may have continued to use distant feeding areas to maintain t e r r i t o r i e s which were valuable for winter s u r v i v a l even though t h e i r reproductive success was lowered. Exchange of information about feeding areas did not appear to be occurring i n the Pender Harbour colony. Prey abundance and d i s t r i b u t i o n was predictable in time and space, based on . r e s u l t s of seine sampling and observations of feeding herons. Peak prey abundance and feeding habitat a v a i l a b i l i t y occurred coincident with peak food demands of young herons i n the colony. Under those circumstances there was obviously no need for any Information exchange to locate good feeding areas. 78 Local food shortages and starvation have been observed in other heronries. Under those conditions, LF herons could follow DF birds to alternate feeding areas. Food shortages and unpredictable supply should be demonstrated before the information exchange hypothesis for c o l o n i a l nesting can be tested using great blue herons. Indirect evidence suggests that males may be wider ranging than females. Because males may spend l e s s time at the nest and leave the nest unattended, reproduction for some females may be l i m i t e d by thi s behavior of the male. Since males i n i t i a t e nesting and colony formation, t h e i r movements may also be responsible for some of the observed colony i n s t a b i l i t y . 79 GENERAL DISCUSSION AND RECOMMENDATIONS Heronries frequently change locations or fluctuate i n size dramatically. Such movements and changes i n size have previously been associated with disturbing influences or pre-dation and were considered unnatural or detrimental occurrences. Great blue herons d i f f e r from other c o l o n i a l species because colonies are not composed of discrete groups of birds which return annually to the same s i t e . Each year varying numbers of herons gather and form colonies near good feeding s i t e s but not necessarily at previously used locations. The lack of mate and nest s i t e f i d e l i t y , observed at one disturbed colony, may be i n d i c a t i v e of herons generally and probably contributes to the observed i n s t a b i l i t y of many heronries. Colonies which relocated bred successfully at new locations and may, at l e a s t temporarily, have avoided some of the predators which frequented established heronries. The greatest distance moved by a heronry was 10 kilometers, but most moves were under f i v e kilometers i n t h i s study. A c t i v i t i e s which may cause relocations should be avoided unless there i s suitable s i m i l a r habitat available within 10 kilometers. The proximity of preferred feeding areas to p o t e n t i a l nesting areas may be important i n determining the maximum distance a heronry might move. Human disturbances had the e f f e c t of increasing the success of natural predators in heronries and frequently resulted i n colony relocations. Although some heronries adapted to human a c t i v i t i e s , others, away from regular human a c t i v i t y where 80 adults f l e d from people, should be undisturbed during the nesting season ( A p r i l - J u l y ) . The percentage of nests successful was the best s t a t i s t i c to assess reproduction i n heronries. The method of determin-ing the numbers of successful and unsuccessful p a i r s , without causing a major disturbance i n the colonies, should be further explored. Colony censuses should include an accurate count of occupied and vacant nest platforms during f l e d g l i n g counts. In colonies adapted to human a c t i v i t i e s the rel a t i o n s h i p between nest occupancy and numbers of breeding pairs could be further examined. Banding of i n d i v i d u a l s , at one colony, has shown that breeding herons have d i f f e r i n g habits which r e l a t e to repro-duction, over-winter s u r v i v a l and food-finding. Males may have greater r e s p o n s i b i l i t y for foraging and food-finding while females may provide more nest defence. Although distant feeding herons had lower reproductive success than l o c a l feeders, they may have gained long-term advantages by main-taining t e r r i t o r i e s important for over-winter s u r v i v a l . Birds which t r a v e l further to feed may also have more options i n choosing colonies i n which to nest. Further studies using banded herons could better define the roles of males and females and the importance of distant feeders to formation of colonies and l o c a t i o n of feeding areas. 81 LITERATURE CITED BAYER, R.D. 1978. Aspects of an Oregon Estuarlne Great Blue Heron Population. In: Wading Birds. Research Report No. 7, National Audubon Society, New York. A. Sprunt, J.C. Ogden and S. Winckler, eds. Pp. 213-217. BAYER, R.D. 1979. Bald Eagle-Great Blue Heron Interactions. Murrelet 60: 32-33. BAYER, R.D. I98I. A r r i v a l and Departure Frequencies of Great Blue Herons at Two Oregon Estuarlne Colonies. The Auk 98: 589-595. BENNING, W.F. 1969. A Survey of Great Blue Heronries 1964-69. Kingbird 19: 85-90. BLUS, L.J., C.J. HENNY and T.E. KAISER. 1980. P o l l u t i o n Ecology of Breeding Great Blue Herons i n the Columbia Basin, Oregon and Washington. Murrelet 61: 63-71. CAMPBELL, R.W., M.G. SHEPHARD and W.C. WEBER. 1973. Vancouver birds i n 1971. Vancouver Nat. His. Soc. 88 pp. CAMPBELL, R.W., M.G. SHEPHARD, B.A. MacDONALD, and W.C. WEBER. 1974. Vancouver birds i n 1972. Vancouver Nat. H i s t . Soc. 96 pp. CANADIAN WILDLIFE SERVICE. 1971. Pesticides and w i l d l i f e . C.W.S. Rept., Information Canada, Ottawa. 24 pp. COOK, D.C. 1978. Grey herons holding feeding t e r r i t o r i e s on the Ythan Estuary. B i r d Study 25 : 11-16. C0ULS0N, J.C. and C.S. THOMAS. I983. Mate choice i n the Kittlwake g u l l . In: Mate Choice. P.P.G. Bateson, ed. Cambridge Univ. Press. CUSTER, T.W. and R.G. 0SB0RN. 1978. Feeding Habitat Use by Colonially-Breeding Herons, Egrets and Ibises i n North Carolina. The Auk 95' 733-743. DES GRANGES, J.L. 1978. Adaptive Value of S o c i a l Behaviour i n the Great Blue Heron (Ardea herodlas). In: Proc. 1978 Conf. of the Colonial Waterbird Group: 192-201. DES GRANGES, J.L. 1979. Abandoned Windmill Used as a Nesting Site by Great Blue Herons. Canadian F i e l d - N a t u r a l i s t 93 : 439-440. 82 DES GRANGES, J.L. I 9 8 O. A Canadian program for surveillance of Great Blue Heron (Ardea herodias) populations. Proc. Colonial Waterbird Group 1979: 59-69. DES GRANGES, J.L. and P. LAPORTE. I 9 8 I . Third Tour of Inspection of Quebec Heronries 1979. C.W.S. Progress Note No. 123. 10 pp. DES GRANGES, J.L. and P. LAPORTE. I 9 8 3 . Fourth and F i f t h Tours of Inspection of Quebec Heronries, I 9 8 O - 8 I . C.W.S. Progress Note No. 139. 11 PP. DUSI, J.L. and R.T. DUSI. I 9 6 8 . Ecological factors con-t r i b u t i n g to nesting f a i l u r e i n a heron colony. Wilson B u l l . 80: 458-466. FORBES, L.S. I98O. Behavioural studies of Great Blue Herons (Ardea herodias) at Pender Harbour and Sechelt, B.C., i n 1980. C.W.S. Manuscript Rept., Delta, B. C. 60 pp. FORBES, L.S., K. SIMPSON, J.P. KELSALL and D.R. FLOOK. I 9 8 3 . Great blue heron colonies i n B r i t i s h Columbia. C. W.S. Rept., Delta, B.C. 66 pp. FRY, K. I98O. Memo regarding eagle attacks at the Crescent colony. C.W.S. F i l e 9050-Herons, Delta, B.C., June 9, 1980. HEDEEN, S. I967. Feeding Behaviour of the Great Blue Heron i n Itasca State Park, Minnesota. Loon 39: 116-120. HENNY, C.J. and M.R. BETHERS. 1971. Population Ecology of the Great Blue Heron with Special Reference to Western Oregon. Canadian F i e l d - N a t u r a l i s t 85: 2 0 5 - 2 0 9 . HENNY, C.J. 1972. An analysis of the population dynamics of selected avian species with sp e c i a l reference to changes during the modern pesticide era. U.S. F.&W. Service W i l d l i f e Res. Rept. No. 1, Washington, D.C. HJERTAAS, D.G. 1982. Great Blue Herons and Racoons at Nl c o l l e F l a t s . Blue Jay 40: 36-41. JENNI, D.A. I969. A Study of the Ecology of Four Species of Herons during the Breeding Season at A l i c e Lake, Alachua County, F l o r i d a . Ecol. Monogr. 39: 245-270 JEREMA, R.S. 1973. Birds of Port Coquitlam. Manuscript Report, Port Coquitlam Municipality. 59 PP« 83 KELSALL, J.P. and K. SIMPSON. I98O. A three year study of the great blue heron In South Western B r i t i s h Columbia. Proc. 1979 Conf. of the Colonial Waterbird Group: 69-74. KOONZ, W.H. I98O. Bald eagle nest i n a Manitoba heron colony. Blue Jay 38: 4-9. KREBS, J.R. 1974. Colonial nesting and s o c i a l feeding as strategies f o r exploiting food resources i n the Great Blue Heron (Ardea herodlas). Behaviour 51: 99-134. MARK, D.M. 1974. Preliminary Results of Some Great Blue Heron (Ardea herodlas) Studies i n the Vancouver Checklist Area. Vancouver Nat. H i s t . Soc. Discovery 3- 38-44. MARK, D.M. 1976. An Inventory of Great Blue Heron (Ardea  herodlas) Nesting Colonies i n B r i t i s h Columbia. Northwest Science 50: 32-40. McALONEY, K. 1973. The Breeding Biology of Great Blue Herons on Tobacco Island, Nova Scotia. Canadian F i e l d - N a t u r a l i s t 87: 137-140. MEYERRIECKS, A.J. i960. Comparative breeding behavior of four species of North American herons. Publ. N u t t a l l Ornithol. Club No. 2: 1-158. MILSTEIN, P. l e S., I. PRESTT and A.A. BELL. 1970. The Breeding Cycle of the Grey Heron. Ardea 58: 171-257. MOCK, D.W. 1976. P a i r Formation Displays of the Breat Blue Heron. Wilson B u l l . 88: 185-230. OWEN, D.F. i960. The nesting success of the heron (Ardea cinerea) i n r e l a t i o n to the a v a i l a b i l i t y of food. Proc. Zool. Soc. London 133: 597-617. PAINE, J.M. 1972. A Study of Colonial Nesting i n the Great Blue Heron (Ardea herodlas). Unpub. B. Sc. t h e s i s . Dept. of Zoology, U.B.C. 61 pp. PARKER, J . 1980. Great Blue Herons (Ardea herodlas) i n Northwestern Montana: Nesting Habitat Use and the Eff e c t s of Human Disturbance. M. Sc. thesis, U. of Montana. 82 pp. PARRIS, R.W. and G.A. GRAU. 1979. Feeding Sites of Great Blue Herons i n Southwestern Lake E r i e . Proc. 1978 Conf. of the Colonial Waterbird Group: 110-113. 84 PIEFER, R.W. 1979. Great Blue Herons Foraging f o r Small Mammals. Wilson B u l l . 91: 6 3 O - 6 3 I . PRATT, H.M. 1970. Breeding Biology of Great Blue Herons and Common Egrets i n Central C a l i f o r n i a . Condor 72: 4 0 7 - 4 1 6 . PRATT, H.M. 1972. Nesting Success of Common Egrets and Great Blue Herons i n the San Francisco Bay Region. Condor 74: 4 4 7 - 4 5 3 . PRATT, H.M. 1980. Directions and Timing of Great Blue Heron Foraging F l i g h t s from a C a l i f o r n i a Colony: Implications for S o c i a l F a c i l i t a t i o n i n Food Finding. Wilson B u l l . 92: 4 8 9 - 4 9 6 . QUINNEY, T.E. and P.C. SMITH. 1979. Reproductive Success, Growth of Nestlings and Foraging Behaviour of the Great Blue Heron (Ardea herodias herodias L . ) . F i n a l Contract Report No. KL229-5-7077 C.W.S., Ottawa, Ontario. RODGERS, J.A. 1978. Breeding behavior i n the Louisiana heron. Wilson B u l l . 9 0 : 4 5 - 5 9 . SANTY, D. 1964. A r e c o l l e c t i o n of an encounter between a Golden Eagle and a Great Blue Heron. Blue Jay 22: 5 5 . SIMPSON, K. and J.P. KELSALL. 1978. Preliminary Studies of Great Blue Heron Colonies 1977. Manuscript Report, C.W.S., Delta, B.C. 20 pp. SIMPSON, K. and J.P. KELSALL. 1979. Capture and Banding of Adult Great Blue Herons at Pender Harbour, B r i t i s h Columbia. Proc. 1978 Conf. of the Colonial Water-b i r d Group: 71-78. SLADEN, W.J.L. 1978. Coded color protocols for great blue heron (Ardea herodias) and brent/brant goose (Branta b e r n l c l a ) . I n t l . Waterfowl Research Bureau Symposium, Tunisia. SOUTHERN, L.K. and W.E. SOUTHERN. 1982. E f f e c t of habitat decimation on Ri n g - b i l l e d g u l l colony and nest-site tenacity. The Auk 9 9 : 3 2 8 - 3 3 1 . STINGLE, S. 1978. Does the Great Blue Heron exhibit t e r r i -t o r i a l i t y during feeding at Bolinas Lagoon, Marin County, C a l i f o r n i a ? Undergrad. paper, U.C.L.A., C a l i f o r n i a . 85 TAYLOR, R.J. and E.D. MICHAEL. 1971. Predation on an inland heronry i n eastern Texas. Wilson B u l l . 83: 172-177. TEMPLE, S.A. 1969. A Case of Turkey Vulture Piracy on a Great Blue Heron. Wilson B u l l . 81: 94. THOMPSON, D.H. 1979a. Declines i n Populations of Great Blue Herons and Great Egrets i n Five Midwestern States. Proc. 1978 Conf. of the Colonial Waterbird Group: 114-127. THOMPSON, D.H. 1979b. Feeding Areas of Great Blue Herons and Great Egrets Nesting Within the Floodplain of the Upper M i s s i s s i p p i River. Proc. 1978 Conf. of the Colonial Waterbird Group: 2 0 2 - 2 1 2 . URHAHN, H.J.M. I 9 6 8 . Feeding Ecology of the Great Blue Heron. Unpub. B. Sc. thesis, Dept. of Zoology, U.B.C. 43 pp. VERMEER, K. 1970. Insular Great Blue Heron Colonies on Large Manitoba Lakes. Blue Jay 28: 84 -86. VERMEER, K. and D.R.M. HATCH. 1972. Additional Information on Great Blue Heron Colonies i n Manitoba. Blue Jay 30: 89-92. VERMEER, K. 1973. Great Blue Herons and Double-crested Cormorant Colonies i n the P r a i r i e Provinces. Canadian F i e l d - N a t u r a l i s t 87: 4 2 7 - 4 3 2 . WARD, P. and A. ZAHAVI. 1973. The importance of certai n assemblages of birds as "information centers" for food-finding. Ibis 115: 517-534. WERSCHKUL, D.F., E. McMAHON and M. LEITSCHUH. 1976. Some ef f e c t s of human a c t i v i t i e s on the Great Blue Heron i n Oregon. Wilson B u l l . 88: 661-662. WERSCHKUL, D., E. McMAHON, M. LEITSCHUH, S. ENGLISH, C. SIBINSKI and G. WILLIAMSON. 1977. Observations on the reproductive ecology of the Great Blue Heron (Ardea  herodias) i n Western Oregon. Murrelet 58: 7-12. WERSCHKUL, D.F. 1979. Nesting mortality and the adaptive significance of early locomotion i n the l i t t l e blue heron. Auk 96: 116-130. WITTENBERGER, J.F. I 9 8 I . Animal Social Behaviour. Duxbury Press, Boston. 705 PP. YOM-TOV, Y. 1974. The ef f e c t of food and predation on breed-ing density and success, clutch size and laying date of the crow (Corvus corone). J . Anim. Ecology 4 3 : 2J-79-/+98. 86 APPENDIX Discriminant function used to determine the sex of banded herons at Pender Harbour Morphological measurements ( b i l l , head plume, tarsus, t a i l and wing length and weight) were obtained from 19 dead and 79 l i v i n g great blue herons. Measurements were taken to the nearest mm using a s t e e l tape, and weight to the nearest 25 g. 1. B i l l : measured from the t i p of the b i l l to the point at which the skin of the forehead joins the b i l l . 2. Head plume:measured from the attachment on the s k u l l to the end of the longest plume. 3. Tarsus: with the l e f t leg extended to approximately 130° at the knee j o i n t , measured from the jo i n t indentation on the ankle to the jo i n t indentation on the knee. 4. T a i l : measured from the base of the pineal gland to the end of the longest t a i l feather. 5. Wing: with the b i r d restrained and l a i d on i t s r i g h t side, measured from the leading edge (elbow) of the l e f t wing to the end of the longest f l i g h t feather. 6. Weight: restrained birds were l a i d i n the cradle of the d i a l scale. Dead specimens were coll e c t e d mainly during the winter i n south coastal B.C. Sex was determined during autopsy by Dr. A.C. MacNeill, Agriculture Canada, Health of Animals Branch. Laparotomies were done to determine sex of three adult l i v i n g birds captured at Pender Harbour i n 1978. 87 Incisions were made i n the l e f t abdominal wall using xylocaine l o c a l anesthetic and gonads observed using a high i n t e n s i t y microscope l i g h t . Using measurements from dead or laparotomized males (eight) and females (13) a l i n e a r discriminant function was developed a f t e r the method of Rao (1973). The function was used to determine the sex of J2 banded herons, which were members of the 18 banded pairs i n the Pender Harbour colony i n 1978 or 1979. Assigned sexes for each paired b i r d were examined to ensure that each p a i r consisted of a male and a female. Additional information such as observations of copu-l a t i o n was used i n correcting the sexual c l a s s i f i c a t i o n of three i n d i v i d u a l s . Following t h i s v e r i f i c a t i o n , measurements of the entire group of now c l a s s i f i e d birds were used to develop a second discriminant function which was applied to the remaining banded bird s . The measurements I used i n determining the sex of banded birds are shown i n Table 1. Two measurements were discon-tinued during the course of the trapping due to d i f f i c u l t i e s i n obtaining consistent measurements ( t a i l length) or obvious large v a r i a t i o n s unrelated to age or sex (head plume). Two of three laparotomies attempted on adult birds were success-f u l . D i f f i c u l t i e s i n r e s t r a i n i n g large herons, poor f i e l d laboratory conditions and extended handling time precluded further attempts. Two of the three birds Involved have been observed since and one successfully raised four young i n 1979. Laparotomy i s a v i a b l e , although probably unnecessary. 88 technique i n determining the sex of l i v i n g herons, given proper equipment and working conditions. Table 2 shows the c o e f f i c i e n t s of the l i n e a r discriminant function developed using measurements of dead (known sex) and paired l i v i n g herons. Both functions are evaluated for each i n d i v i d u a l and the one with the lower value determines the sex (Rao 1973) Of 53 individuals of known sex (dead o paired birds) 52 (9&%) were c o r r e c t l y c l a s s i f i e d . Table 3 shows the morphological measurements of c l a s s i f i e d herons. Using posterior proba-b i l i t i e s over 70% of the birds could be c l a s s i f i e d with 95% confidence. I used t h i s technique to assign sexes to a l l the banded herons at Pender Harbour. LITERATURE CITED RAO, C.R. 1973. Linear S t a t i s t i c a l Inference and I t s Applications. Second Edition, Wiley, New York: 574-581. 0 89 Table 1. Morphological measurements of 79 great blue herons captured at Pender Harbour i n 1978 and 1979. N Min. Max. Mean SD B i l l length (mm) 79 116 146 132 8.34 Tarsus length (mm) 79 133 180 160 11.02 Wing length (mm) 79 462 544 500 18.01 T a i l length (mm) 68 170 230 195 12.56 Weight (kg) 79 1.87 2.97 2.34 .25 Table 2. Coe f f i c i e n t s of the l i n e a r discriminant function based on measurements of known sex and paired great blue herons i n B r i t i s h Columbia, 1979. Variable Male Female Constant -855.71 -745.97 B i l l length 3.07 2.64 Tarsus length - 0.43 - 0.49 Wing length 2.65 2.56 N 24 29 90 Table 3 . Morphological measurements of known male and female great blue herons i n south coastal B r i t i s h Columbia. Sex Min. Max. Mean SD N B i l l •M 129 146 137.0 4.43 24 length (mm) F 112 131 123.9 4.72 29 Tarsus M 155 177 165.9 6.72 24 length (mm) F 135 165 152.0 7.04 29 Wing M 470 544 505.7 13.75 24 length (mm) F 465 500 483.0 8.69 29 Weight (kg) M 1.53 3.07 2.48 0.29 24 F 1.90 3.27 2.11 0.34 29 PUBLICATIONS SIMPSON, KEITH and J.P. KELSALL. 1978. Capture and Banding of A d u l t Great Blue Herons at Pender Harbour, B r i t i s h Columbia. P r o c e e d i n g s of the C o l o n i a l W a t e r b i r d Group, 1978 : 71 - 78. and J.P. KELSALL. 1978. P r e l i m i n a r y S t u d i e s of Great Blue Heron C o l o n i e s , 1977. Canadian W i l d l i f e S e r v i c e . M a n u s c r i p t R e p o r t . 20 p. and J.P. KELSALL. 1978. The Wilmer N a t i o n a l W i l d l i f e A r e a ; W i l d l i f e and H a b i t a t s . Canadian W i l d l i f e Serv. M a n u s c r i p t R e p o r t . 39 p. KELSALL, J . P r and K. SIMPSON. 1979. A Three Year Study of the Great Blue Heron i n Southwestern B r i t i s h Columbia. P r o c . C o l o n i a l W a t e r b i r d Group, 1979. V o l . 3 : 6 9 - 7 4 . KELSALL, J.P., E.S. TELFER and K.- SIMPSON. 1980. I n d i r e c t D e t e r m i n a t i o n of Ungulate Foot A r e a . Canadian J o u r n a l of Zoology. 58(3) : 464 - 466. 

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