REPRODUCTION IN 'l'HE CATTLE EGRET (BUBULCUS IBIS IBXS): 'l'BE FUNCTION OF BREEDING PLUMES

by

@ Elizabeth Ann Krebs

Department of Biology McGill University August 1991

A dissertation submitted to the Faculty of Graduate Studies and Research, McGill University - in partial fulfillment of the requirements for the degree of Master of Science (Biology) • ii

( GENERAL ABSTRACT This study investigates the colonization of Barbados, the seasonality of breeding, and the function of breeding plumes in the cattle egret Bubulcus ibis ibis. Colonization occurred around 1956, and the island now has 4 colonies.

Numbers of birds a~ each colony are still increasing. Colonies are evenly spaced around the island, perhaps to minimise local food competition. Plumage scores varied seasonally, and males had higher scores than females. Breeding activity peaked in July-August and November-March. Conspecific interference may force poorly plumed birds to breed when environmental conditions are poor. Chick mortality was primarily from starvation during periods of low breeding activity, and primarily from coaspecific attack during high breeding activity. Fledging success did not increase with breeding activity. Plumage scores correlated

positively in breeding pairs, indicating strong fe~ale preference for weIl plumed males. WeIl plumed males fed chicks more often than poorly plumed males and had the higher fledging success. Males contributed more ta nest and nestling attendance when their mates were weIl plumed. Females accepted extra-pair copulations only from males whose plumage scores were equal to or greatar than their mates. Fpmale preference for weIl plumed mates and weIl plumed partners for extra-pair copulations suggests that plumes indicate the potential paternal care and the genetic ( quality of a mate. iii

RESUME GENERAL

Cette étude porte sur la colonisation à la Barbade de l'aigrette Bubulcus ibis ibis, sur son accouplement saisonnier et sur le rôle du plumage d'accouplement. La colonisation de l'île par ces oiseaux s'effectua vers les années 1956 et il existe présentement 4 colonies distinctes.

Le nombre d'oiseaux par colonie augmente et les colonies sont espacées de façon régulière probablement afin de limiter la compétition alimentaire. Le compte (score) du plumage variait avec les saisons et les mâl.es possèdaient des scores plus élevés. Les périodes d'accouplement les plus actives ont été enregistrées de juillet à août et de novembre à mars. Cependant, la mortalité des oi3illons

était plus élevée, ce qui suggère que le succés reproductif n'était pas associé a l'activité de reproduction.

L'intervention des conspécifiques semble amener les individus dont le score de plumage est !aible, à s'accoupler seulement lorsque les conditions environementales sont mauvaises. Les individus d'un couple possède généralement un score de plumage semblable. De plus, les mâles à score

élevé, nourissent plus fréquemment leurs petits et contribuent ainsi au succès de la portée. Les femelles acceptent les copulations extra-conjugales provenant seulement des mâles dont le score est égal ou supérieur au leur. La préférence des femelles pour un mâle à beau plumage suggère que le plumage est une indication du iv potentiel des capacités paternelles et des qualités génétiques des partenaires. ·._------

v

• ACKNOWLEDGEMENTS

In retrospect, it seerns that a cast of thousands has contributed directly and indireetly to the complet ion of this thesis - so many, many thanks to the diverse set of people who have helped me puzzle and play rny way throuqh

life as an egretter. I thank rny supervisee Wayne Hunte for his constant encouragement and enthusiasm, ec,pecially for

certain theoletical topies. For stoieal field assistance 1 would like to thank: Fred; Nicky Drayton; David Green; Stephane Perrault; Zena Tooze; and Janet Dickie. Thanks also to Julie Horrocks and Hazel Oxenford for their advice, moral support and wonderful Sunday night dinners. On the

6th floor front, 1 thank aIl my fellow lab rats especiaIly, Karen Burke, Jim Mountjoy, Danny Weary and Todd Winquist for tolerating my incessant interruptions, and for their fine ornithological and statistical advice.

1 would also like to thank both rny parents for my early introduction to nature and the joys of fieldwork, as weIl as

my family and David for their support of me dur 1ng my endless peregrinations. Support for this project was provided by an NSERC postgraduate scholarship to EK and a NSERC research grant to Wayne Hunte. vi

PREFACE

1.) statement of contribution to original knowledge:

Ta the best of my knowledge, the material presented in this thesis is an original contribution to knowledge of the patterns of variation and function of breeding plumes in cattle egrets.

2.) Historical statement of relevant work:

A historical review may be found in the general introduction and it is supplemented by more extensive background material in the introduction and text of individual chapters.

3.) Format:

The thesis is presented in paper format consisting of 4 chapters, each of which will be submitted for publication.

( vil

TABLE OF CONTENTS

GENERAL ABSTRACT · ..... · ... · .... i i RESUME ...... · ..... iii ACKNOWLEDGEMENTS · ..... v PREFACE · ...... · ..... vi LIST OF FIGURES · ..... ix LIST OF TABLES .... · .... xii GENERAL INTRODUCTION 1 · ..... · ..... ,- LITERATURE CITED · ..... · .... :.>

CHAPTER 1. The colonization of Barbados by cattle egrets (Bubulcus ibis) 1956-1990. ABSTRACT · .... 6 INTRODUCTION · .... 7 METHODS · .... · ..... · ...... 9 RESULTS · ..... · .... 10 DISCUSSION ..... 13 LITERATURE CITED · ..... 18

CHAPTER 2. Seasonal variation in plumage, breeding activity and breeding success ABSTRACT · .... 20 INTRODUCTION · . . . . 21 METHODS · ..... · .... 23 RESULTS · ..... · .... 34 DISCUSSION ...... · .... · ..... 47 LITERATURE CITED · ..... · .... 58

CHAP'l'IR 3. Breeding plumes, mating preferences, parental care and reproductive success in the cattle egret ABSTRACT ...... · .... · ..... 61 INTRODUCTION ...... · . . . . 62 METHODS · .... 66 RESULTS · ...... · .... · ..... 72 ,..... DISCUSSION ...... · . " ... · ...... 97 LITERATURE CITED 106 .... ,t · ...... · .. .. . · .... · ..... viii

CHAPTER 4. Breeding plumes and extra-pair copulations in the cattle egret

ABSTRACT ••••• 109 INTRODUCTION •••••• 110 M1'2THODS 112 RESULTS 116 DISCUSSION ••• 122 LITERATURE CITED 130

SUMMARY ...... 134

( - ._------

ix

LIST OF FIGURES

CHAPTER 1

Figure

1 Location of Barbados, and locations of cattle egret colonies in the island. Circles indjcate the average foraging range of a cattle egret (5.68 km radius; Custer and Osborn 1978) ......

2 Size of cattle egret colonies in Barbados, 1956-1990. ') 1 <.

CHAPTER 2

Figure

1 Cattle egret in Barbados with full breeding colours (red bill and legs) and breeding plumes (plumage score=12) • • • ...... 2'1

2 Plumage characteristics used to quantify the degree of plumage development (see Table 1). Numbers indicate the plumage score assigned ...... 28

3 Frequency distribution of male and female total plumage scores in the population over the whole study period...... -j()

4 Mean total plumage scores in each sampling period, presented separately for males and females ...... 40

5 Seasonal variation in numbers of occupied nests plotted dS 5 day averages. The temporal locations of sampling periods 1, 2 and 3 are indicated ...... 41

6 Seasonal variation in the to~. l number of chicks plotted as 5 day averages. The temporal locations of sampling periods 1, 2 and 3 are indicated ...... 42

7 Seasonal variation in the number of red-billed birds plotted as 5 day averages. The temporal locations of sampling periods 1, 2 and 3 are indicated...... 43

8 Percentage of breeding pairs that hatched and fledged 0, l, 2 or 3 chicks, presented separately for hatching and fledging and for each sample periode ...... 4G

"'.\ x

CHAPTER 3 j .. Figure 1 Female plumaC}e scores vs. male plumage scores for aIl breeding pairs of cattle egrets across aIl sample periods. N~170, sorne points represent more than one pair ...... 74 2 Copulation rates vs. plumage scores for male cattle egrets across aIl sample periods...... 77 3 Male ne st dttendance during pre-eqglaying vs. female (a) and male (b) plumage scores, across aIl sample periods...... 80 4 Male incubation vs. female (a) and male (b) plumage scores, across aIl sample periods...... 83 5 Male nestling attendance during the non-continuous attendance phase vs. female plumage score, across aIl sample periods...... 87 6 Female nestling attendance during the non-continuous attendance phas~ vs. female (a) and male (b) plumage scores, across aIl sample periods...... 89 7 Male feeding rate (bolusesjhourjchick) vs. male plumage scores across aIl sample periods...... • . • . . . 91 8 Proportion of boluses delivered by males during the nestling period vs. male (a) and female (b) plumage scores, across aIl sample periods... • . . . . . • ...... 94

CHAPTER 4 Figure 1 The distributions of plumage scores for the male population (N=41), for males who successfully attempted EPCs (N=l3), and for males who unsuccessfully attempted EPCs (N=9) .•...... ••... 117 2 The distributions of plumage scores for the male population (N=43) and for males whose mates accepted EPCs (N=8) .•••...... •••...•••..•••....•••.•.••... 119 3 The distributions of plumage scores for the female population (N=43) and for females with whom males attempted EPCs (N=9) •..••••.•••••.•••••..••....••... 120 1 xi

LIST OF TABLES

CHAPTER 2

Table

1 Scoring system used to assign plumage scores to breeding plumes on the head, chest and back of individual cattle egrets. (see Figure 2) ...... 27

2 Spearman Rank Correlation matrix illustrating the degree of inter-correlation between an indiviàual male's head, chest, back and combined (total) plumage scores. Correlations are presented for the entire study, and separately for each sarnple perjod. A total of 170 males were scored over the whole study: 13 in period li 59 in Period 2i and 98 in Period 3...... 35

3 Spearman Rank Correlation matrix illustrating the degree of inter-correlation between an individual female's head, chest, back and cornbined (total) plumage scores. Correlations are presented for the entire study, and separately =or each sample periode A total of 170 females were scored over the whole study: 13 in period li 59 in period 2; and 98 in period 3...... 36

4 Mean, range and coefficient of variation (C.V.) of plumage scores, presented for the entire study and separately for each sampling periode ...... 37

5 Measures of breeding success for pairs of cattle egrets, present~d separate!y for each sample period and for the whole study. X is the mean, sd the standard deviation, and N the number of pairs ...... 45

6 Nestling mortality (t of chicks hatched who died during nestling period), broken down into different mortality causes, and presented separately for each sample period and the whole study combined ...... 48

7 The distribution of nestling deaths arnong known mortality causes (% by each cause) presented separately for each sample period and the whole study •...... •...... " ...... '" 4 9

8 Comparison of the breeding success of cattle egrets at different locations. .1 If fledg~ng success is calculated 35 days after hatchingi. If fledging success is calculated 14 days after hatching ..•••... 54 xii

( CHAPTER 3 Table

1 Mean copulation rates (* copulations/hour) for cattle egret pairs, for the whole study and separately for each sample period; and Spearman rank correlations between plumage scores and copulation rates for both malas and females. Probabilities are indicated by asterisks (see leqend below)...... 76 2 Mean male and female attendance rates during pre-egglaying and early incubation, for the whole study combined, and by sample period. N is sample size...... 78

3 Spearman rank ~orrelations between male plumage scores, female plumage scores, and ne st attendance. AlI pairs who reached egglaying are included, and aIl sample periods are combined. Probabilities are indicated by asterisks (see legend below) .••••...•.• 79 4 Mean male and female nestling attendance during continuous (1-8 days old) and non-continuous (9-21 days old) attendance for aIl pairs who fledged chicks; for the entire study combined and by sample period...... 85 5 Spearman rank correlations between male plumage scores, female plumage scores, and nestling attendance during the continuous phase (a), and the non-continuous phase (b). AlI pairs who fledged chicks are included, and aIl sample periods are combined. Probabilities are indicated by asterisks (see legend below)...... 86 6 Male and female feeding rates for the whole study combined and separately for each sample periode N is the sample size...... 90 7 Spearman rank correlations between male plumage scores, female plumage scores and feeding rates per chick (a), and the proportion of boluses delivered to the nest by the males (b). Pairs who fledged chicks are included in the feeding rates. AlI pairs whose chick survived to direct feeds are included in the calculations of proportional feeds. AlI sample periods are combined ••••••••••••••••••••..••• 92 ( 8 Mean male plumage score, mean female plumage score xiii

and mean total plumage score for (a) the number of chicks hatched by a pair and (b) the number of ';': chicks fledged by a pair. AlI sample periods are combined. The results of Kruskal-Wallis test~ to detect differences between categories are presented. 96

CBAPTER .. Table 1 Mean male attendance rates, chick feeding rates and fledging success for males whose mates had accepted extra-pair copulations and for the whole male population...... 122 1

GENERAL INTRODUCTION

The cattle egret Bubulcus ibis belonqs to the heron family Ardeidae. There are 2 major subspecies, Bubulcus ibis coromandus which is found in Asia and Australia, and Bubulcus ibis ibis which is native to Africa and Europe but has expanded its range into South America, North America and the Caribbean (Crosby 1972). Colonization of the West Indies probably occurred after the 1940's from a population in South America. Colonization of Barbados occurred between 1956 and 1960. Several studies have shown that the size (Werschkul et al. 1977; Gibbs et al. 1987) and spacing (Lack 1954; Fasola and Barbieri 1978; Furness and Birkhead 1984) of colonies are strongly influenced by the distribution of local food resources. These studies attempted to infer the original sequences of colony formation from present patterns of colony distribution. In Chapter 1, l characterise the colonization of Barbados by cattle eqrets from initial settlement to the present, by describing population growth and the formation sequence and location of colonies on the island. In the tropics, seasonal variation in rainfall, and hence in food availability, is a major factor determininq breeding seasonality, but the variation is typically less marked, and the breeding season longer than in temperate regions (MacArthur 1964). Individual breedinq success is ,(. assumed to be higher when environmental factors are most ,,

2

favourable and breeding activity is highest. However, increased breeding activity may increase irtraspecific competition and decrease breeding success. Whether individual breeding success varies with levels of breeding activity, and whether the causes of chick mortality vary seasonally, have not been investigated for colonial birds in the tropics. In its native Africa, the cattle egret breeds once a

year (Siegfried 1972), but seasonal breeding patterns vary geographically. In the neotropics the birds have been reported to breed from as little as once per year to almost

continuously (Lowe-McConnell 1967; Lancaster 1970). Cattle egrets develop buff coloured filamentous plumes in the breeding season. In Chapter 2, l investigate seasonal variation in breeding plumes, breeding activity and breeding success in a colony of cattle egrets in Barbados; and l assess whether the mortality factors affecting breeding success differ seasonally. Despite their visual prominence, the functians of breeding plumes in the heron family have nat been specifically investigated. Darwin (1871) first noted that plumes were ornamental, and likely to function in mate choice, however the role of plumes in the pairing patterns of herons or egrets has not been examined. If plumes are a criterion of mate choice, females May prefer mates with longer plumes because of mating advantages accruing ta their

male offspring (Fisher 1958), because plumes are correlated 3

( with the genetic quality of the bearer (Zahavi 1975; Kodric­ Brown and Brown 1984; Andersson 1986), and/or because plumes signal capacity for parental care (Hoelzer 1989). Cattle egrets are monogamous, and both sexes exhibit weIl developed breeding plumes. In Chapter 3, 1 investigate whp.ther plume quality is a criterion of mate choice, and whether it is correlated with parental care and/or offspring viability for both sexes in cattle egrets. It is now evident that extra-pair copulations (EPCs) are common in monogamous colonial birds (Gladstone 1979; Fujioka and Yamagishi 1981: Frederick 1987; McKilligan 1990; Morton et al. 1990). The advantages of EPCs to males are clear, but females may also benefit from EPCs (Mineau and Cooke 1979; Roskaft 1983; Moller 1988; Smith 1988; McKilligan 1990); and female cooperation in EPCs may be necessary for successful fertilization (FitCh and Shugart 1982). Potential benefits to females include increased fertilization (McKinney et al. 1984), increased genetic diversity of the brood (Williams 1975), and better genes for their offspring (Gladstone 1979: Westneat et al. 1990). Females obtain genes, but no paternal care from EPCs. This may allow determination of whether a character such as breeding plumes signaIs genetic or immediate (paternal carel benefits. In Chapter 4, 1 address this by investigating the effects of breeding plumes on the distribution of extra-pair copulations in cattle egrets. ( 4

LITERATURE CITED

Andersson, M. 1986. Evolution of condition-dependent sex­ ornaments and mating preferences: sexual selection based on viability differences. Evolution 40:804-816.

Crosby, G. T. 1972. Spread of the cattle egret in the western hemisphere. Bird Banding 43:205-211.

Darwin, C. 1871. The descent of man and selection. in relation to sex. Murray. London.

Fasola, M. & F. Barbieri. 1978. Factors affecting the distribution of heronries in northern Italy. Ibis 120:537-540.

Fisher, R. C. 1958. The genetical theory of natural selection. 2nd ed. Dover. New York.

Fitch, M. A. and G. W. Shugart. 1984. Requirements for a m~xed reproductive strategy in avian species. Am. Nat. 124:116-126.

Frederick, P. 1987. Extra-pair copulations in the mating system of white ibis (Eudocimus albus). Behaviour 100: 170-201.

Fujioka, M. and S. Yamagishi. 1981. Extramarital and pair copulations in the cattle egret. Auk 98:134-1 ;4.

Furness, R. W. and T. R. Birkhead. 1984. Seabird colony distributions suggest competition for food supplies during the breeding season. Nature 311:655-656.

Gibbs, J. P., S. Woodw~rd, M. L. Hunter & A. E. Hutchinson. 1987. Determinants of great blue heron colony distribution in coastal Maine. Auk 104:38-47.

Gladstone, D. E. 1979. Promiscuity in monogamous colonial birds. Am. Nat. 114:545-557.

Hoelzer, G. A. 1989. The good parent process of sexual selection. Anim. Behav. 38:1067-1078

Kodric-Brown, A. and J. Brown. 1984. Truth in advertising: the kind of traits favoured by sexual selection. Am. Nat. 124:309-323.

Lack, D. 1954. The stability of the heron population. Brit. Birds 47:111-121.

Lancaster, D. A. 1970. Breeding behaviour of the cattle egret in Columbia. Living Bird 9:176-194. 5 Lowe-McConnell, R. H. 1967. Bio1ogy of the immigrant ( cattle egret (Ardeola ibis) in Guyana, South America. Ibis 109:168-179. MacArthur, R. A. 1964. Geographical ecology. Princeton University Press. Princeton, New Jersey.

McKilligan, N. G. 1990. Promiscuity in the cattle egret (Bubulcus ibis). Auk 107:334-341. McKinney, F., K. M. Cheng, and D. J. Bruggers. 1984. Sperm :ompetition in apparently monogamous birds. In: Sperm competition and the evolution of anim~l mating systems (R. L. Smith, ed.). pp 523-545. Acade'mic Press. New York. Mineau, P. and F. Cooke. 1979. Rape in the 1esser snow goose. Behaviour 70:280-291. Moller, A. P. 1988. Female choice selects for male sexual tail ornaments in the monogamous swallow. Nature 332:640-642.

Morton, E. S., L. Forman and M. Braun. 1990. Extrapair fertilizations and the evolution of colonial breeding in purple Martins. Auk 107: 275-283. Roskaft, E. 1983. Male promiscuity and fema1e adultery by the rook Corvus frugilegus. Ornis Scand. 14:175-179. Siegfried, W. R. 1972. Breeding success and reproductive output of the cattle egret. ostrich 43:43-55.

Smith, S. M. 1988. Extra-pair copulations in blacJ-:--capped chickadees: the role of the female. Behaviour 107:15- 23. Werschkul, D., E. McMahon, M. Leitschuh, S. English, C. Skibinski & G. Williamson. 1977. Observations on the reproductive ecology of the great blue heron (Ardea herodias) in western Oregon. Murrelet 58:7-12. Westneat, D. F., P. W. Sherman and M. L. Morton. 1990. The ecology and evolution of extra-pair copulations in birds. I~: Current Ornitho1ogy, Vol. 7 (D. M. Power, ed.). pp. 331-369. Plenum Press. New York.

Williams, G. 1975. Sex and evolution. Princeton University Press. Princeton.

Zahavi, A. 1975. Mate selection - a selection for a handicap. J. Theor. Biol. 53:205-214. 6

CHAPTER 1: The colonization of Barbados by cattle egrets (Bubulcus ibis) 1956-1990.

ABSTRACT

This chapter describes the colonization of Barbados by the cattle egret (Bubulcus ibis) from initial settlement until the present. Egrets first established a breeding

colony on the south-west coast of Barbados between 1956 and 1960. Two additional breeding colonies were formed in 1978 and 1980, and 1 permanent roosting colony in 1984. Suitable colony sites are unlikely to be limiting, as colonies ean be established in very different vegetation types. Proximity to water is not a requirement for colony location, sinee only 2 of the colonies were situated over water. Space at the colony site does not influence the formation of new colonies, since only 10% to 30% of available space at a site

was used in the 4 colonies. The colonies are evenly spaced around the island, suggesting that new colonies may be formed to minimize local food competition. Population size

in the island, and in aIl 4 colonies is still increasing, although growth in the oldest and largest colony may be slowing. Weekly counts indicated that the number of birds in the roosting colony was negatively correlated with that in the breeding colonies, suggesting some short-term movement of birds between colonies. 7 ( INTRODUCTION The cattle egret (Bubulcus ibis) belongs to the family Ardeidae. Two major subspecies have been identified, the Asian cattle egret (Bubulcus ibis coromandus) which is found in Asia and Australia, and the African cattle egret (Bubulcus ibis ibis) which is considered a native to Africa and Europe but is now established in the New World (Hanebrink 1971). 8. ibis ibis first appeared in the New World around 1877 in Surinam and then extended its range into South America, North America and the Caribbean (Crosby 1972). The speed and scale of the egret's range expansion are among the greatest recorded for any species (Weber 1975). Colonization of the West-Indies occurred after the colonization of North America in the 1940's, probably from a population in South America (Crosby 1972).

Barbados is a coral-limestone island, 431 km2 in area and approximately 34 km long and 23 km wide. It is located on the eastern edge of the Lesser Antilles, (130 l' N, 590

35' W), some 400 km north of the Venezuelan mainland (Figure 1). Fifty-one percent of the land is cultivated in sugar cane, fruits, vegetables or pasture. As recorded by naturalist, M. Hutt, the cattle egret arrived in Barbados in 1956 and had established a breeding colony by 1960 (M. Hutt, pers. comm.). The cattle egret is the most conspicuous and { one of the most common birds on the island. Cattle egrets •

8

Figure 1. Location of Barbados, and locations of cattle egret colonies in the island. Circles indicate the average foraging range of a cattle egret (5.68 km radius; Custer and Osborn 1978)

• ..... __'ê ---W':1" •• ( ~~,..,.- , L-..-/- f, l .~ 0 - o ~. \) ~

o l'> :." Barbados LE,>SER • -. o

-- --1.-.

Veneluela

• BrHdlng/Rooltlng Slt •• ~ • Rool'Ing Sit. / ' N

\ "" ,

! 1

\",

o 1 2 3 (' Kil i MM 9 are the only colonially nesting ardeid in Barbados. There are now 4 cattle egret colonies on the island, ) permanent sites used for breeding and roosting and one permanent roosting site (Riven-Ramsey 1988). Several authors have examined factors affecting the distribution of ardeid colonies. consistently, colony spacing (Lack 1954: Fasola and Babieri 1978: Furness and Birkhead 1984: Gibbs et al. 1987) and size (McCrimmon 1978;

Werschkul et al. 1977: Burger 1981: Gibbs et al~ 1987) have been found to relate to the distribution of local food resources. AlI of these studies attempted to infer the original settlement sequences from the current patterns of colony distribution. The objective of this chapter is to describe the colonization of Barbados by the cattle egret, Bubulcus ibis ibis from the time of its arrivaI on the island in 1956. l characterize the colonization of the island by the population growth of the egret and the sequence and location of colony establishment.

METHODS The time and sequence of colony establishment by cattle egrets in Barbados was determined by interviewing local residents and naturalists (Riven-Ramsey 1988). The conspicuousness of the egret in the context of the avian fauna of Barbados, and the small size and high human population density of the island, made this approach to 1 ~

10

( determining colony establishment feasible and accurate. The numbers of birds in each of the 4 colonies was determined between 1981 and 1989 by censusing each colony twice annually or biannually. For each census, the number of resident birds in the colony was estimated by counts from a fixed location using 9x24 binoculars. The number of incoming birds at sun set was also counted and added to the number of resident birds to give the population size for each colony. The availability of apparently suitable

habitat was visually ass~ssed for each colony. To assess the possibility of movement of birds between the colonies, short-term changes in colony size were determined for 3 of the colonies by conducting weekly counts between March and

August, 1989.

RESULTS The first egret colony established on Barbados was in Graeme Hall swamp in a tract of white mangroves

(Laguncularia racemosa) between 1956 and 1960 (M. Hutt, pers. comm). The swamp is 3? hectares and is located on the south coast, in a relatively urbanized area (Figure 1). The nesting colony occupies about 15% of the mangroves at the swamp. A second c010ny, Hannay' s, formed in the north of the

is1and in 1978 in a sma11 mixed stand of bearded fig trees (Ficus citrifolia) and whitewood (Tabebuia pa11ida) (Figure 11

1). The birds are positioned high in mature trees which emerqe out of a deep gully and are relatively inaccessible. The nesting colony occupies about 30% of the total tree area and is located in a predominantly agricultural area. Frizers, the most recent breeding colony, formed in

1980 about halfway up the east coast of the island (Figure 1) in a bed of reeds (Arundo donax) which are 15 to 25 feet tall. The reeds are situated around a stream which only runs seasonally. The nests are weIl hidden because they are 5-10 feet down from the top of the reeds. The nesting colony occupies about 10% of the reed bed and is surrounded by pasture. A permanent roosting site (Holetown Swamp) was established halfway up the west coast of the island (Figure

1) in 1984 in a small grove of white mangroves. As of 1990, no nesting has been observed, although large numbers of roosting birds ~re continuously present. The roosting egrets occupy about 25% of the mangrove area at the swamp .. The site is adjacent to a major road and is located on the edge of a small town. The population size of cattle egrets in Barbados appears to be growing exponentially sorne 35 years after colonization (Figure 2). This growth is occurring at aIl four locations. colony size is predicted by age of colony, as might be expected in an expanding population (Figure 2). 12

Figure 2. Size of cattle egret. colonies in Barbados, 1956-

1990. • Total population; --C!- Graeme Hall; • Frizers; • Hannays; Holetown.

- 12000

10000 en 8000 "0.... :c 0 -.... 6000 Q) .0 E ::J 4000 Z

2000

o+---~~~------~~--~~~~~--~ 1950 1960 1970 1980 1990

Year 13

The correlation of colony size with colony aqe sugqests that colonies function largely as independent sub-populations. However, the 3 colonies censused over the 5 month period (Frizers, Hannays, Holetown) aIl showed weekly variation in population size. The number of birds at Frizers (breedingsite) was not correlated with that at Hannays (breeding site) (Spearman's Rank Correlation; rs=O.29, P>O.5, N=8), and the numbers at Holetown (roosting site) were not correlated with either that at Hannays (rs=-O.42, P>O.2, N=10) or that at Frizers (rs=-o.34, P>O.2, N=8). However, the number of birds at Holetown was neqatively correlated with the total number at Hannays and Frizers combined (rs=-O.76, P

DISCUSSION Proximity to water is not apparently a prerequisite for the colony location of cattle eqrets in Barbados. Although the first and last colonies formed are over permanent water masses, colonies 2 and 3 are not. Inaccessibility to the colony May be important, but can be provided by means other than underlyinq water. Proximity to water is not advantaqeous for cattle egrets from a foraginq perspective since, unlike other ardeids, they rarely foraqe in wetlands r (Custer and Osborn 1978). The frequent suggestions that in " 14

mixed species colonies, egrets favour sites over water (eg., .. ' Blaker 1969; Siegfried 1971; Telfair 1981) May indicate heterospecific attraction of colonial nesters rather than attraction to water per se. The only other heron species in Barbados is the green heron (Butriodes virescens). It ncsts only sOlitarily in Graeme Hall swamp, site of the first egret colony. Availability of space at the colony location does not appear to influence the development of new colonies of egrets in Barbados. Only 10% to 30% of the available tree space is used by birds in the 4 colonies on the lsland. Several previous studies have suggested that availability of space in the celony influences neither the distribution nor the population dynamics of cattle egrets and ether ardeids (Burger 1981; Custer and Osborn 1978; Gibbs et al. 1987;

McCrimmon 1978; Werschkul et al. 1977). Moreever, cattle egrets can nest in very different types of vegetation: white mangroves, black mangroves (Avicennia germinans) and red

mangroves (Rhizophora mangle) (Maxwell and Kale 1977;

Rodgers 1987; Arendt and Arendt 1988: this study): sedges

(Phragmites §RP.) (Burger 19,8: Burger 1982); marsh grass

(Spartina~) (custer and Osborn 1978): wild cane (Arundo donax) (this study): Florida eIder (Sambucus simpsonii)

(Weber 1975); and bamboo (Bambusa var.) (Lancaster 1970). This suggests that suitable sites for colony establishment 15

are not limiting, since colonies can be established in very different vegetation types. The most striking aspect of the sequence of development and location of the egret colonies in Barbados is their even spacing around the island (Fig 1). The first colony was formed in the south, the second in the north, and the last two in the east-central and west-central portions of the island, mid-way between the first two colonies. Gibbs et al. (1987) reported even spacing of great blue heron colonies in Maine and suggested that this may minimize overlap between the foraging ranges of the colonies, allowing for greater access to resources. The mid-way point

between adjacent colonies in Barbados (Figure 1) corresponds closely to the average daily foraging range of 5.68 km reported for cattle egrets in North Carolina (Custer and Osborn 1978). Colonies may only be evenly spaced when food availability is not patchily distributed. ror example, heronries in northern Italy are irregularly spaced, reflecting the distribution and size of wetlands (foraging areas) (Fasola and Barbieri 1978). Other studies have observed that local food abundance influences colony distributions (e.g., Lack 1954; Furness and Birkhead 1984). The diet width and opportunistic feeding habits of cattle egrets ensures that they can utilize many different types of feeding sites. Consequently, potential feeding sites are 16

"'li- evenly spread throughout the island. This is consistent with the regular spacinq of egret colonies observed in Barbados. The population size of egrets in Barbados is still increasinq rapidly. Maintenance of the rapid qrowth rate over an extended period may have been facilitated by the even colony spacing observed, if spacing minimizes food competition. Although colony spacinq may minimize food competition, precisely what triqgers the formation oI new colonies remains unclear. population qrowth of the original and larqest colony (Graeme Hall) has slowed over the past 3 years, perhaps reflecting food competition near the colony

site. However, the second colony (Hannays) was formed 12

years ago, weIl before ë' reduction in qrowth of the Graerne Hall colony was observed. While the correlation of colony size with colony age may suggest that colonies function largely as independent sub-populations, movement of individuals does occur between colonies. Most obviously the increase in colony size at Holetown (the most recent colony) must be due to immigration, since Holetown is not a nesting colony. Moreover, the 3 colonies censused weekly varied 1n population size, the number of birds at Holetown (roosting site) correlated negatively with the number at Frizers and •.. Hannays (roostinq, breeding sites) combined. This suggests 17

that birds at the roosting site move between their colony and colonies at breedinq sites. The movement May allow birds to obtain information on potential breedinq sites, in the context of the quality of available nest sites as weIl as foraginq habitat around the colony. since population qrowth at the oldest and largest colony (Graeme Hall) has finally slowed, it will be of interest to see whether and when Holetown becomes a breedinq colony, when population qrowth at Hannays, Frizers and Holetown will beqin to slow, whether additional colonies will be formed and where, and what ultimately will be the carrying capacity and spatial organization of the cattle egret population in Barbados.

{ r ! 18 LXTERATURE CXTED

Arendt, W. & A. Arendt. 1988. Aspects of the breeding biology of the cattle egret CBubulcus ibis) in Montserrat, West Indies, and its impact on nest vegetation. Colonial Waterbirds 11:72-84.

Blaker, D. 1969. Behaviour of the cattle egret Ardeola ibis. Ostrich 40:75-129.

Burger, J. 1978. The pattern and mechanism of nesting in mixed species heronries. Pp. 45-58 in Wading Birds, Natl. Aud. Soc. Res. Rep. No. 7 CA. sprunt, J. Ogden, and S Winckler, Eds.). New York, National Audubon Society.

Burger, J. 1981. A model for the evolution of mixed­ species colonies of ciconiiformes. Quart. Rev. Biol. 56:143-167.

Burger, J. 1982. On the nesting location of cattle egrets Bubulcus ibis in South African heronries. Ibis 124: 523-529. Crosby, G. T. 1972. Spread of the cattle egret in the western hemisphere. Bird Banding 43:205-211.

Custer, T. W. & R. G. Osborn. 1978. Feeding habitat use by cOlonially-breeding herons, egrets and ibises in North Carolina. Auk 95:733-743.

Fasola, M. & F. Barbieri. 1978. Factors affecting the distribution of heronries in northern ItaIy. Ibis 120:537-540. Furness, R.W. & T.R. Birkhead. 1984. Seabird colony distributions suggest competition for food supplies during the breeding season. Nature 311:655-656.

Gibbs, J. P., S. Woodward, M. L. Hunter & A. E. Hutchinson. 1987. Determinants of great blue h":'!ron colony distribution in coastai Maine. Auk 104:38-47.

Hanebrink, E. L. 1971. Food, feeding behaviour and extension of the range of the cattie egret. The Migrant 42:49-56. Lack, D. 1954. The stability of tbe heron population. Brit. Birds 47:111-121. 19 Lancaster, D. A. 1910. Breeding behaviour of the cattle egret in Columbia. Living Bird 9:176-194. McCrimmon, D. A. Jr. 1978. Nest site characteristics arnong five species of herons on the North Carolina coast. Auk 95:267-279. Maxwell, G. R. & H. W. Kale. 1977. Breeding biology of five species of herons in coastal Florida. Auk 94:689-700. Riven-Ramsey, D. 1988. Foraging and breeding behaviour of the cattie egret (Bubulcus ibis) in Barbados, West Indies. M.Sc. Thesis, University of the West Indies. Rodgers, J. A. Jr. 1987. Breeding chronology and reproductive success of cattle egrets and Iittle blue herOllS on the west coast of Florida, U.S.A. Colonial Waterbirds 10:38-44. Siegfried, W. R. 1971. Communal roosting of the cattle egret. Trans. roy. soc. S. Afr. 39:419-443. Telfair, R. C. 1981. Cattle egrets Ardeola ibis ibis inland heronries and the availability of crayfish. Southwest Nat. 26:37-42. Weber, W. J. 1975. Notes on cattle egret breeding. Auk 92:111-117. Werschkul, D., E. McMahon, M. Leitschuh, S. English, C. Skibinski & G. williamson. 1977. Observations on the reproductive ecology of the great blue heron (Ardea herodias) in western Oregon. Murrelet 58:7-12.

( 20

CBAPTER 2. S8asonal variation in plumage, breedinq activity and breeding suceess

ABSTRACT

This chapter investigates seasonal variation in the breedinq plumes, breeding activity and breeding success in a colony of cattle egrets (Bubulcus ibis) in Barbados. The development of breeding plumes by individuals was characterised by assigning plumage scores to individuals. Plumage scores for both males and females varied seasenaIIy, but males had hig~er scores than females. As 1ndicated by both number of nests and number of chicks, cattle egrets bred almost year round in Barbados but with periods of peak activity in July to August and November to March. These peaks coincide with the onset and end of the annual rainy season. Plumage scores were lowest when breeding activity was lowest. Poorly plumed birds May be subordinate te weIl plumed birds, and may be forced to breed when environmental conditions are less favourable. Chick mortality was primarily from starvation during the period of low breeding activity, when environmental conditions May be poorest and primarily from conspecific attack during the period of high breeding activity. Fledging success is therefore not highest when breeding activ~ty is greatest. optimal breeding time may be a trade-off between low food 21

availability but Iow conspecific interference, and high food availability but high conspecific interference.

INTRODUCTION For animaIs in temperate regions the strong seasonal changes in temperature typically dictate the seasonality of breeding. In the tropics, where variation in temperature is lower, seasonai variation in rainfall, and hence in food availability, appears ta be a major factor determining breeding seasonality (MacArthur 1964). Aithough seasonal variation in breeding occurs in the tropics, the variation is less marked and the breeding season is often longer than in temperate regions (MacArthur 1964). Investigations of breeding seasonality in the tropics have focussed mainly on correlations between variation in breeding activity and variation in environmentai factors. Breeding success is typically assumed to be highest in months in which breeding activity is highest since environmental factors are MOSt favourable during these periods. In colonial birds, breeding success May also vary between colonies (Wittenberger and Hunt 1985). The costs and benefits to breeding may differ with colony size and the levels of breeding activity. Costs such as the rates of parasitism (Hoogland and Sherman 1976: Brown and Brown 1986; r Moller 1987), infanticide (Moller 1987), interference 22

(Siegfried 1972; Hoogland and Sherman 1976) and nest site competition (Siegfried 1972; Burger 1978) tend to increase with colony size and breeding density. However, benefits such as increased foraging efficiency (Emlen and Demong 1975; Brown 1988) and decreased predation rates (Nisbet 1975; Birkhead 1977) May also increase in large colonies. Colonially breeding birds in the tropics May be able ta maximize their breeding success both by selecting when and in what size of colony to breed (Brown et al. 1990). Whether individual breeding success varies with leveis of breeding activity, and whether the causes of chick mortality vary seasonally, have no~ been investigated for colonial birds in the tropics. The cattle egret (Bubulcus ibis) is a small colonially breeding Ardeid which, due to its opportunistic terrestriai feeding habits, has undergone a large geographical range expansion, and is now found over much of the globe (Weber 1972). Although in its native Africa the cattle egret breeds once a year (Siegfried 1972), it adapts readiIy ta new environmental conditions, and its seasonal breeding patterns now vary geographically. In the neotropics, cattle egrets have been reported to breed from as little as once a year to almost continuously (Lowe-McConnell 1967; Lancaster 1970). They are therefore a good species in which to estimate whether seasonal variation in individual success correlates with seasonal variation in breeding activity. 23

( Cattle egrets develop buff coloured filamentous plumes in the breeding season. Breeding plumes are one of the distinguishing features of the heron family, Ardeidae (Hancock and Kushlan 1984). Their appearance only during the breeding season, and prominent role in courtship displays, support Darwin's (1871) suggestion that plumes are a sexually selected character. Among the monogamous Ardeids, 56% of the species develop breeding plumes or a distinctive breeding plumage, typically in both sexes (Hancock and Kushlan 1984). Although plumes are a distinguishing feature of the Ardeids, few studies have attempted to quantify plumage variation within a population or ta relate seasonal variation in plumage to seasonal variation in breeding activity. In this chapter I investigate seasonal variation in breeding plumes, in breeding activity, and in breeding success in a colony of cattle egrets (Bubulcus ibis ibis) in Barbados: and l assess whether the mortality factors affecting breeding suc cess differ seasonally.

METHODS When breeding, cattle egrets develop reddish brown filamentous feathers on their head, chest and back (Figure 1) • Immediately prior to pairing, they also acquire distinctive red colouration in the beak, irises, lores and legs. These intense breeding colours fade to the typical ( 24

Figure 1. Cattle egret in Barbados with full breeding colours (red bill and legs) and breeding plumes (plumage score=12). (

(~ 25 ye110w during ear1y nestbui1ding. The co1our changes therefore al10w new breeding birds to be identified. l monitored seasonal variation in plumes, breeding aetivity and breeding success in of cattle egrets in a co1ony of approximately 2500 roosting and breeding birds situated in a large bearded fig (Ficus citrifolia) in the northeast of

Barbados. The co10ny was monitored by 3 observers positioned 10 to 30 Metres away using binoculars and te1escopes. Breedinq activity Breeding activity was monitored by biweekly censuses between 8 September 1989 and 28 September 1990. Individuals were categorized as: displaying; nestbuilding; incubating; or with nest1ings. The number of birds in eaeh of these categories was counted. In addition, the number of red-bil1ed egrets was counted to measure the rate of new breeding attempts in the co1ony. It was impossible to accurate1y count aIl birds nesting in the co1ony from a single position. However, sinee counts were always made from the same location, each count should represent a consistent subsamp1e of nesting birds, and therefore reflect breeding activity in the whole eolony. Ta standardize for variation in counts caused by visibility differences between days, 5-day averages were ca1culated for each category and these have been used in aIl figures and analyses. Breeding occurred in aIl months. The Mean number 26

( of breeding pairs counted was 85~32 in the colony (range 16- 147 pairs). Breedinq plumes Breeding plumes were measured by weekly recordings during 3 sample periods. These were, period 1 - 15 September to 28 November 1989, period 2 - 22 January to 15 May 1990, and period 3 - 21 May to 7 August 1990. Breeding plumes were censused separate1y for each sex. The sex of each member of a pair was initial1y determined by behaviour and confirmed by copulation position. The following criteria were used to assiqn sex: Males - Red-billed bird performing vigorous courtship display in colony, while unpaired. - Bird observed to be consistently in the top position durinq copulation. - Bird observed to brinq most of the sticks to the nest Females - Bird observed to submission mou nt a disp1aying, unpaired red-bill. - Bird observed consistently in bottom position during copulation. - Bird observed to perform most of the nest building Reverse copulations were occasional1y observed. Sex was therefore assigned to members of a pair after a minimum of 3 copulation attempts were seen. Reverse copulations occurred in on1y 3 pairs, aIl of which fai1ed to lay (N=2), or to hatch (N=l) eggs. These pairs were excluded from aIl analyses. The plumes of each courting, or newly paired bird, were { visual1y assessed, as described in Table 1 and Figure 2. 1 27 Table 1. Scorinq system used to assign plumage scores to breedinq plumes on the head, chest and back of individual cattle eqrets (see Figure 2) .

AREA SCORE DESCRIPTION

HEAD 0 - no plumes 1 - sparse plumes in front of head * 2 - sparse plumes extendinq down back of head 3 - lush plumes in front of head 4 - lush plumes extending down back of he ad

CHEST 0 - no plumes 1 - < 1/3 lenqth of chest, pale in colour 2 - > 1/3 lenqth of chest, pale in col our 3 - < 1/3 lenqth of chest, dark in colour 4 - > 1/3 length of chest, dark in colour

BACK 0 - no plumes 1 - < 1/2 lenqth of back, pale or dark 2 - > 1/2 length of back, pale or dark 3 - > 3/4 length of back, pale in col our 4 - > 3/4 length of back, dark in col our

* plumes not extending down back of the he ad (see Figure 2) 28

Figure 2. Plumage characteristics used ta quantify the degree of plumage development (see Table 1). Numbers indicate the plumage score assigned. .'

HEAD

1 2 3 4

CHEST BACK

3 2

4 3 4 29

( The plumes observed on the head, chest and back of each egret were scored. Head plumes were scored based on the distance they extended over the head, as weIl as on their density: sparse (few filaments, intermittent and wispy appearance), or lush (many filaments, dense brush-like appearance). Plumes on the chest and back area were scored based on their length relative ta the bird's body, and their colour: dark (red-brown) or pale (light orange to white) (Table 1). Generally, higher scores reflect increasing length, and colour or density of plumes. However, l categorized chest plumes primarily on colour and secondarily on length. Unlike back plumes, whose length is clearly apparent in courtship displays, erected chest plumes are distinct on a displaying bird from the contrast against white chest feathers, and it appeared that shorter dark plumes had more visual impact than longer pale plumes. Factors such as light, wind conditions and bird behaviour (plumes are fluffed when displaying) can affect the assessment of plumes. To avoid biases due to external conditions, the score for each bird was re-assigned by different observers over severaI days and verified by one observer. Plumage scores were assessed for a total of 340 individuals over the whole study.

( 30

Breeding success Breeding success was measured during aIl 3 sample periods (see breeding plumes). It was monitored was monitored daily except during late incubation when nest checks were made every 3 to 4 days. Breeding success was measured for new breeding focal pairs chosen from a small, intensively monitored section of the colony. 1 was able to consistently identify and monitor pairs in the colony using several criteria. These were: (i) ne st position, (ii) stage of breeding (e.g. displaying, nestbuilding, incubating, small downy chicks, large chicks), (iii) individual plume development, and idiosyncrasies in the plumage or face. One member of a pair continuously attended the nest from early pairing until the chicks were at least 9 days old and intruders were rarely observed ta enter the nest site. Characteristic pair displays such as mutual backbiting, and greeting calls further ensured that focal birds could be consistently followed. Focal pairs of birds were followed from early pairing through to fledging (or nest failure) to obtain measures of breeding success for each of the 3 sample periods. A total of 81 pairs were monitored over the 12 months of the study: 21 pairs in period l, 43 pairs in period 2, and 17 pairs in period 3. Most pairs were followed from pairing onwards (N=61). However, some were followed only from hatching onwards (N=20), to compensate for the reduction in sarnple 31

( size due to pairs that failed before hatchinq chicks. Sample sizes therefore vary, dependinq on the variable under consideration. In cattle eqrets, it is difficult to decide when to consider a chick fledged. Many studies on Ardeids consider chicks to have fledqed when they begin to wander from the nest at around 2 weeks oid. However, due to the hiqhly asynchronous breeding at the study colony, adjacent nests did not generally contain chicks of similar ages. Consequently, chicks rarely strayed from their nests until they were free-flying, allowing them to be monitored until close to independence. Chicks are free-flying between 30

and 40 da ys old and may continue to be fed by their parents for as long as 56 days after hatching (Blaker 1969). In this study, l considered a chick who survived to 35 days after hatching as successfully fledged. After this age chicks can fly short distances, are often absent from the nest, and are no longer fed exclusively at the nest. Hatching success, fledging success and nestling mortality were used as indices of breeding success. No counts of clutch size or measures of incubation mortality were attempted, since it was difficult to see into nests to accurately determine clutch size. Hatching success was the brood size per nest at hatching for aIl pairs who completed a clutch. Fledqinq success was the number of chicks in the ( ne st 35 days after hatching. It was recorded for 2 sets of 32 birds: aIl pairs who successfully hatched chicks (Fledging success 1), and aIl pairs who laid a clutch (Fledging success 2). Nestlinq mortality was the percentage of chicks hatched that died during the nestling period (1 to 35 days after hatching). Nestling mortality was attributed to siblicide, starvation, red-bill attack, nest collapse or unknown causes. A chick was considered to have died from siblicide if criteria (i) or (ii), and (iii) were met: (i) chick was attacked by its sibling during feeding; (ii) clear evidence of physical damage to the chick was observed (i.e., loss of feathers on head; and cuts and blood around he ad and bill); or (iii) chick was observed battered and dead hanging from the nest or immediately below the nest, with other siblings unharmed in nest. starvation was considered to be the cause of death if: (i) chick was stunted in size compared with chicks of a similar age; and (ii) chick's movements, especially during feeding, appeared weak. Deaths of younger chicks through their inability to compete for food with older, larger siblings was considered starvation if no aggression was observed between the chicks. Mortality throuqh attacks by red-billed birds was readily identifiable because attacks were conspicuous and resulted in rapid death or displacement of nestlinqs. Chicks were considcred to have died from a red-bill attack 33

( if one of the following criteria was met: (i) the attack was observed; (ii) chicks who appeared healthy disappeared overnight and were replaced on the nest by a displaying red­ bill male or new pair; or (iii) chicks were found to be dead or displaced immediately below the nest with a new red-bill pair on the nest. Red bill attacks typically resulted in

1055 of aIl chicks in the nest. Broken branches also led to the collapse and loss of 2

nests i~ the colony. The "unknown" cateqory contains deaths that did not fall unambiguously into one of the above categories. Many of these deaths were of very young chicks where it was difficult to tell if the chick was abnormal or if the parents were not caring for it adequately. Data analysis Many of the variables under consideration had distributions that deviated significantly from normal. This was particularly true for plumage scores. standard transformations did not adequately normalize aIl of the data. Therefore, non-parametric statistics were used for most analyses. Dunn's non-parametric multiple range tests were employed to identify which groups differed siqnificantly in Kruska1-Wallis tests. Since in this test the probabilities are "shared" between groups, two groups are considered to be significantly different at values greater than 0.05, depending on the number of groups being 34

tested (Neave and Worthington 1988). All tests are two­ tailed unless otherwise indicated. Data were analyzed using the SYSTAT statistical software (Wilkinson, 1989).

RESULTS Breedinq plumes The scores assigned to breeding plumes on an individual's head, chest and back were significantly intercorrelated for both males and females, i.e., birds which were well plumed on the head were also weIl plumed on the chest and back (Table 2: 3). This was true over the whole study, as well as within each sample period. Only in period 1 were scores not consistently inter-correlated between body parts, due mainly to the small sample size (N=13). Given the strong intercorrelations observed, plumage scores from each body part were summed to give a total plumage score for each individual. Only total plumage scores have been used in subsequent analyses. Breeding plumes varied considerably, total plumage scores ranging from 1-12 and coefficients of variation ranging from 0.13 to 0.85 over the whole study period (Table

4). Plumage scores differed between the sexes (Table 4; Mann-Whitney Test, U=20816, P

Table 2. Spearman Rank Correlation matrix illustrating the degree of inter-correlation between an individual male's head, chest, back and combined (total) plumage scores. Correlations are presented for the entire study, and separately for each sample periode A total of 170 males were scored over the whole study: 13 in Period 1; 59 in period 2; and 98 in Period 3.

8A11PLE UEA TOTAL READ CREST BACK PERlOD SCOUD

whole study TOTAL 1 1 1 2 1 3 1

whole study READ 0.89:** 1 1 0.59*** 1 2 0.90*** 1 3 0.70 1 *** whole study CREST 0.92*** 0.74*** 1 1 0.88 0.39~:* 1 2 0.94*** 0.76 1 3 0.73*** 0.27** 1

aIl BACK *** O. "6*** 0.81*** 1 0.89*** ns 1 0.91 0.44*** 0.76** 1 2 0.87*** 0.91* 0.71*** 1 3 0.60*** 0.20 0.39*** 1

ns P>0.05 * P<0.05 ** P

,t 36

Table 3. Spearman Rank Correlation matrix illustrating the degree of inter-correlation between an individual female's head, chest, back and combined (total) plumage scores. Correlations are presented for the entire study, and separately for each sample periode A total of 170 females were scored over the whole study: 13 in period 1; 59 in Period 2; and 98 in period 3.

SMPLB AUA TOTAL HEAD CREST BACR PERXOD SCORED whole study TOTAL 1 1 1 2 1 3 1 whole study HEAD 0.85*** 1 ns 1 0.39*** 1 2 0.51 1 3 0.63*** 1 *** whole study CREST 0.92*** 0.73*** 1 23 ns 1 1 0.84*** o • *** 2 0.94 0.46** 1 3 0.86*** 0.32 1 whole study BACR 0.92*** 0.73*** 0.79*** 1 1 0.73** -0.05ns 0.39ns 1 2 0.71*** 0.69*** 0.54*** 1 3 0.80*** 0.36*** 0.53*** 1 ns P>0.05 * P<0.05 ** P

SEX AREA MEAN RANGB C.v. DOLE STUDY (N=170) MALES TOTAL 8.50 1-12 0.47 HEAD 2.97 1-4 0.40 CHEST 2.79 0-4 0.52 BACK 2.75 0-4 0.58

FEMALES TOTAL 4.95 1-12 0.67 HEAD 1.84 1-4 0.47 CHEST 1.58 0-4 0.89 BACK 1.52 0-4 0.93

PERIOD 1 (N=13) MALES TOTAL 10.23 4-12 0.23 HEAD 3.61 2-4 0.21 CHEST 3.31 0-4 0.33 BACK 3.31 2-4 0.26

FEMALES TOTAL 4.69 2-9 0.49 HEAD 2.15 1-4 0.32 CHEST 1.39 0-3 0.86 BACK 1.15 0-4 1.11

PERIOD 2 (N=59) MALES TOTAL 4.10 1-12 0.85 HEAD 1.81 1-4 0.59 CHEST 1.29 0-4 0.98 BACK 1.03 0-4 1.36

FEMALES TOTAL 1.52 1-7 0.82 HEAD 1.05 1-2 0.21 CHEST 0.31 0-3 2.30 BACK 0.17 0-2 3.13

PERIOD 3 (N=98) MALES TOTAL 10.92 5-12 0.13 HEAD 3.58 2-4 0.19 CHEST 3.62 1-4 0.18 BACK 3.71 2-4 0.16

FEMALES TOTAL 7.04 1-12 0.35 HEAD 2.28 1-4 0.36 CHEST 2.37 0-4 0.49 BACK 2.39 0-4 0.46 38 but only 2.5% of females, fell into the highest plumage category (Figure 3). Breeding plumes of males and females varied seasonally, plumage scores differing significantly between sample periods (Figure 4: Kruskal-Wallis test, for males, H=91.46, P

P<0.05; 2 vs. 3, T=9.28, PO.10). For females, plumage scores differed in each sample period (Dunnls multiple comparison: 1 vs. 2, T=3.41, P

Figure 3. Frequency distribution of male and female total plumage scores in the population over the whole study periode 40

.. MALES---~ 35 rzza FE MALES ,.....--... ~ c o 30 +-' o :::J 0.. 25 o 0.. CR 20 "----'" >-, u 15 c Q) :::J CT 10 Q) L LL 5

o o 3 4 5 6 7 8 9 10 11 1/

Plumage scon=; 40

Figure 4. Mean total plumage scores in each sampling period, presented separately for males and females. ...

N=LS Ne 9R 13 12 1 Cl) 1 1 L 1 0 10 •1 u • N -:)9 1- (f) 9 - Cl) 8 1 CJl 0 7 E 6 -::J 5 Cl 1 4 Cl) • 3 0 :2 2 j 1 0 0 ') 3 1.3 -

Cl) 12 ~ 0 1 1 U 10 (f) 9 Cl) CJl 8 0 E 7 ::J 6 - Cl 5 Cl) 4 0 E 3 Cl) LL ~t _____ 1 0 2 ,) ,,,, Breeding per iod +' 41

Fiqura 5. Seasonal variation in numbers of occupied nests plotted as 5 day averages. The temporal locations of samplinq periods 1, 2 and 3 are indicated. J\.'

Period Perlod 2 Perlod 3 150 140 130 120

(f) 110 4-' (f) 100 Cl) c 90 4- 0 80

L 70 Cl) D 60 E 50 :J Z 40 30 20 10

_L_----1-___ L __ J ___ j 0 - j S 0 N 0 J ~- M A M ,j ,J 1\ () 1989 t9DO DoLe 42

Figure 6. Seasonal variation in the total number of chicks plotted as 5 day averages. The temporal locations of sampling periods l, 2 and 3 are indicated. 80

Period 1 Perlod 2 IJcrlod j 70 ------(f) ~ u 60 ..c u 4- 50 0

L (J) 40 D E :J 30 c

0 4-' 20 0 1- 10

0 -1_-----..L_....J_-1____ L _ S 0 N 0 J F M A tv1 J .J 1\ C; () 1989 1990 Dote 43

Fiqure 7. Seasonal variation in the number of red-billed birds plotted as 5 day averaqes. The temporal locations of sampling periods l, 2 and 3 are indicated. Period 1 Period 2 rcrlod 3 50

45

40 (f)

D 35 -0 (J) 30 L

4- 0 25

L (J) 20 D E 15 :J Z 10

5

0 L - 1 ,1 ~ (' s 0 N D J M ,J ,J 1\ ) ( ) 1989 19BO Dote 44 a minimum of 1 week before densities of occupied nests are highest. Red-bill numbers were negatively correlated with the number of chicks in the colony (rs=-O.29, P

Mean hatching success was 1.49 chicks/pair and did not vary significant1y between sample periods (Table 5; Figure

8; Kruska1-Wallis test, H=1.29, P=O.52). Mean f1edging success for pairs who had successfu1ly hatched chicks

(Fledging success 1) was 0.92 chicks/pair (Table 5). It differed significantly between samp1e periods (Kruskal­

Wallis test, H=10.89, P=O.004), decreasinq with each subsequent period (Table 5; Figure 8). The difference between periods was caused primari1y by the low fledging success in period 3 (Dunn's multiple comparison: 1 vs. 2,

T=1.48, P>O.30i 2 vs. 3, T=2.20, P

5). It also declined with subsequent sample periods (Table 45 Table 5. Measures of breeding success for pairs of cattle egrets, presenteà separately for ~ach sample period and for the whole study. X is the mean, SD the standard deviation, and H the number of pairs.

period 1 Period 2 period 3 Whole study

Ratebinq i 1.52 1.43 1.59 1. 49 sueeess SD 1.03 0.78 1.00 0.90 H 21 35 17 73

Fle4qinq f 1.31 0.95 0.44 0.92 sueeess SD 0.79 0.72 0.51 0.75 1 H 16 43 16 75

Fle4qinq X 1.00 0.74 0.35 0.73 sueeess SD 0.89 0.74 0.49 0.77 2 H 21 35 17 73 46

Figure 8. Percentage of breeding pairs that hatched and fledged 0, 1, 2 or 3 chicks, presented separately for hatching and fledging and for each sample

periode Il Period 1; ~ period 2; CJ period 3. " 60

Ul 50 L [ 40

4- o 30 -+-' C Cl) U 20 L Cl) 0... 10 o .JJ o 2 3 60

Ul 50 L o Cl 40 4-o 30 -+-' C Cl) U 20 L Cl) 0... 10 o o 70

60 Ul L o 50 Cl '+-o 40 ~ 30 Cl) ~ 20 Cl) 0... 10 o o 2 Num ber of chicks 47

( 5: Figure 8), but the difference between periods was not statistically significant (Figure 8: Kruskal-Wallis test, H=4.46, P=O.108). Overall nestling mortality was 51% and differed significantly between sample periods (Table 6; X2=19.48, P

deaths could be attributed to specifie causes. The 3 major causes were: starvation, attack by red-bills and siblicide (Table 7). Mortality from red-bill attacks was almost twice as common in period 3 as in Periods 1 and 2, and mortality from starvation was twice as common in Period 2 as in periods 1 and 3 (Table 7). Nestling mortality from siblicide was similar in aIl periods (Table 7).

DISCUSSION Variation in breeding plumes has previously been noted in studies of cattle egrets (Lancaster 1970: McKilligan 1985; Haddock 1989). However, this is the first attempt to systematically quantify the variation in plumes and follow their changes through an annual cycle. For both males and females, breeding plumes on the head, chest and back of an

individual were strongly inte~correlated, inà1cating that birds who were weIl plumed on one part of the body were weIl plumed throughout. The extent of plume development differed

, i between the sexes, males having higher mean plumage scores, 48 Table 6. Nestling mortality (% of chicks hatched who dicd during nestling period), broken into different mortality causes, and presented separately for each sample period and the whole study combined.

period 1 Period 2 period 3 Whole study

Ro. chicks hatched 32 74 33 139 siblicide 3.1 4.1 12.1 5.8

._------starvation 3.1 12.2 15.2 10.8

. - - - -- Red Bill attack 3.1 5.4 24.2 9.4 _.------. Rest collapse 3.1 0.0 3.0 1.4

Unknown 21.9 24.3 24.2 23.7 Total nestling mortality 34.4 45.9 78.8 51.1 49 Table 7. The distribution of nestling deaths among known mortality causes (% by each cause) presented separately for each sample period and the whole study.

Period 1 period 2 period 3 Whole stUdy

Siblicide 25.0 18.7 22.2 21.1

starvation 25.0 56.3 27.7 39.5

Red-bill Attack 25.0 25.0 44.4 34.2

Hast collapse 25.0 0.0 5.6 5.3

,( 50

... but lower variance, than females . Plumage scores for breeding birds were high in sample period 1 (September to November), low in sample periad 2 (February to April) and high in sample period 3 (May to August) (Figure 4). Seasonal variation in number of occupied nests indicated two annual peaks of breeding activity in Barbados, the first between November and March, the second between July and August (Figure 5). Breeding activity was low between March and June (Figure 5). Thus, plumage scores of breeding birds tended to be high dur ing periods of high breeding activity and low during periods of low breeding activity. Poor plumage on breeding birds may indicate that they are first time breeders. Maddock (1989) observed that one year old breeders had poorer plumes than two year old breeders in the Australian cattle egret (Bubulcus ibis coromandus), but plume quality in first year breeders varicd from none ta "full" plumes. Juvenile cattle egrets can generally be distinguished from adults by the black leg colour of the former, an0 yellow leg colour of the latter

(Lancaster 1970). In this study, 21% of birds with a plumage score of 1 had black legs, and 79% had yellow legs. Thirty-nine percent of birds with plumage scores greater than 1 also had black legs. This suggests considerable variation in the breeding plumage of birds of a given age, 51

and makes it unlikely that aIl birds breeding with poor plumes are necessarily first year breeders. Whether or not aIl poorly plumed birds are first year breeders, there is evidence suggesting that poorly plumed birds are behaviourally subordinate and in poorer physical condition than weIl plumed birds. Woolfenden et al. (1976) studied food competition among cattle egrets on an island in Florida. Food was limiting, and mortality due to starvation was high. WeIl plumed egrets were more aggressive, occupied better feeding areas, and had lower mortality rates than poorly plumed egrets. This suggests that the poorly plurned birds breeding in rnonths of low breeding activity in t~~ present study may be the weaker competitors. Although space per se does not appear limiting in the colony (Chapter 1), competition for nest sites apparently occurs. New breeding pairs of egrets frequently occupied old nests, or took over nests by forcibly evicting nest residents. This is an opportunistic strategy if nestbuilding is costly. Cattle egrets of poor competitive ability may be forced to breed during months of low breeding activity to avoid conspecific interference and competition for ne st sites in the colony. At high breeding densities, new pairs searchinq for nest sites were responsible for 55% of aIl nest 1ailures, compared to 17% at low breeding densities. Data on seasonal variation in occupied nests suggests that cattle egrets in Barbados breed almost year-round, but with periods of peak activity (November-March: July-August). Other studies of cattle egrets in the neotropics have also reported two annual peaks in reproductive activity (Lowe­ McConnell 1967: Visscher 1977). However, in South Africa (Blaker 1969; Siegi.a::ied 1972), North America (Burger 1978;

Rodgers 1987), Japan (Fujioka 1984), and Australia

(McKilligan 1985), where climate is more seasonal and temperature-driven, cattle egrets breed only once a year and breeding is restricted ta about a 4 month periode Rains in Barbados are markedly seasonal, with the rainy season typically extending from July to December (Riven­

Ramsey 1988). The two periods of peak breeding activity therefore coincide with the onset and end of the rainy season. The period of low breeding activity (April to June) is within the dry season. In the tropics, food abundance for terrestrial animaIs is generally higher in the rainy

season than the dry season (Janzen and Schoener 1968; Wolda 1980). The seasonal pattern of breeding activity of cattle egrets in Barbados suggests that breeding is lowest during months of low food availability. The seasonal variation in plumes of breeding birds may reflect the fact that poorly plumed, and perhaps subordinate birds, are forced to breed when environmental conditions are least favourable for reproduction. 53

It is not known whether the bimodal distribution of breeding activity observed indicates that individual cattle egrets can breed twice in one year. Lowe-McConnell (1967) suggested that the bimodal breeding peaks observed for cattle egrets in Guyana indicate that individuals breed twice, but she did not follow marked birds. Several authors have observed re-nesting by individuals (Siegfried 1972; Fujioka 1986; Maddock 1989; this study). However, this typically follows a failed nesting attempt and is not suggestive of biannual breeding by individual birds. In this study the large roosting population in the colony (cf. 2500) compared to the average monthly nesting population (cf. 85 birds) suggests that the bimodal breeding activity may reflect individuals breeding once, but at different times of the year. The fledging success of cattle egrets in Barbados was low compared to other locations (Table 8). This is due partly to small clutch sizes, since mean brood size at hatching (hatching success) is low (Table 8). A pattern of declining clutch sizes with latitude is typical for many avian species (Lack 1954). Arendt and Arendt (1988) summarize clutch sizes of cattle egrets from a variety of latitudes. Clutch size ranged from 4.6 eggs/nest at 46 0 north to 1.9 eggs/nest at 70 south. In Barbados, at 13 0 north, clutch size is expected to be low. r The low fledging success in Barbados is also due to 54 Table 8. Comparison of the breeding succesï of cattle egrets at different locations. * If fledging success is calculated 35 days after hatching; .2 If fledging is calculated 14 days after hatching. {a) This study: (b) Siegfried (1972): (c) Maddock (1989): (d) Burger (1978); (e) Fujioka (1985); (f) Rodgers (1987): (g) Weber (1975); (h) Maxwell and Kale (1977): and (i) Jenni (1969).

Location HatchiDC) Ple4qinC) P1e4qinC) NestlinC) success suceess success mortality 1 2 (~±sd) ($ë±sd) (5l±sd) (averall %) Barbados 1.49±.9 O.92±.75 0.73±.77 51.1 (a) 1.03±.68 0.80±.68 44.2 South 2.59 1.83 1.08 36.0 Africa (b) Australia 2.4 1.7 (c) New Jersey 3.6±.9 2.48±.9 (d) Japan 3.4 2.56 25.8 (e) Florida ( f) 2.05±.05 1.3±.O6 51. 5 (g) 1.8 28.6 (h) 2.6±.16 2.3 2.1 11. 0 (i) 2.9 17.9 55

( high rates of nestling mortality (Table 6). However, comparisons of nestling mortality are confounded by differences between studies in the time at which fledging is considered as occurring. With an ongoing risk of mortality, the longer the fledging period used, the higher the mortality prior to fledging. Most studies estimate fledging success from survival until 14 days (eg. Weber 1975: Burger 1978: Rodgers 1987), even though siblicidal attacks in broods can occur up to 25 days after hatching (Fujioka 1985: this study). Nestling mortality in this study remains higher than reported values, even when re-calculated at 14 days after hatching (Table 8). In this study, hatching success did not vary between sample periods, while nestling mortality and consequently fledging success did. Nestling mortality was highest and fledging success lowest in period 3: whereas mortality was lowest and fledging success highest in period 1. Fledging success is therefore not necessarily highest during months in which breeding activity is highest, and in which environmental conditions appear most favourable for reproduction. Periods 1 and 3 were rainy months with high breeding activity: period 2 was in the dry season and there was little breeding activity. Twice as many chicks died of starvation in period 2, compared to Periods 1 and 3. This is consistent with the suggestion that food availability was 1 ~ 56 lowest in this period, and hence that limited food availability was the cause of the lower breeding activity observed in this period. In contrast, nearly twice as many chicks died from attacks by red-billed birds in period 3, than in Periods 1 and 2. Conspecific attacks, presumably motivated by competition for nest sites, may therefore constitute a major cost to birds breeding in months of high breeding activity. In particular, birds of poor competitive ability should avoid breeding in such months. If poor breeding plumes indicate poor competitive ability in egrets, this is precisely what was observed in the present study. Nest-density per se was not the best predictor of competition for nest-sites, and hence of nestling mortality caused by conspecific attacks. Nest density was high in Periods 1 and 3, but nestling mortality was lowest in period 1 and highest in period 3. Specifically, mortality from conspecific attacks was high in period 3 and low in Period 1. The most striking difference in the colony between Periods 1 and 3 was the abundance of red-billed birds present (Figure 7). There were twice as many red-billed birds in the colony in period 3 as in Period 1 (18.8% vs. 9.8%), and red-billed birds were responsible for aIl but one case of mortality due to conspecific attack. The seasonal variation in plumage, breeding activity and breeding success observed for cattle egrets in this study emphasizes that there are costs and benefits to 57

breeding in the colony at different times. The nature of these costs may vary with time of year, and cost/benefit trade-offs may differ for different birds. Consequently, given that there is seasonal variation in breeding activity which tracks favourable environmental conditions but which intensifies intraspecific competition during the favourable conditions, individual breeding success may not be lowest in months in which environmental conditions appear least favourable for reproduction (see Fretwell 1972 for similar arguments re habitat selection as an ideal free distribution). Optimal breeding times may therefore reflect trade-offs between food availability and conspecific interference, and the optimal time may differ for different individuals (see Brown et al. 1990 for similar suggestions). The results of this study indicate that tropical colonial birds are excellent candidates for investigating breeding decisions. Future studies should attempt ta quantify fledgling weight and post-fledging survival. Chicks may differ in both paramnters, and numbers of chicks fledged may therefore overestimate breeding success at times of year when food availability is low. 58

LXTERATURB CITED Arendt, W. and A. Arendt. 1988. Aspects of the breeding biology of the cattle egret (Bubulcus ibis) in Montserrat, West Indies, and its impact on nest vegetation. Colonial Waterbirds 11:72-84. Birkhead, T. R. 1977. The effect of habitat and density on breeding success in the common guillemot (Uria aalge). J. Anim. Ecol. 46:751-764. Blaker, D. 1969. Behaviour of the cattle egret Ardeola ibis. Ostrich 40:75-129. Brown C. R. and M. B. Brown. 1986. Ectoparasitism as a cost of coloniality in cliff swallows (Hirundo pyrrhonota). Ecology 67:1206-1218. Brown, C. R. 1988. Enhanced foraging efficiency through information centres: a benefit of coloniality in cliff swallows. Ecology 69:602-613. Brown, C. R., B. J. Stutchbury, and P. D. Walsh. 1990. Choice of colony size in birds. TREE 5:398-403. Burger, J. 1978. Competition between cattle egrets and native North American herons, egrets and ibises. Condor 80:15-23. Darwin, C. 1871. The Descent of Man and Selection in Relation to Sex. Murray, London, U.K. Emlen S. T. and N. J. Demong. 1975. Adaptive significance of synchronized breeding in a colonial bird: a new hypothesis. Science 188:1039-1041. Fretwell, S. D. 1972. Seasonal environments. Princeton University Press, Princeton, New Jersey. Fujioka, M. 1984. Asynchronous hatching, growth and survival of chicks of the cattle egret Bubulcus ibis. Tori 33:1-12. Fujioka, M. 1985. Sibling competition and siblicide in asynchronously hatching broods of the cattle egret (Bubulcus ibis). Anim. Behav. 33:1228-1242. Fujioka, M. 1986. Infanticide by a male parent and a new female mate in colonial egrets. Auk 103:619-621. 59 Hancock, J. and J. A. Kushlan. 1984. The Herons. Croom Helm, London, U. K. Hoogland, J. L. and P. W. Sherman. 1976. Advantages arJd disadvantages of bank swallow (Riparia riparia) cOloniality. Ecol. Monogr. 46:33-58.

Janzen, D. H. and T. W. Schoener. 1968. Differences in insect abundance and diversity between wetter and drier sites during a tropical dry season. Ecology 49:96-109.

Lack, D. 1954. The Natural Regulation of Animal Numbers. Oxford University Press, London, U.K.

Lancaster, D. A. 1970. Breeding behav5.our of the cattle egret in Columbia. Living Bird 9:176-194.

Lowe-McConnell, R. H. 1967. Biology of the immigrant cattle egret (Ardeola ibis) in Guyana, South America. Ibis 109:168-179.

MacArthur, R. A. 1964. Geographical ecology. Princeton University Press, Princeton, New Jersey. 269 pp.

McKilligan, N. G. 1985. The breeding success of the Indian cattle egret Ardeola ibis in eastern Australia. Ibis 127: 530-536.

Maddock, M. 1989. Colour and first age of breeding in cattle egrets as determined from wing-tagged birds. Corella 13: 1-8.

Moller, A. P. 1987. Advantages and disadvantages of coloniality in the swallow, Hirundo rustica. Anim. Behav. 35:819-832.

Nisbet, l. C. T. 1975. Selective effects of predation in a tern colony. Condor 77:221-226.

Neave, H. R. and P. L. worthington. 1988. Distribution­ Free Tests. Unwin Hyman, London, U.K.

Riven-Ramsey, D. 1988. Foraging and breeding behaviour of the cattle egret (Bubulcus ibis) in Barbados. M.Sc. thesis, University of the West lndies.

Rodgers, J. A. Jr. 1987. &~~eding chronology and reproductive success of cattle egrets and little blue herons on the west coast of Florida, U.S.A. Colonial Waterbirds 10:38-44. f1 60

siegfried, W. R. 1972. Breeding success and reproductive output of the cattle egret. ostrich 43:43-55. visscher, M. N. de. 1977. A mixed colony of egrets and magnificent frigatebirds in Venezuela. Gefaut 67:203- 223.

Weber, W. J. 1972. A new world for the cattle egret. Nat. Hist. 81:56-63.

Weber, W. J. 1975. Notes on cattle egret breeding. Auk 92:111-117.

wilkinson, L. 1989. SYSTAT: The system for Statistics for the PC. SYSTAT Inc., Illinois, U.S.A.

wittenberger, J. F. and G. L. Hunt, Jr. 1985. The adaptivc significance of coloniality in birds. In: Avian Biology Vol. VIII (Eds. D. S. Farner, J. R. King and K. C. Parkes). pp. 1-78. Academie Press, New York.

Wolda, H. 1980. Seasonality of tropical insects. I. Leafhoppers (Homoptera) in Las Cumbres, Panama. J. Anim. Ecol. 49:277-290.

Woolfenden, G. E., S. C. White, R. L. Mumme, and W. B. Robertson Jr. 1976. Aggression among starving cattlc egrets. Bird Banding 47:48-~3. 61

( CHAPTER 3: Breedinq plumes, .atinq preferences, parental care and reproductive success in cattle egreta

ABSTRACT

This chapter investigates whether plumage is a criterion of mate choice and is correlated with either parental care or offspring viability in the cattle egret (Bubulcus ibis). Plumage scores were positively correlated in breeding pairs, largely because of female preference for weIl plumed males. Neither male nest attendance, incubation, nor nestling attendance were correlated with male plumage. Copulation rate was positively correlated with male plumage, but is unlikely to be the basis of the female preference. WeIl plumed males fed chicks more often than poorly plumed males and had the higher fledging success at times of year when food was limiting. This may be the basis of female preference for male plumage. The pairing pattern is consistent with both the good genes and good parent hypotheses for sexual selection but not with the Fisherian hypothesis. Neither female copulation rate, nest attendance, incubation, nestling attendance nor fledging success were correlated with female plumage. Male nest and

nestling attendance were positively correlated ~jth their mate's plumage. This suggests that males value weIl plumed females more highly. It may indicate that plumes signal high genetie quality, since weIl plumed females do not r G2

provide more maternaI care. The parental behaviour of females was not influenced by the plumage of their mates.

INTRODUCTION

Darwin (1871) first proposed that the elaborate

secondary sexual characteristics observed in many males hilVC evolved by sexual selection through female mating preferences. Studies have now clearly demonstrated that females in many species prefer to mate with males with

elaborate characters (Anderssor 19B2; Moller 198B: Norris

1990a; Petrie et al. 1991). However, why females prefer elaborate male traits, and consequently the evolution of female mating preferences is less clear. Females may prefer males with elaborate traits solely because of the mating advantages accruing to their male

offspring (the Fisherian hypothesis; Fisher 1958). However, the advantages to the male offspring depend on female preference for the trait being already in the population. Consequently, preference for the trait may have initially provided sorne benefit to the female or her offspring other

than enhanced mating success of sons (Fisher 1958) . Elaboration of the tra1t in males is believed to be favoured until the mating advantage conferred by the trait is countered by the survival cost of bearing it. Several genetie models have demonstrated che plausibility of this

process (Lande 1981; Kirkpatriek 1982). 63

Females rnay choose mates with exaggerated traits because the trait is correlated with the genetic quality of the bearer (t}' 'good genes' hypothesis). Females thus obtain good genes, increasing the survival or fecundity of their offspring. Male characters could signal genetic quality in several ways. Zahavi (1975) originally proposed that elaborate traits signal male quality because they are handicaps to the survivorship of the bearer. Whether this process can lead to the elaboration of traits has been extensively debated, since offspring inherit the handicap as weIl as the high quality genes (Maynard Smith 1985; but see P0miankowski 1988). Elaborate traits may indicate genetic quality because they are condition-dependent (Kodric-Brown and Brown 1984; Andersson 1986), i.e. the traits are only fully expressed in males of high genetic quality. Male characters may also indicate genetic quality by signalling the age of the bearer, since older males have demonstrated their viabllity (Manning 1985; 1987). Elaborate male traits may not signal overall genetic quality, but rather the quality of specifie male characteristics. For example, Hamilton and Zuk (1982) suggested that the traits signal the level of r~rasitic infection. Females choosing bright males thus obta_i relatively parasite-free mates and genetic resistance for their young (see Moller 1990a for review). Mottro (1982) modelled the evolution of elaborate male traits which signal heritable variation in parental quality. Females preferring the traits obtain good male parental care for their offspring, and produce offspring who will be high quality parents. Female mating preferences may not be based on genetie benefits, but exclusively on immediate benefits whieh increase female survival, fecundity, or paternal care for her offspring. Males in good condition may display traits which advertise this, and hence their resource provisioning capacity. Studies have recently demonstrated that male plumage quality is correlated with subsequent male parental care in several monogamous passerinns (Norris 1990b; Hill 1991). Hoelzer (1989) has demonstrated that female preferences for non-heritable male parental care can lead ta the evolution of traits signalling parental quality (the 'good parent' hypothesis). He suggested that females will be selected to improve the accuracy with which they differentiate between males varying in non-heritable parental quality. Consequently, males with genes for a trait that honestly predicts the subsequent quality of male parental care will be preferred over males without genes for the trait. The Fisherian, good genes, and good parent hypotheses for the evolution of elaborate male traits are not mutually exclusive. They may aIl occur in nature and may interact in a given mating system. Separating their importance in any 65

( mating system is complicated by the fact that they often make similar predictions. AlI three predict that, if the trait has evolved through female mate choice, females will prefer males in which the trait is better developed. The Fisherian hypothesis does not predict a correlation between the trait development and future viability of offspring. The good genes hypothesis predicts a correlation between trait development and offspring viability. It does not predict a correlation between trait del'elopment and parental care, except in the case where the choice is for genes for

good parental care. ~l~ good parent hypothesis predicts a correlation between trait development, and parental care and between trait development and offspring viability, the enhanced viability arising from better parental care. Cattle egrets are a monogamous, colonial ardeid, in which both sexes exhibit lush filamentous breeding plumes. Considerable variation in the length and colour of the plumes exists between individuals within both sexes (Chapter 2). Since both sexes possess breeding plumes, the processes for the evolution of traits based on mate preference described above may apply equivalently to males and females. It is therefore possible to investigate the relationship between plume quality, mate preferences, parental care and offspring viability for both sexes in cattle egrets. The objectives of this chapter are to investigate whether plume quality is a criterion of mate choice, and whether plume 66 quality is correlated with parental care and offspring viability: and thereby ta comment t"n the relative importance of the Fisherian, good genes, and good parent hypotheses for the evolution of breeding plumes in cattle egrets.

METRons l monitored the behaviour of 67 breeding pairs of cattle egrets in a colony in Barbados, West Indies. In the breeding season, both sexes develop reddish-brown filamentous plumes on the head, back and chest. Irnmediately prior to pairing cattle egrets also acquire a distinctive red coloration in the legs, bill, lores and irises. Thus, unpaired birds ready to breed, and newly paired birds were easily identified. l assessed the length of plumes of each bird visually and assigned each a plumage score (see Chapter 2 for details). courtsbip and pairing An unpaired red-billed male cattle egret acquires and de fends a small display territory in the colony. This territory ~ay be an area along a branch, or an old nesting site often with a nest in various stages of dccay. The male defends this site vigorously from both male and fernale intruders, and perforrns a series of courtship displays, especially during the early morning and late afternoon. six male courtship displays were observed: twig shakei forward; stretch: wing touch: flap flighti and snap (see ------

67

, i Blaker 1969 for a complete description of each display). Males continue to display from a site for up to ! week, although sorne males pair on their first afternoon of displaying. Female egrets appear to exhibit more evident mate choiee than males. Red-billed females, eonspicuous by their

sleeked plumage (Blaker 1969), move through the colony and wateh males display. l have seen up to 6 females simultaneously watching nearby displaying males. Pairing is initiated by females, who attempt to land on the back of a displaying male (submission mount). The male often displaces the female, and the female may mount the same male many times before pairing with him. Male resistance to

female submission mounts varied, and may b~ a mechanism of male mate choice. Two birds were considered to have paired when mutual backbiting was observed. Sex Determination The sex of eaeh member of a pair was determined from stereotypie behaviour3 (e.g. male stick eollecting, female nestbuilàing) and copulation position following the criteria outlined in Chapter 2. Once sex had been deterrnined, differences in plume development and/or idiosyncrasies of the face or plumes were used to continue di fferentiating the pair. One pair was discarded from analyses because of difficulty in

i" eonsistently distinguishing the sexe ,1 68 samplinq periods Individual pairs were followed from courtship or early pairing, through nestbuilding, egglaying, incubation, and the nestling stage until the chicks were free-flying (between 35-45 days old). This represented a maximum observation period of 79 days. Pairs of breeding birds were monitored during 3 different times of the year: Period 1 - 15 september to 30 November 1989; Period 2 - 22 January to 15 May 1990; period 3 - 21 May to 7 August 1990. New pairs were chosen during approximately the first month of each periode Pre-eqqlayinq and early incubation vatches Cattle egrets attend their nest site almost continuously after pairing and begin nestbuilding the day after pairing. l monitored newly paired birds from pairing untii clutch completion. Clutch size in Barbados varied between 1-3 eggs, with 2-egg clutches being the most common. The date of egglaying was determined visually, when possible, and behaviourally when visibility was poor. Pairs were monitored for 4-5 days after the first egg was laid, allowing for a maximum clutch of 3 eggs to be laid. The monitoring perjod therefore extended over a female's fertile period and over the first 4 to 5 da ys of incuba~ion. l used focal sampling on the minute to monitor attendance at the nest and incubation rates for both members 69

( of a pair. Copulations were recorded continuously over the focal watch. Focal watches were conducted in 2 different observation regimes during the study. In periods 1 and 3, observations

alternated between morning (700 - 1200 hours approximately)

watches one day and afternoon (1200 - 1800 hours) watches the next day. In Period 2, observations continued over aIl daylight hours, except for a 2 hour lunch hour (alternating

within 1100 and 1400 hours). This gave a total focal watch

of 10 hours/day in period 2. Observers were able to watch

up to 4 pairs at a time. Focal watches on nests (N=67)

totailed 7699.46 nest-hours of observation over the whole

study. l conducted 714.64 nest-hours (13 nests) of

observation in Period l, 4942.59 nest-hours (34 nests) in

Period 2, and 2042.23 nest-hours (20 nests) in period 3. Feeding watcbes Hatching occurred 23-25 days after egg-Iaying. Nests were periodically checked during this period to document nest failure. The watches began 1 day after hatching. Focal watches recorded the attendance of nestlings by each parent, aIl feedings, and the number of boluses delivered to each chick were recorded. Feeds were divided into 2 categories: Nest feeds - parent regurgitates bolus directly into the bottom of the nest, and typically re-eats any remaining food (chicks 1-5 days old) 70 Direct feeds - chicks grab the parents bill crosswise and pull it down ta receive a bolus directly ln • their mouth (chicks >5 days old). Often during direct feeds, 2 chicks grabbed the parent's bill at once. The chick who had the uppermost position, and who was observed chewing or swallowing after the bolus was regurgitated, was considered successful. Data Handling and Analysis Data for aIl discrete behaviours (e.g. copulation,

feeding) are reported as rates (f observedjhour) for each bird. Attendance is reported as the proportion of time spent at the nestjday (total number of minutes at the nestjnumber of minutes of observation). Pre-egglaying attendance refers to attendance until the laying of the first egg. Early incubation refers to attendance rates

calculdted from the laying of the first egg until 5 days later.

Attendance of nestlings declines after hatching (rs~-

0.82, P

( hatching. AlI parental attendance of chicks, except feedings, ceased by 21 days old. Non-continuous attendance was therefore calculated from 9 to 21 days after hatching. Feeding rates were calculated from 5 days after hatching onwards (direct feeds). Earlier feeding rates (nest feeds) were not considered reliable because parents often re-ate much of the food remaining in the nest and regurgitated it later, confounding estimates of parental feeding effort. Total feeding rates to chicks declined from 5 days to 40 days after hatching (rs=-O.55, P

Fledging success was recorded as the number of chicks surviving to 35 days old for nests which successfully hatched chicks (Fledging success 1~ see Chapter 2). Pairs who were observed only after hatching were excluded from estimates of hatching but not fledging success. The distribution of many of the variables deviatcd significantly from normality. standard transformations failed to consistently normalize variables. Therefore, non­ parametric statistics were used to analyze aIl data. AIl correlations are Spearman rank correlations unless otherwise stated. Non-parametric partial correlations were calculated by using ranked variables in a partial correlation, as suggested by Ward (1988). The probabilities from aIl statistics are two-tailed. l used the SYSTAT statistical software for aIl analyses (Wilkinson 1989).

RESULTS Matinq preferences Males had higher plumage scores than females in 79% of aIl pairs (N=170), equal scores with females in 20% of pairs, and lower scores than females in only 1% of pairs. Male plumage scores were significantly greater than female scores within pairs (Wilcoxon paired-sample test, z=8.096, P

Figure 1. Female plumage scores vs. male plumage scores for aIl breeding pairs of cattle egrets across all

sample periods. N=170, some points represent more than one pair. 12 • 1 1 • • 10 0) • • • L 0 9 u • • • (f) 8 0) • • • • • ~ 0 7 E • • • • • • ~ 6 - CL • • • • • 5 -0) • • • • • 0 E 4 • • • 0) LL 3 • • • • • • • 2 • • • • • • • • 1 • • • • • • • • • 0 1---1__ -1_____ L __ -1_ 0 1 2 3 4 5 6 7 8 9 10 11 12

Male plumage score 75

( egglaying was 0.09tO.06 copulations/hour (N=52 pairs). Copulation rates varied between sample periods (Table 1; Kruskal-Wallis Test, H=14.36, P0.3; 3 vs. 1, T=2.86, pO.20, N=45). Nest attendance Neither male nor female nest attendance varied significantly between sample periods, but the variation for males approached significance (Table 2; Kruskal-Wallis Tests; males, H=5.76, P=0.06, N=45; females, H=0.588, P=O.75, N=45). Males spent more time at the nest than females (Table 2; Mann-Whitney Test, U=1694, P

Sample N Mean SD Correlations period Male Pemale Plumes Plumes

Whole study 52 0.09 ±0.06 0.39*** 0.16ns period 1 11 0.18 ±0.08 0'91**** 0.23ns period 2 24 0.06 ±0.03 -0.03ns O.Olns period 3 17 0.07 ±0.02 0.37ns 0.19ns

ns P>O .10 T 0.05

Figure 2. Copulation rates vs. plumage scores for male

cattle egrets across aIl sample periods.

-;. -J.' 0.30

0.25 • ~ => 0 0.20 • ..c • ~ • • (f) c • 0 0.15 • -+-' 0 => 0... 0.10 • 0 • • • • u • • 1 • • 1 0.05 1 • • • • 1 • • • • • 0.00 • L ___ L ___ 1 _ _ L _1 0 2 3 4 5 6 7 8 9 10 1 1 12 Male plumage score 78

Table 2. Mean male and female attendance C# minutes at ( nestjtotal minutes of observation) durinq pre­ eqglaying and early incubation, for the whole study combined, ar.1 by sample periode N is sample size.

sample Male Pellale Attendanca Attendance

(pre-eqqlayinq)

Whole study 45 O.72±O.15 O.53±C.14 period 1 8 O.75±O.ll O.57±O.16 period 2 23 O.67±O.l5 O.5l±O.13 period 3 14 O.79±O.l3 O.53±O.l5

(early incubation)

Whole study 45 O.57±O.20 O.45±O.19 period 1 8 O.57±O.25 O.49±O.24 period 2 23 O.53±O.ll O.50±O.ll period 3 14 O.66±O.27 O.43±O.26 79

Table 3. Spearman rank correlations between male plumage scores, female plumage scores, and nest attendance. AlI pairs who reached egglaying are included, and aIl sample periods are combined. Probabilities are indicated by astericks (see legend below). a) PRE-EGGLAY:ING (N=45)

Male Pemale Male Femlrole plUlles plwae. attend. attend. Male plume. 1.00 p.male plumes 0.78**** 1.00 Male attend. 0.27T 0.41 ** 1.00 pellale attend. 0.18ns 0.06ns -0.50**** 1.00

b) BARLY :INCUBATION (N=45)

Male Pellaie Male Female plumes plumes attend. attend.

Kale--~--~------~------plumes 1.00 ------Fellaie plumes 0.78**** 1.00 Kale attend. 0.33* 1.00 Fellaie attend. -O.H;ns -0.21ns -0.89**** 1.00 ns P>O .10 T O. 05

Fiqure 3. Male nest attendance durinq pre-eqglaying vs. female (a) and male (b) plumaqe scores, across aIl sample periods. a) 0.910 f • • • • • Cl) • u • • 1 • c 0.8 i 1 • • Cl) "0 0.7 l' C • • • 1 Cl) 0.6 -+..J • • -+..J o 05 • -+..J • Cf) • Cl) 04 c •

Cl) 0.3 o • 2 0.2 0.1 0.0 '------"'---'-----'---1---L---'--_L-L -1.__ L __ l _ j 0 23456789101117

Female plumage score

b) 1 0 09 • 1 • 1 • Cl) • 1 u 08 • • • c 1 • Cl) 0.7 • • """0 • c 1 • • • 1 Cl) 0.6 • ...... • • -+..J • 0 05 • -+..J • • UJ Cl) 04 c • Cl) 0.3 - 0 • 2 0.2 0.1 0.0 --1-J__ J 0 2 3 4 5 6 7 8 9 10 11 1/

Male plumage score *' -'U> 81

arise through the fa ct that male and female plumes were positively correlated within pairs. When the effects of female plumage scores were controlled by partial correlation analysis, male attendance was no longer correlated with male

plumage (rp=-O.09, P>O.5J, N=45). However, when effects of male plumage were controlled, male attendance remained positively correlated with female plumage (rp=o.36, PO.10, N=43). When the effect of female plumage was controlled, male mate attendance was not correlated with male plumage scores (rp=o.15, P>O.5, N=43). The results suggest that male mate attendance is higher only when both the male and female have high plumage scores, but that female plumage has the stronger effect. Female nest attendance was not correlated with either female or male plumage scores during pre-eqglaying (Table

3) • ( 82

Behavioural co~relates with breedinq plumes durinq early incubation lleither male nor female incubation rates varied significantly between sample periods, but the variat~on for males approached significance (Table 2; Kruskal-Wallis Test; males, H=5.63, P=O.06, N=45; femaleo, H=2.81, P=O.246, N=45). Males spent more time incubating th an females during early incubation (Table 2; Mann-Whitne. Test, U=1294.5, P=O.02, N=90). Male incubation tended to be positively correlated with male plumage scores, and was significantly positively correlated with female plumage scores (Figure 4; Table 3). When the effect of female rlumage was controlled, male incubation was not correlated with male plumage scores

(rp~O.02, P>O.5, N=45). When the effect of male plu~age was controlled, male incubation was not correlated with female plumage scores (rp =O.20, P>O.l, N=45). The results suggest that male incubation is higher only when both the male and the female have high plumage scores, but that female plumage has the stronger effect. Female incubation was not correlated with either female or male plumage scores (Table 3).

Behavioural correlates with breedinq plumes durinq the nestlinq phase continuous attendance Neither male nor female nestling attendance during the 83

Fiqure 4. Male incubation vs. female (a) and male (b) plumage scores, across aIl sample periods. ,.. iJ a) 1.0 • • 0.9 • • 0.8 • c • • • 0 0.7 • -+-' • • • 0 0.6 • • • ..0 • • ::J u 0.5 • • c 1 • • 0.4 • Q) • 0 0.3 • :2 •• • • 0.2 0.1 L_L 0.0 -----L_. __ 0 2 3 .,.A 5 6 7 8 9 10 11 12

Femole plumage score

b) 1.0 • • • 0.9 • • 0.8 • c • • • 0 0.7 • • -+-' • 0 0.6 • • • ..0 • • ::J u 0.5 • • c 1• • • 0.4 • Q) • • 0 0.3 • :2 • • • 0.2

0.1 [ 0.0 1 1 0 2 3 4 5 6 7 8 9 10• 11 17 .. Mole plumage score oU 84

( continuous phase (chicks 0-8 days old) varied between sample periods (Table 4; Kruskal-Wallis Test; males, H=0.716, P=O.70, N=23; females, H=0.224, P=O.89, N=23). Males tended to attend nestlings more than females (Table 4; Mann-Whitney

U-test; U~349.0, P=0.06, N=46). Male and female nestling

attendance rates were negatively correlated (rs=-0.82, P<0.001, N=23). Neither male nor female nestling attendance was correlated with male or female plumage scores (Table 5) • . Non-continuous ne st attendance Neither male nor female nestling attendance during the non-continuous attendance phase (chicks 9-21 days old) differed significantly between sample periods (Table 4; Kruskal-Wallis Test; males, H=4.39, P=O.11, N=25; females, H=4.24, P=O.12, N=25). Nestling attendance did not differ between males and females (Table 4; Mann-Whitney Test; U=312.0, P>O.2, N=50). Male and female nestling attendance rates were positively correlated (rs=0.42, P0.5, t, 85

Table 4. Mean male and female nestling attendance during continuous (1-8 days old) and non-continuous (9- 21 days old) attendance for aIl pairs who fledged chicks; for the entire study combined and by sample periode

sample Mala .astliDCJ Female NastliDg period attendance atteDdanca continuous attendance Whole study 23 O.53±O.12 O.48±O.12 period 1 4 O.53±O.O4 O.47±O.O5 period 2 13 O.51±O.14 O.49±O.14 period 3 6 O.55±O.12 O.45±O.13

Non-continuous attendance Whole study 25 O.30±O.15 O.31±O.16 period 1 4 O.41±O.09 O.44±O.20 period 2 15 O.25±O.16 O.25±O.15 period 3 6 O.36±O.lO O.34±O.07 86 Table 5. Spearman rank correlations between male plumage ( scores, female plumage scores, and nest1ing attendance durinq the continuous phase Ca), and the non-continuous phase (b). AlI pairs who fledqed chicks are inc1uded, and aIl samp1e periods are combined. Probabilities are indicated by astericks (see leqend below).

a) CONTlNUOOS NBS'l'LING A'l'TBNDAlfCB (N=23).

Kale l'_ale Male l'emale pluaa. plUII•• attend. attend. Kale plume. 1.00 Female plume. 0.67**** 1.00 Kal. attend. 0.07ns -o.Oons 1.00 l'ellale attend. o.OOns O.lSns -0.91**** 1.00

b) NON-CONTINOOUS HESTLINa AT'l'ENDANCE (N:25)

Kal. Pemale Male l'emale plumes plUlle. attend. attend. Male plumes 1.00 Fellale plume. 0.64**** 1.00 Male attend. 0.40* 1.00 l'allale attend. 0.3ST 0.42* 1.00

ns P>O.lO T 0.05

)

Figure 5. Ma~~ nestlinq attendance durinq the non- continuous attendance phase vs. female plumage score, across aIl sample periods.

• (

0.8 -

0.7 (J) () c (J) 0.6 u c • • Q) • ..j--' 0.5 +-' 0 CJl 0.4 c • • -1--' • • (J) 0_3 • • (J) • • c • • • -(J) 0.2 0 2 • 0.1 • • 0.0 • • 0 1 2 3 4 5 6 7 8 9 10 1 1 12 Female plumage score

( 88

.." N=25). When the effect of female plumes on female attendance was controlled, female attendance was no longer correlated with male plumage scores (rp=o.24, P>O.20, N=25). The results suggest that female attendance is higher only when both the female and male have high plumage scores, but that male plumage has the stronger effect. Feeding rates Male feeding rates (#boluses/hour/chick) varied between sample periods (Tablf'. 6; Kruskal-Wallis Test; H=8. 28, P=O.02, N=36): the variation resulting from a difference

between periods 1 and 2 (Dunn's multi~le comparison: 1 vs. 2, T=2.87, PO.5; 3 vs. l, T=1.88, P>O.3). Female feeding rates did not vary between sample periods (Kruskal-Wallis Test: H=3.17, P=O.20, N=36). Male feeding rates tended to be higher for males with higher plumage scores (Figur.e 7; Table 7). When effects of sampling period were controlled, the correlation between

male feeding rates and male plumage score becarue ~ignificant

(rp=O.46, PO.5, N=23). The proportion of boluses delivered by males did not vary significantlY between sample periods (Table 6; Kruskal­ Wallis Test, H=4.94, P=O.09, N=35: but note the P-value). The proportion of boluses delivered by males was '.41· 89

Fiqure 6. Female nestling attendance during the non­ continuous attendance phase vs. female Ca) and

male Cb) plum~ge scores, across aIl sample periods. ~ 0) o!lIl' 08

0.7 'TI c Q) 0.6 -+-' • -+-' 0 0.5 01 • • c 1 • -+-' 0.4 • Cf) Q) • c 1 0.3 • • Q) - • 0 • E 0.2 • • • Q) LL • 0.1 • • 0.0 • -...L---L----L __ J _ _ j a 2 3 4 5 6 7 8 9 10 11 17

Female plumage score

b) 0.8

0.7 'TI c Q) 0.6 +-' • +-' 0 0.5 01 • • C 1 • +-' 0.4 • Cf) Q) • c 1 0.3 • • Q) • 0 1 E 0.2 • • • Q) IL 0.1 • • • • 0.0 • L __ L_J 0 2 3 4 5 6 7 G 9 la 11 1/1

Mole plumage score ~

~ 90 ( Table 6. Male and female feeding rates for the whole study combined and separately for each sample periode N is the sample size.

sample N Male Female perioeS Rata Rate

(Rate per chick - ,boluaea/chick/hour)

Whole study 35 0.32±0.21 0.32±O.18 period l 4 0.54±0.12 O.38±O.09 period 2 20 0.23±0.10 O.31±O.18 period 3 Il 0.40±0.30 O.31±O.20

(Proportion of boluses - 'bolusa. by parent/total 'bolusas)

Whole study 35 0.50±0.19 O.50±O.19 period 1 4 0.58±0.10 O.42±O.10 period 2 20 0.45±0.19 O.55±O.19 period 3 Il 0.55±0.19 O.45±O.19

( 91

Figure 7. Male feeding rate (boluses/hour/chick) vs. male plumage scores across aIl sample periods. 1.0 0.9

...Y- u 0.8 • ...c u 0.7 • "-... (J) t-' 0 0.6 • l- D) 0.5 - c • • -n • • Q) 0.4 • • (J) • • -t - • O.j • • (J) -- - • • 1 0 0.2 • • 3- 1 • • • o 1 • • • 00 0 2 3 4 5 6 7 8 9 10 1 1 12 Male plumage score

( r 92 Table 7. Spearman rank correlations between male plumage scores, female plumage scores and feeding rates per chick Ca), and the proportion of boluses delivered to the ne st by the males (b). Pairs who fledged chioks are included in the feeding rates. AIl pairs whose chiok survived to direc~ feeds are included in the calculations of proportional feeds. AlI sample periods are combined.

a) FEEDING RATES PER CUICK Cf DOLUSES/HOUR/CHICK) CN=23)

Male Female Male Female plumes plumes Feedinq Feeding

Male plumes 1. 00

Female plumes 0.62*** 1.00

Male Feedinq 0.36T 0.24ns 1.00

Female Feedinq 0.09ns 0.2Sns 0.32ns 1.00

b) PROPORTION OF BOLUSES DELlVERED BY MALES (N=35)

Male l'emale Male Female plumes plumes feedinq feeding

Male plumes 1. 00

l'emale plumes 0.64**** 1.00

Male feedinq 0.39* 0.33T 1.00

ns P>O.lO T 0.05

significantly correlated with male plumage scores, and tended to be higher with females of higher plumage scores (Figure 8; Table 7). When the effect of female plumes was controllcd, the correlation between proportion of boluses delivered and male plumes was not significant (rp=o. 22, P>O.20, N=35). When the effect of male plumes on proportion of boluses delivered was controlled, the correlation between proportion delivered and female plumes was not significant

(rp=O.15, P>0.5, N=35). The results suqgest that the proportion of boluses delivered by males is highE.:i..- only when both the male and female have hiqh plumage scores, but that male plumage has the stronqer effect. Female feeding rates were not correlated with either female or male plumaqe scores (Table 7). Correlate. bat.een plume. and br.edinq suceess Hatchinq success Hatching success of pairs ranged between 0 to 3 chicks and did not vary between sample periods (Chapter 2). The limited range in hatching success made it impractical to conduct correlation analyses between hatching success and plumage scores. However, neither mean male nor female plumage scores differed between pairs who hatched 0, 1, 2 or 3 chicks (Table 8) • Fledqinq success Fledging success varied between sample periods, due to siqnificantly lower fledginq success in period 3 (Chapter

" 94 )

Fiqure 8. Proportion of boluses delivered by males during the nestling period vs. male (a) and female (b) plumage score, across aIl sample periods. (

a) 1 CI) Cl) • • CI) 0.8 ~ • • • • .8 0.6 • • • • • • i • • • • E 0.4 • • 1 c • • • .2 • • ~ 0.2 8. • 0 0 1 1 1 1 1 1 •1 1 1 1 1 1 1 1 1 1 1 1 1 Q: 0 2 4 6 8 10 12

Male plumage score

b) ~ CI) • • .=z 0)- .8 • • • • • • ~ 0.6 • • • «S • • • E ; • • • • c: 0.4 • • • • 1 ~ 0.2 • 8. • ~ • °0 2 4 6 8 10 12 Female plumage score

( ','

95

2). For period l, n.san male plumage score was higher (X=11.4±0.89, N=5) for pairs who fledged chicks than those who did not (5. 5±2 .12, N=2); but the dl fference was not

statisti~ally significant at the small sample size (Mann­ Whitney Test, U=O.O, P=0.12, N=7). In period 2, male plumage scores were significantly higher for pairs who successfully fledged chicks (1=4.55±3.57, N=3l) than those

who did not (~=2.75±2.56, N=12) (Mann-Whitney Test, U=115.0, P<0.05, N=43). In period 3, male plumage scores did not differ between those who fledged chicks (i=lO.0±2.77, N=7)

and those who did not (~=11.0±1.22, N=9) (Mann-Whitney Test: U=31.5, P=l.O, N=16). Period 3 was characterized by high chick mortality resulting from high rates of conspecific interference (Chapter 2). If period 3 is excluded from the analysis, male plumage scores were significantly higher for pairs who successfully fledged chicks (i=5.5±4.11, N=36) than those who did not (x=2.85±2.48, N=14) (Mann-Whitney Test; U=133.0, P

U=O.OO, P=0.12, N=6; period 2, U=167.0, P=0.50, N=43~ period 3, U=34.0, P=0.77, N=16) 96 Table 8. Mean male plumage score, mean female plumage ( score and mean total plumage score for (a) the number of chicks hatched by a pair and (b) the number of chicks fledged by a pair. AlI sample periods are combined. The results of Kruskal­ Wallis tests to detect differences between categories are present.ed.

a)

__~ ______~~~ er .atched 012 3 Kale plwaes 7.l7±4.28 5.l8±4.05 6.14±4.60 10. 5±2 .12

Sample size 12 Il 35 2 Kruskal-Wallis statistic=2.64, p=0.45 Fa.ala plumes 4.92±4.14 2.90±2.42 3.69±3.44 5. 50±2 .12

Sample size 12 Il 35 2 Kruskal-Wallis statistic=2.02, p=0.57 Total plumes l2.08±7.84 8.09±5.52 9.83±7.58 l6.0±0.00 Sample size 12 Il 35 2 Kruskal-Wallis statistic=2.57, p=0.46

b)

I!!lml2.z: lJ:e~g.4 0 1 2 Xale plumes 6.l8±4.57 6.50±4.02 5. 62±4. 82 Sample size 22 3(, 13 Kruskal Wallis statistic=l. 03, ns, p=0.60 remale plumes 3.96±3.55 3 .10±2. 75 2.08±1.89 Sample size 22 30 13 Kruskal Wallis statistic=2 .25, ns, p=O.33 Total plwaes 10.l4±7.79 9. 60±5. 90 7.69±6.22 Sample size 22 30 13 Kruskal Wallis statistic=l. 22, ns, p=O.54 r 97

• DISCUSSION To understand the opera'cion of sexual selecti on through mating preferences, it is necessary to identify which traits are preferred and why they are preferred. Breeding plumes in cattle egrets are used primarily in courtship and pair displays and are molted after the breeding season. The length of breeding plumes was positively correlated in breeding pairs. This may imply preference by both sexes for mates with longer plumes. However, females were more selective. Long plumed females mated exclusively with long plumed males: long plumed males mated with females varying considerably in plume length. Intrasexual competition for mates can also produce assortative mating patterns (Burley 1983). However, aggressive interactions between males and between females were rare in the present study, and males were never observEd to be displaced from display terri tories. Even in monogamous cattle egrets, female preference for long plumed males may increase male reproductive success, and hence be the mechanism for the evolution of the se traits. In Barbados, only a portion of the egrets in a colony breed in any breeding season. Evidence suggests that a number of birds that are ready to breed can not find mates, and must delay breeding and/or move between colonies (Chapter 2). Thus preferred birds may have a higher probability of obtaining mates. Preferred birds may aiso 98

breed earlier in any season, and plumage scores of breeding birds did vary throughout the year (Chapter 2). If food near colonies is limiting (Chapter 1), breeding earlier may increase parental access to food and hence fledging success. Moreover, breeding early may increase the chance of producing 2 broods in a season (e.g. Moller i988: 1990b for swallows), and increase the probability of re-mating if the first breeding attempt is unsuccessful. Finally, preferred males may have higher reproductive success because their

mates provide more parental care (Burley 1986). However, neither female ne st attendance, incubation, nestling attendance nor feeding rate of chicks was significantly correlated with male plumage in this study (but note a weak tendency for female nestling attendance to be correlated with male plumes). Being preferred by females may increase the reproductive success of males in several ways, but why has female preference evol ved? Copulation rate increased wi th male plumage score in this study, but is unlikely to be a basis for female preference. Clutch size is small and sperm availability is unlikely to be limitinq. This is supported by the observation that hatching success was not correlated with male plumage in this study. If plumage is an index of male condition, weIl plumed males may copulate more frequently simply because they are in better condition. Al ternatively, the correlation between male plumage and (.. 99 copulation frequency may be female-driven. Females with poorly plumed males may limi t intra-pair copulations, thereby increasinq the competitiveness of sperm from extra­ pair copulations. Extra-pair copulations are common in eqrets, weIl plumed males havlnq greater success in extra­ pair copulations than poorly plumed males (Chapter 4) . Wejther male nest attendance in the pre-egqlaying phase of the breeding cycle, male incubation, nor male nestling attendance were correlated with male plumage, indicating that these behaviours are not the basis of female preference for weIl plumed males. However, weIl plumed males fed chicks at a higher rate than poorly plumed male. Moreover, perhaps because of this, weIl plumed males fledqed more chicks than poorly plumed males at times of year when the major cause of chick mortality was starvation. Higher provisioninq rates and higher fledging success of weIl plumed males may therefore be the basis of the female preference for these males. These resul ts suggest that female preference and the evolution of male plumage in cattle egrets is not based solely on the production of attractive sons (the Fisherian hypothesis). Females preferring weIl plumed males may produce more attractive sons, but females are also obtaining the immediate benefit of better paternal care and higher fledg inq success. 100

It is possible that, as suggested by Hoelzer (1989: the good parent process), the immediate benefit of better paternal care is the sole basis of the female preference, the preference for non-heritable parental quality leading to the evolution of male plumage. The resul ts of this study are consistent wi th the good parent hypothesis for the existence of female preference and the evolution of male plumage in cattle egrets. Norris (1990b) also found that a preferred trait male stripe size, in great tits, was correlated with male feeding rates and nest attendance. The issue of whether the present results are also consistent with mate choice for good genes is more complexe In its simplest form, this hypothesis predicts a correlation between male plumage and offspring viability: and fledging success was higher for weIl plumed males in this study. The hypothesis does not predict the observed correlation between plumage and paternal care, since the higher fledging success is predicted to be the consequence of better genes and not greater paternal care. However, fathers with good genes may be in better condition (Andersson 1986). Good condition may correlate with characters such as longer plumes, brighter coloration (Woolfenden et al. 1976 for cattle egrets: Millinski and Bakker 1988 and Hill 1991 for other species) and more parental care. Moreover, the basis for the female preference may be good genes for parental care. In this

{ '. 101

.~ case, the observed correlation between plumage and parental care is speeifically predicted by the good genes hypothesis. Age may indicate genetic quality, sinee oIder males have demonstrated their viability. Reproductive effort, and hence parental care, should also increase with age (Pianka and Parker 1975) and the quality of the care may improve with parental experience (Ollason and Dunnet 1988). FemaIes preferring older males may therefore obtain both better parental care and better genes for viability. Interestingly, the size of sexually selected traits increases with age in several speeies (eg. deer antlers,

Clutton-Brock et al. 1982; peaeoek tails, Manning 1987: in

press; pheasant spurs, Schanz et al. 1990). Maddock (1989) found that first-year breeders had poorer plume development than second year breeders in the Australian eattle egret (Bubulcus ibis coromandus), suggesting that plume development may increase with age in egrets. In addition,

long-term studies of ospreys (Poole 1984): kittiwakes

(Thomas and Coulson 1988) and fulmars (Ollason and Dunnet

1988) have shown that breeding success increases with age. It is evident from the above that the patterns of female mate ehoice described in this ehapter can be explained exelusively on the basis of preference for non­ genetie benefits. It is not possible to deterrnine whether female preference for good genes is also a component of mate choice and sexual selection in egrets. An investigation of 102

( female preference for males in extra-pair copulations, when males supply only genes and no paternal care, may allaw further clarification (Chapter 4). The positive correlation between plumage scores in breeding pairs, and the fa ct that the new mates of re­ pairing males have better plumes than the original mates, may suggest male preference for well plumed females. However, males of a given plumage score mate with females who vary considerably in plumage. The weak male mating preference is consistent with the observation that female plumage did not predict female behaviour. Female plumage was not correlated with copulation rates, female nest attendance, incubation, nestling attendance or feeding rate to chicks, and was not carrelated with ber subsequent fledging success. Although male preference for better plumed females is weak at pairing, aspects of their post-pairing behaviour suggest that males may value well plumed females more highly. Male nest attendance during pre-egglaying and nestling attendance were positively correlated with female plumage. Male nest attendance may reduce the risks of his mate's participating in extra-pair copulations, and of nesting failure througb conspecific theft of nest material. Nestling attendance could reduce nestlinq mortality during periods when mortality from conspecific interference is ( high. Since maternaI care is not correlated with female 103 plumage, males do not obtain greater immediate benefits from weIl plumed females. The suggestion that well plumed females are valued more highly by males may therefore indicate that plumes signal genetic quality. Males may be prepared to incur a greater cost to a given act of reproduction if the genetic quality of the offspring being produced is expected to be high. Interestingly, although females preferred weIl plumed males at pairing, their post­ pairing behaviour was not affected by male plumage. Neither female ne st attendance, incubation, nestling attendance nor feeding rate to chicks were correlated with their mates plumage score. Females may not be increasing their maternal care in response to their matels plumage simply because their chicks are already receiving increased parental care from the weIl plumed mates (Burley 1988). The role of plumage in mate choice and the behavioural correlates of plumage differ between the sexes in cattle egrets. Females show a strong preference at pairing for weIl plumed males; males show little preference at pairing for well plumed females, but appear to value them more highly after pairing. Given that egrets are biparental, and that both sexes invest heavily in nest building, incubation, and rearing of young, it is nct immediately evident why females are the choosier sex (Trivers 1972). If egg production makes reproduction more costly for females, and the rate at which females can re-mate following reproduction 104

( is slower than males, the availability of females may be limiting for male reproductive success. The observation that many 'ready to breed' birds can not find mates (Chapter 1; 2) supports the assertion that mates may be limiting. Most of these birds in search of mates may be males. Given this scenario, females should show preference for higher quality mates; but males may have to mate whenever opportunities are available, perhaps independently of mate quality. If female mate choice is more stringent than male, selection to accurately signal mate quality will be stronger on males than females. This may explain why it is males rather than females that exhibit the correlation between plumage and parental care. Once the correlation predictably exists for males, females in breeding pairs can modify their maternaI care opportunistically. The more stringent female mate choice for plumes will also create greater variance in male than female mating success, and intensify selection for plumage in males. This is consistent with the observation that plumes in egrets are better developed in male than females. However, the sexual dimorphism in egret plumes is slight compared ta many passerines. since male egrets show little plumage based preference for females at pairing, and female plumes are not correlated with maternaI care, why are female plumes weIl developed? ( One possibility is that female plumes are a vestigial trait 105 maintained by selection on males, i.e. they are the partial expression of genes for male characteristics carried by the female (Huma and Weatherhead 1989). However, the strong development of female plumage in cattle egrets suggests that direct selection on female plumage has also occurred. A possible mechanism is that there is weak male preference for long plumed females at pairing, creating some variance in female reproductive success based on plumage. A more likely mechanism is based on the post-pairing behaviour of males observed in this study. Although the proposed limited availability of females may make males non-selective at pairing, their post-pairing behaviour suggests that the y value better plumed females more highly. Specifically, male ne st and nestling attendance increased with increasing female plumage. A possible consequence of this behaviour is increased variance in female reprOductive success based on plumage (i.e. direct selection _or female plumes). l did not detect a correlation between female plumage and fledging success in this study. However, plumage-based variance in female reproductive success, generated by variation in male nest and nestling attendance, May emerge only when social and ecological conditions favour strong conspecific interference with nests and nestlings. The possibility that plumage-based variation in reproductive success may change seasonally should be considered in future studies of mate chaice and sexual selection. 106 LITERATURE CITED ( Andersson, M. 1982. Female choice selects for extreme tail length in a widowbird. Nature 299:818-820. Andersson, M. 1986. Evolution of condition-dependent sex­ ornaments and mating preferences: sexual selection based on viability differences. Evolution 40:804-816. Blaker, D. 1969. Behaviour of the cattle egret Ardeola ibis. ostrich 40:75-129. Burley, N. 1983. The meaning of assortative mating. Ethol. and Sociobiol. 4:191-203. Burley, N. 1986. Sexual selection for aesthetic traits in species with biparental care. Am. Nat. 127:415-445. Burley, N. 1988. The differentiaI-allocation hypothesis: an experimental test. Am. Nat. 132:611-628. Clutton-Brock, T. H., S. D. Albon, and F. E. Guiness. 1982. Red Deer: Behaviour and ecology of two sexes. University of Chicago Press. Chicago. Darwin, C. 1871. The Descent of Man and Selection in Relation to Sex. Murray. London. Fisher, R. C. 1958. The Genetical Theory of Natural Selection. 2nd ed. Dover. New York. Hamilton, W. D. and M. Zuk. 1982. Heritable true fitness and bright birds: a role for parasites? Science 218:384-387. Hill, G. E. 1991. Plumage coloration is a sexually selected indicator of male quality. Nature 350:337-339. Hoelzer, G. A. 1989. The good parent process of sexual selection. Anim. Behav. 38:1067-1078

Kirkpatrick, q. 1982. Sexual selection and the evolution of fem,_~ choice. Evolution 36:1-12. KOdric-Brown, A. and J. Brown. 1984. Truth in advertising: the kind of traits favoured by sexual selection. Am. Nat. 124:309-323. Lande, R. 1981. Models of speciation by sexual selection on polygenic traits. Proc. Nat. Acad. Sei. USA 78:3721- 3725. ( 107 Maddock, M. 1989. Colour and first age of breeding in cattle egrets as determined from wing-tagged birds. Corella 13:1-8. Manning, J. T. 1985. Choosy females and correlates of male age. J. Theor. Biol. 116:349-354. Manning, J. T. 1987. The peacock's train and the age­ dependency model of female choice. J. World Pheasant Assoc. 12:44-56. Manning, J. T. in press. Age advertisement and the evolution of the peacock's train. J. Evol. Biol. Maynard Smith, J. 1985. Mini review: sexual selection, handicape and true fitness. J. Theor. Biol. 57:239- 242. Millinski, M. and T. Bakker. 1988. Female sticklebacks use male coloration in mate choice and hence avoid parasitized males. Nature 334:330-333. Moller, A. P. 1988. Female choice selects for male sexual tail ornaments in the monogamous swallow. Nature 332:640-642. MaIler, A. P. '1990a. Parasites and sexual selection: current status of the Hamilton and Zuk hypothesis. J. Evol. Biol. 3:319-328. MaIler, A. P. 1990b. Male tail length and female mate choice in the monogamous swallow (Hirundo rustica). Anim. Behav. 39:458-465. Mottro, U. 1982. The courtship handicap - phenotypic effect. J. Theor. Biol. 97:319-324. Muma, K. E. and P. J. Weatherhead. 1989. Male traits expressed in females: direct or indirect sexual selection. Behav. Ecol. Sociobiol. 25:23-31. Norris, K. J. 1990a. Female choice and the evolution of the conspicuous plumage colouration of monogamous male great tits. Behav. Ecol. Sociobiol. 26:129-138. Norris, K. J. 1990b. Female choice and the quality of parental care in the great tit Parus major. Behav. Ecol. Sociobiol. 27:275-281. l 108 Ollason, J. C. and G. M. Dunnet. 1988. Variation in ( breedinq success in fu1mars. In: Reproductive Success: Studies of Individua1 Variation in Contrastinq Breedinq Systems (T. H. Clutton-Brock, ed.). pp. 263-278. University of Chicago Press. Chicago. petrie, M., T. Ha11iday and C. Sanders. 1991. Peahens prefer peacocks with e1aborate trains. Anim. Behav. 41:323-331. Pianka, E. R. and W. S. Parker. 1975. Age-specifie reproductive tactics. Am. Nat. 109:453-464. pomiankowski, A. 1988. The evolution of fema1e mate preferences for male geneti~ quality. Oxford Surveys evo1. Bio10qy. 5:136-184 Poole, A. F. 1989. Ospreys: a Natural and Unnatura1 History. Cambridqe University Press. Cambridge, U.K. Schanz, T. von, G. Goransson, G. Andersson, I. Froberg, M. Grahn, A. He1qee, and H. Wittzell. 1989. Fema1e choice selects for a viability-based male trait in pheasants. Nature 337:166-169. Thomas, C. S. and J. C. Coulson. 1988. Reproductive success of kittiwake gu11s, Rissa tridactyla. In: Reproductive Success: Studies of Individua1 Variation in contrastinq Breeding systems (T. H. C1utton-Brock, ed.). pp. 251-262. University of Chicago Press. Chicago. Trivers, R. L. 1972. Parental investment and sexual selection. In: Sexual Selection and the Descent of Man 1871-1971 (B. Campbell, ed.). pp. 136-179. Aldine. Chicaqo.

Ward, P. I~ 1988. Sexual dichromatism and parasitism in British and Irish freshwater fish. Anim. Behav. 36:1210-1215. Wilkinson, L. 1988. SYSTAT: the system for statistics for the PC. SYSTAT Inc. Evanston, Illinois. Woolfenden, G. E., S. C. White, R. L. Mumme and W. B. Robartson Jr. 1976. Aggression among starving cattle eqrets. Bird Bandinq 47:48-53. Zahavi, A. 1975. Mate selection - a selection for a handicap. J. Theor. Biol. 53:205-214. ( 109

CBAPTER 4. BreediDg plumes an4 extra-pair copulations in cattle agrata

ABSTRACT

This chapter examines the effects of breeding plumes on the distribution of extra-pair copulations (EPCs) in the cattle egret (Bubulcus ibis). Seventeen percent of aIl observed copulations in the colony were extra-pair. Males who attempted extra-pair copulations had higher plumage scores than the mean for the male population. Females accepted EPCs only from males whose plumage scores were greater than or equal to their mates. Males sought EPCs opportunistically; the plumage scores of females with whom males attempted EPes did not differ from the mean for the female population. Males whose mates had accepted EPes tended to attend nestlings less, may have fed nestlings less, and tended to have lower fledging success than typical for the male population. This suggests that males reduce parental care in response to EPCs. Despite this, the fact that females selectively cooperate in EPCs suggests a net benefit, providing the EPC is with a weIl plumed male. WeIl plumed males provide more parental care ta their nest mates, but females receive no parental care from EPC partners. This may suggest that plumes signal high genetic quality, and that females accept EPCs ta obtain high quality .... offspring. Plumes in cattle egrets may signal bath 110

( potential parental care and genetic viability of a mate by indicating the mate's age.

INTRODUCTION Bright plumage and ornaments are common among monogamous bird species. Darwin (1871) believed that these characters were sexually selected, evolving through mating preferences. It is now known that several factors can create the variance in mating success necessary for the operation of sexual selection in monogamous species. These include sex ratio biases at the time of breeding (Birkhead et al. 1985: Johnson 1988a; 1988b), differences in mate

quality (O'Donald 1983; Burl~y 1986: Moller 1988b) and/or extra-pair copulations (Gladstone 1979; Moller 1988b). Moreover, mating preferences by both sexes have now been demonstrated in many monogamous species (O'Donald 1983: Burley 1986: Johnson 1988a; Moller 1988b; Norris 1990: Hill 1991). Determining the functional basis of mate preference for elaborate traits has proved more difficult. According to the Fisherian hypothesis, sexually selected characters are arbitrary traits which are preferred by females because of the mating advantages accuring to their male offspring (Fisher 1958; Lande 1981; Kirkpatrick 1982). Others have argued that secondary sexual characteristics have evolved ( b~cause they indicate the genotypic quality of potential 111 mates (the good genes hypothesis; Zahavi 1975; Hamilton and Zuk 1982; Kodric-Brown and Brown 1984; Andersson 1986). The traits may indicate overall genetic viability (Zahavi 1975; Kodric-Brown and Brown 1984; Andersson 1986), genetic resistence to parasites (Hamilton and Zuk 1982), or rnay signal age and hence genetic viability and the quality of parental care (Manning 1985; 1987), the latter because both reproductive effort and experience should increase with age. Finally, Hoelzer (1989) has suggested that mate preference based on non-heritable variation in parertal care can lead to the evolution of traits that accurately indicate the current capacity of the mate for parental care (the good parent hypothesis). The cattle egret (Bubulcus ibis) is a monogamous colonial heron in which both sexes develop distinctive reddish-brown filamentous plumes on the head, back and chest during the breeding season (Chapter 2). Cattle egrets paired assortatively with respect to the development of breeding plumes, primarily because of female preference for weIl plumed males at pairing (Chapter 3). Females who paired with weIl plumed males benefited directly by increased levels of male parental care, and by higher fledging success at times of year when starvation was a major cause of chick mortality. This suggests that female preference and the evolution of male plumage is not based solely on the production of attractive sons. However, the 112

( results of Chapter 3 did not allow determination of whether male plumes indicate only non-heritable capability for parental care, or whether they indicate high overall genetic viability leading to good condition and hence increased paternal care. Female plumes were not correlated with paternal care, but post-pairing behaviour suggested that males may value weIl plumed females more highly (Chapter 3). This is weak evidence indicating that plumes may signal genetic quality. A more robust way or determining whether plumes signal a genetic or phenotypic benefit is by examining the distribution of extra-pair copulations (EPCs) in the population. EPCs are common in cattle egrets and occur freuquently in the absence of the male (Fujioka 1981). Since a female only obtains genes from an EPC, female preference for weIl plumed males in EPCs would suggest that plumes signal genetic quality (see Westneat et al. 1990). The primary purpose of this chapter is to investigate the effects of breeding plumes on the distribution of extra-pair copulations in cattle egrets.

METRODS Observations in a breeding colony of cattle egrets were made between 21 May and 4 June 1990 in Barbados, West Indies. The colony is situated in the north of the island in bearded fig trees (Ficus citrifolia) emerging from a 113 gully. Observers, 10 m to 30 m away from the colony, were able to monitor the behaviour of breeding pairs using binoculars. The development of breeding plumage in cattle egrets varies between individuals and seasonally (Chapter 2). Both sexes have breeding plumes, although male plumes are typically more developed. within most pairs, the male's plumes are longer than the female's (Chapter 3). Plumes were assessed visually by scoring the head, chest and back plumes on an individual (see Chapter 2 for details). Each individual was then assigned a total plumage score obtained by summing scores from plumes on aIl three body areas. Plumage scores of focal birds (16 pairs) ranged from 4 to 12 during the period sampled for EPCs. Plumage scores of aIl breeding pairs in the colony were also monitored over the 2 week observation periode The 16 focal pairs were observed from pairing through early incubation (i.e. over the female's fertile periodi 7- 13 days of observation). Pairs were observed from sunrise until 12 noon each day, and from noon until dusk on alternate days (973.0 nest-hours). Watches consisted of noting the presence or absence at the nest of each member of a focal pair each minute, as weIl as continuous scanning for copulations, aggressive interactions and nestbuilding activities. Individuals were easily recognizable by their plumage characteristics and the locations of their nests. 114

The sex of each member of a pair was confirmed by copulation position. To obtain a population estimate of copulation rate in the colony, aIl other pairs in view were scanned opportunistically for copulations. During watches, aIl intra-pair and extra-pair copulations and the plumage and/or identity of the male and female involved were noted. AlI copulations observed occurred at or within 1 metre of the ne st site. Copulations were termed "successful" if the female was observed to lift her tail, and/or lateral wipes were performed by the male. Unsuccessful copulations fell into ona of two categories. If the female struggled, vocalized and attempted to displace the male, copulations were termed "resisted" copulations. Males did not appear to be able to achieve lateral wipes and cloacal contact was not made in resisted copulations. These copulations were therefore likely to be unsuccessful in transferring sperme If the male fell off the back of the female, or if the female was unable to remain on the branch, copulations were termed "unsuccessful". Within pairs, only successful and unsuccessful copulations were observed, i.e. females were never seen to resist a copulation attempt by their mate. AlI three types of copulations were observed during EPCs. Intra-pair copulations usually occurred at the return to the ne st of one member of the pair. Typically, a pair would greet, 'backbite' (Blaker 1969), and the returning 115

bird would preen or nestbuild for several minutes before a copulation occurred. Within pairs, females occasianal1y solicited males, but often the male climbed on the back of the female with no solicitation and began treading. After intra-pair copulations, the male usually remained in the area, preening near the nest while the female engaged in nestbuilding activity. Extra-pair copulations were very distinctive. Typicelly a male attempting an EPC would swoop onta the back of a female lying on her nest and immediately begin treading. The female would look at the male and either begin vocalizing loudly and struggling, or simply lift her tail and allow the male to copulate. After comp1eting an EPC or EPC attempt, the male would immediately return ta his

,i own nest area. In this study, aIl EPC attempts occurred in i \ .' the absence of the female's mate. AlI EPCs observed were at the female's nest site. Most females involved in EPCs were mounted by several males over several days. AlI EPCs were attempted weIl within the female's fertile period (range of EPCs: 3 days before the first egg ta l day after), except for 2 which occurred during incubation and were excluded from analysis. As in Chapter 3, nestling attendance was measured as the proportion of observation time that a parent was at the nest, feeding rates as the number of boluses delivered per 116

chick per hour, and fledging success as the number of chicks alive 35 days after hatching. Because plumage scores were not norrnally distributed and sample sizes were low, nonparametric statistics were used in aIl analyses. Statistical tests are two-tailed unless otherwise specified. Means and standard deviations are reported. AlI statistics were performed using SYSTAT (Wilkinson, 1988).

RESULTS Approximatp.ly 17% (26/157) of aIl observed copulations were extra-pair, involving a total of 10 different females. Whether calculated from copulations observed on focal pairs or those sighted opportunistically in the colony, the proportion of copulations that were EPCs was similar (focal=16%, 16/103: oppportunistic=18%, 10/54). Males with plumage scores of <10 did not attempt EPes (Figure 1). Males who attempted EPCs had siqnificantly higher plumage scores than the population Mean (males attempting EPes, x=11.5+0.7 N=22; whole male population x=10.5+1.7, N=41: Mann-Whitney Test, U=587.5, P

Figure 1. The distributions of plumage scores for the male population (N=41), for males who successfully attempted EPCs (N=13), and for males who unsuccessfully attempted EPCs (N=9).

) (

80 • Whole male population

CI Males with unsuccessful EPCs

60 121 Males with successful EPCs -'#. -en Q) (ij E 0 40 -~ 0 c: Q) ::J 0" Q).... LL 20

o+------~--~~--~ 3 4 5 6 7 8 9 10 11 12

Plumage score

( 118

Test, U=235.5, P=O.18). Males with low plumage scores may not attempt EPCs because of the low probability of success. Females accepted EPCs only from males whose plumage scores were greater than or equal to those of their mates (Wilcoxon paired-sample test, successful EPC males vs. mates: T=3, p

Figure 2. The distributions of plumage scores for the male population (N=43) and for males whose mates accepted EPCs (N=8). 50 • Whole male population III Males whose mates accepted EPCs 40 -';I!. -CI) 0) as 30 E -0 u~ c: 0) 20 ~ C- O) u.~ 10

0-+------3 4 5 6 7 8 9 10 11 12

Plumage score

- 120

Figure 3. The distributions of plumage scores for the female population (N=43) and for females with whom males attempted EPes (N=9). 30 • Who le female population

[J Females with whom males attempted EPCs

~ -0 20 -(1) caQ) E Q) -0 ~ c: Q) 10 ::l CT Q) ~ LL

o 3 4 5 6 7 8 9 10 11 12

Plumage score 121

( typical for the population (Table 1; Mann-Whitney Test, U=15.0, P=O.80). Males who attempt EPCs may attend their mates less (x=64.6%) than typical for the male population, but the sample size did not permit statistical analysis. Males whose mates had accepted EPCs tended to attend nestlings less than the mean for the male population (Table li Mann-Whitney test, U=37.0, P=0.12). They may also feed their chicks less often, but the difference did not approach statistical significance at the small sample size (Table 1; Mann-Whitney test U=27.0, p=0.72). In nests where females had accepted EPCs, the me an number of chicks fledged tended to be lower than the mean for the population (Table 1; Mann­ whitney Test, U=36.5, P=0.08). In contrast to males whose mates accepted EPCs, males who engaged in EPCs fledged more chicks from their nests (x=1.0) than the mean for the population, but the sample size did not permit statistical analysis.

DISCUSSION Extra-pair copulations in cattle egrets were not randomly distributed in the population. WeIl plumed males were more likely to attempt EPCs than poorly plumed males,

and females accepted EPCs onl~ from males with equal or better plumes than those of their mates. Male distribution

of EPC atte~pts was apparently not influenced by the plumage ( quality of females. 122 Table 1. Mean male attendance rates, chick feeding rates émd fledging success for males whose mates had accepted extra-pair copulations and for the whole male population.

Mates with EPCs Whole population (N=5) (N=12 )

Mate attendance O.80±O.08 O.79±O.12

Nestling attendance O.49±O.IO O.57±O.10

Feeding rate O.33±O.08 O.42±O.22

Fledging success O.25±O.50 O.83±O.58 123

( Many studies have now documented non-random distributions of EPCs in bird populations (Fujioka and Yamaqishi 1981; Buitron 1983; Bjorklund and Westman 1983; Roskraft 1983; Birkhead et al. 1985; Hatch 1987; Frederick 1987a; McKilligan 1990; Moller 1988b; 1990), and have suggested that they are caused by aggressive male-male interactions, and/or female selectivity. The former have been noted during EPC attempts in colonial birds (Fujioka and Yamagishi 1981; Frederick 1987a). In white ibises

(Eudocimus albus), Frederick (1987a) found that 53% of aIl EPCs observed involved fights between males, and that the number of EPCs attempted by males was positively correlated with their fighting ability. Fujioka and Yamagishi (1981) reported that fighting ability (dominance status) in cattle egrets also predicted a male's success in EPCs. Woolfenden et al. (1976) suggested that the quality of breeding plumes in cattle egrets reflects dominance status. Male-male aggressive interactions were not observed during EPC attempts in this study. However, weIl plumed males attempted more EPes than poorly plumed males, presumably because the former are more likely to be successful. The higher probability that weIl plumed males will be successful in EPC attempts is the consequence of female selectivity in this study. Female selectivity can in principle occur through active solicitation of EPCs or ( through female cooperation. Females have been observed to 124 directly solicit EPCs in commOll murres (Uria aalge) (Birkhead et al. 1985: Hatchwell 1989), northern fulmars (Fulmarus glacialis) (Hatch 1987), black-capped chickadees (Parus atricapillus) (Smith 1988), and cattle egrets

(McKilligan 1990). Even in species where active solic~ting of EPes by females is not common, most studies note that females actively resist or accept certain EPC attempts (Fujioka and Yamagishi 1981; Bjorklund and Westman 1983: Buitron 1983; Roskaft 1983: Frederick 1987a: McKilligan 1990; Westneat 1987a; this study). Since forced copulations are unlikely to successfully transfer sperm in species without intromittent organs (Fitch and Shugart 1984), some female cooperation during EPCs may be necessary for fertilization. Female resistance appeared to be successfu1 in preventing cloacal contact in 39% (9/23) of aIl observed EPes in this study. This falls between the 36% (23/64) reported by Fujioka and Yamagishi (1981) and the 50% (14/28) reported by McKilligan (1990) for cattle egrets. Westneat et al. (1990) suggest that separating female cooperation and resistance in EPC attempts is difficult. Females may coopera te with EPC attempts because of benefits, or because the costs of resisting are high (eg. female in jury, egg damage). Cooperation therefore does not necessariIy indicate that females benefit from EPes. Similarly, femaie resistance may be viewed as either an attempt to avoid the costs associated with the EPC, or as a 125

strategy to stimulate male-male competition for the ultimate

benefit of the female (Westneat et ~ 1990). In this study male harassment was not apparently a siqnificant cost to females resistinq EPCs. Males did not physically attack or harass females in EPC attempts, and females were never apparently injured during EPC attempts. The EPCs in this study occurred prior to eqqlayinq, so succumbinq to avoid the cost of egq damaqe is unlikely. Selective female cooperation in EPC attempts may therefore suqqest that females benefit from EPCs with certain males. Males in monoqamous speeies who enqaqe in EPCs clearly increase their reproductive success, if EPCs fertilize eqqs. Genetic studies have now confirmed that EPCs do fertilize eqqs, with reported rates of extra-pair paternity varying from 4-71% (Westneat 1987b; Wrege and Emlen 1987; Morton et al. 1990; Sherman and Morton 1988; Moller 1988a; Brooker et al. 1990). In contrast, many studies have concluded that females do not benefit, and possibly suffer from enqaqing in EPCs (e.g. Barash 1976; Fujioka and Yamaqishi 1981;

Bjorklund and Westman 1983; Birkhead et ~ 1985), since clutch size in females is not increased and sinee there may be a cost due to detection and 'punishment' by their mate. However, it is not clear how costly EPCs are to females. Cuckolded male indiqo buntinqs (Passerina cyanea) were less likely to feed nestlinqs, but other aspects of paternal care { did not differ (Westneat 1988). Young male purple martins 126

(progne S11bis), who have low rates of paternity, fed chicks less than their mates; but older males, who have higher rates of paternity, participated equally in chick feeding (Morton et al. 1990). In contrast , Frederick (1987b) suggests that male white ibises do not withhold parental care in response to EPCs. However, he did not determine paternity, and élll females in his sample had engaged in at least 1 EPC, making it impossible to compare parental care against males who had not been cuckolded. Males in this study May also ha·,e reduced parental care in response to EPCs. They tended to attend their chicks less, and may also feed chicks less, if their mate had accepted an EPe. Consequently, their nests tended to have lower fledging success than the mean for the population. There is therefore increasing evidence of a cost to females of engaging in EPCs. However, the fact that females selectively cooperate in EPCs suggests that there may be a net benefit to females, providing the EPC is with an appropriate male. Potential benefits for females accepting EPCs include increased fertilization (McKinney et al. 1984), increased genetie diversity of the brood (Williams 1975), and good qenes for their offsprinq (Hamilton and Zuk 1982; Kodric-Brown and Brown 1984; Andersson 1986). Cattle egrets in Barbados lay a maximum of 3 eggs per clutch, and unhatched eggs were rarely seen. Thus it seems unlikely that a malels ability to fertilize eggs is limited. 127

( Increased genetic diversity of a brood may be beneficial, but does not explain the tendency for females to selectively accept EFCs with males who are better plumed than their mates. The results of this study therefore suggest that female cattle egrets engage in EPCs to produce offspring of higher quality. Hamilton (1990) suggested that bright plumage in monogamous birds may have evolved to signal individual genetic quality to potential EPC partners, and recent studies have indicated that females use sexually selected characters as a criterion for choosing an EPC partner. Moller (1988b) artificially lengthened the tail feathers of male barn swallows (Hirundo rustica) and found that they were more successful at obtaining EPCs than males who had their tails shortened. Houtman (1990) found that female zebra finches (Poephila quttata), when presented with males of varying plumage in the laboratory, selectively engaged in EPCs with males who were brighter than their current mate. The results of Chapter 3 indicated that female cattle egrets prefer to pair with weIl plumed males, and that they obtain better male parental care and higher fledging success by doing so. This suggests that female preference and the evolution of male plumage is not based solely on the production of attractive sons (the Fisherian hypothesis). However, it does not allow determination of whether plumes { indicate non-heritable capacity for parental care (the good 128 parent hypothesis), or whether they indicate high genetic viability (the gaod genes hypothesis) and hence better condition and greater parental care. The present results indicate that female cattle egrets selectively accept EPCs with weIl plumed males. This suggests that plumes indicate high genetic quality, sinee females receive no parental care from these males. The fact that males appear to value weIl plumed females more highly, although female plumage is not correlated with maternal care (Chapter 3), supports that suggestion that plumes indieate genetic quality. A simple way of indicating overall genetic viability is by use of an indicator trait that is age-dependent (Manning 1985). Many sexually selected characters increase with age (e.g. deer antlers (Cervus elaphus), Clutton-Broek et al. 1982; peacock's tail (pavo cristatus), Manning 1987; spurs in pheasants (Phasianus colchicus), Schanz et al. 1989). Moreover, age aften determines which males pursue EPCs. Morton et al. (1990) showed that aIder males successfully fertilized 71% of the offspring of younger males through EPCs in a colony of purple martins, whereas younger males fertilized only 4% of the offspring of aIder males. Older male rooks (Corvus frugilegus) attempted more EPCs than first time breeders (Roskaft 1983), and Buitron (1983) observed only 4 year old male magpies (Pica pica) attempting EPCs. In cattle egrets, the plumes of first year breeders 129

are Iess developed than those of older birds (Haddock 1989), and McKilIigan (1990) noted that no first year males in his study attempted EPCs. Female preference for weIl plumed males may therefore occur in cattle egrets because plumes indicate age and hence genetic viability. Moreover, reproductive effort, including parental care, should increase with age (Pianka and Parker 1975), and the quality of parental care may improve with greater breeding experience. The correlation between plumes and paternal care in cattle egrets may be tlle consequence of males of higher genetic quality beinq in better condition and therefore providing more care, and/or from the direct effect of age on paternal cure. The fa ct that weIl plumed females do not provide more care may simply resul t from their being paired with well plumed males who do (Burley 1988). The females therefore obtain greater parental care for their offsprinq at no additional cost to themselves. Selection on female preference should favour females who prefer male traits that advertise both proximate benefits (paternal carel and genetic quality. Age-dependent characters may meet both criteria in many species.

( 130 .. LZTERATURB CITED Andersson, M. 1986. Evolution of condition-dependent sex­ ornaments and mating preferences: sexual selection based on viability differences. Evolution 40:804-816. Barash, D. P. 1976. Male response to apparent female aduitery in the mountain bluebird (Sialia currucoides): an evolutionary interpretation. Am. Nat. 110:1097-1101. Birkhead, T. R., S. D. Johnson and D. N. Nettieship. 1985. Extra-pair matings and mate guarding in the common murre Uria aalge. Anim. Behav. 33:608-619. Blaker, D. 1969. Behaviour of the cattle egret Ardeola ibis. Ostrich 40:75-129. Bjorklund, M. and B. Westman. 1983. Extra-pair copulations in the pied flycatcher (Ficedula hypoleuca): a removal experiment. Behav. Ecol. Sociobiol. 13:271-275.

Brooker, M. G., l. Rowley, M. Adams an~ P. R. Baverstock. 1990. Promiscuity: an inbreeding avoidance mechanism in a socially monogamous species? Behav. Ecol. Sociobiol. 26:191-199. Buitron, D. 1983. Extra-pair courtship in black-billed magpies. Anim. Behav. 31:221-220.

Burley, N. 1986. Sexual selection for aesthetic ~raits in species with biparental care. Am. Nat. 127:415-445. Burley, N. 1988. Tha differentiaI-allocation hypothesis: an experimental test. Am. Nat. 132:611-628. Clutton-Brock, T. H., S. D. Albon, and F. E. Guiness. 1982. Red Deer: Behaviour and ecology of two sexes. University of Chicago Press. Chicago. Darwin, C. 1871. The Descent of Man and Selection in Relation to SeXe Murray. London. Fisher, R. C. 1958. The genetical theory of natural selection. 2nd ed. Dover. New York. Fitch, M. A. and G. W. shugart. 1984. Requirements for a mixed reproductive strategy in avian species. Am. Nat. 124:116-126. ------

131 Frederick, P. 1987a. Extra-pair copulations in the mating system of white ibis (Eudocimus albus). Behaviour 100: 170-201. Frederick, P. 1987b. Responses of male white ibises to their matels extra-pair copulations. Behav. Ecol. Sociobiol. 21:223-228. Fujioka, M. and S. Yamagishi. 1981. Extramarital and pair copulations in the cattle egret. Auk 98:134-144. Gladstone, D. E. 1979. promiscuity in monogamous colonial birds. Am. Nat. 114:545-557. Hamilton, W. D. and M. Zuk. 1982. Heritable true fitness and bright birds: a role for parasites? Science 218:384-387. Hamilton, W. D. 1990. Mate choice near or far. Am. Zool. 30:341-352. Hatch, S. J. 1987. copulation and mate guarding in the northern fulmar. Auk 104:450-461. Hatchwell, B. J. 1989. Intra-specific variation in extra­ pair copulation and mate defence in common guillemots Uria aalge. Behaviour 107:157-185. Hill, G. E. 1991. Plumage coloration is a sexually selected indicator of male quality. Nature 350:337-339. Hoelzer, G. A. 1989. The good parent process of sexual selection. Anim. Behav. 38:1067-1078 Houtman, A. M. 1990. Zebra finches choose males more attractive than their mates for extra-pair copulations. unpubl. ms. Johnson, K. 1988a. Sexual selection in pinyon jays 1: female choice and male-male competition. Anim. Behav. 36:1038-1047. Johnson, K. 1988b. Sexual selection in pinyon jays II:male choice and female-female competition. Anim. Behav. 36:1048-1053. Kirkpatrick, M. 1982. Sexual selection and the evolution of female choice. Evolution 36:1-12. KOdric-Brown, A. and J. Brown. 1984. Truth in advertising: the kind of traits favoured by sexual selection. Am. Nat. 124:309-323. ,( 132 Lande, R. 1981. Models of speciation by sexual selection on polygenic traits. Proc. Nat. Acad. Sei. USA 78:3721- 3725. Maddock, M. 1989. Colour and first age of breeding in cattle egrets as determined from wing-tagged birds. Corella 13:1-8. Manning, J. T. 1985. Choosy females and correlates of male age. J. Theor. Biol. 116:349-354. Manning, J. T. 1987. The peacock's train and the age­ dependency model of female ch"~ice. J. World Pheasant Assoc. 12: 44-56. MCKilligan, N. G. 1990. Promiscuity in the cattle egret (Bubulcus ibis). Auk 107:334-341. McKinney, F., K. M. Cheng, and D. J. Bruggers. 1984. Sperm competition in apparently monogamous birds. In: Sperm competition and the evolution of animal mating systems (R. L. Smith, ed.). pp 523-545. Academie Press. New York. MaIler, A. P. 1988a. Paternity and parental care in the swallow, Hirundo rustica. Anim. Behav. 36:996-1005. MaIler, A. P. 1988b. Female choice selects for male sexual tail ornaments in the monogamous swallow. Nature 332:640-642. MaIler, A. P. 1990. Sexual behaviour is related to badge size in the house sparrow. Behav. Ecol. Sociobiol. 27:23-29. Morton, E. S., L. Forman and M. Braun. 1990. Extrapair fertilizations and the evolution of colonial breeding in purple martins. Auk 107: 275-283. Norris, K. J. 1990. Female choice and the evolution of the conspicuous plumage colouration of monogamous male great tits. Behav. Ecol. Sociobiol. 26:129-138. O'Donald P. 1983. The arctic skua: a study of the ecology and evolution of a seabird. Cambridge University Press. Cambridge, U. K•• pianka, E. R. and W. S. Parker. 1975. Age-specifie reproductive tactics. Am. Nat. 109:453-464. Roskaft, E. 1983. Male promiscuity and female adultery by the rook Corvus frugilegus. Ornis Scand. 14:175-179. 133 Schanz, T. von, G. Goransson, G. Andersson, l. Froberg, M. Grahn, A. Helgee, and H. Wittzell. Fema1e choice selects for a viability-based male trait in pheasants. Nature 337:166-169. Sherman, P. W. and M. L. Morton. 1988. Extra-pair fertilizations in mountain white-crowned sp\rrows. Behav. Ecol. Sociobiol. 22:413-420. Smith, S. M. 1988. Extra-pair copulations in black-capped chickadees: the role of the female. Behaviour 107:15- 23. Westneat, D. F. 1987a. Extra-pair copulations in a predominantly monogamous bird: observations of behaviour. Anim. Behav. 35:865-876. Westneat, D. F. 1987b. Extra-pair fertilizations in a predominantly monogamous bird: genetic evidence. Anim. Behav. 35:877-886. Westneat, D. F. 1988. Parental care and extra-pair copulations in the indiqo buntinq. Auk 105:149-160. Westneat, D. F., P. W. Sherman and M. L. Morton. 1990. The ecology and evolution of extra-pair copulations in birds. In: Current Ornithology, Vol. 7 (D. M. Power, ed.). pp. 331-369. Plenum Press. New York. Wilkinson, L. 1988. SYSTAT: the system for statistics for the pc. SYSTAT Inc. Evanston, Illinois. Williams, G. 1975. Sex and evolution. Princeton University Press. Princeton. Woolfenden, G. E., S. C. White, R. L. Mumme and W. B. Robertson Jr. 1976. Aggression amonq starving cattle eqrets. Bird Bandinq 47:48-53. Wrege, P. H. and S. T. Emlen. 1987. Biochemical determination of parental uncertainty in white-fronted bee-eaters. Behav. Ecol. Sociobiol. 20:153-160. Zahavi, A. 1975. Mate selection - a selection for a handicap. J. Theor. Biol. 53:205-214. 134

SOMMARY In Chapter 1, colonisation of Barbados by the cattle egret Bubulcus ibis ibis was described. Egrets first established a breeding colony in Barbados between 1956 and

1960. Two additional breeding colonies were formed in 1978 and 1980, and one permanent roosting colony in 1984. The colonies have been established in very different vegetation types, suggesting that the availability of suitable colony sites is not limiting population size of egrets in the island. only between 10% and 30% of available tree space is used at the sites, in1icating that limited availability of space at a site is not the cause of new colonies forming. The 4 colonies were evenly spaced across the island, one in the south, one in the north, one in the east and one in the est. This suggests that new colonies are formed to minimise local food competition. Population size in the island, and in each of the 4 colonies, is still growing, but growth in the oldest and largest colony is slowing. Weekly counts showed that the number of birds in the roosting colony was negatively correlated with that in the breeding colonies, indicating short-term movement of birds betwee~ colonies. In Chapter 2, seasonal variation in breeding plumes, breeding activity and breeding success in the northern calony of egrets was investigated. The development of plumage was quantified by assigning plumage scores to individuals. Males had higher plumage scores than females, 135 and scores varied seasonally in both sexes. Seasonal changes in the number of new nests, chicks, and red-billed (ready to breed) birds indicated that breeding occurred almost year round in Barbados, with peaks of activity in JUly-August and November-March. These periods coincide with the onset and end of the annual rainy season in Barbados. seasonal variation in plumage scores of breeding birds was correlated with seasonal variation in breeding activity. It seems likely that poorly plumed birds are subordinate to weIl plumed birds, and may be forced to breed when environmental conditions are poor and breeding activity is therefore lowest. The causes of chick mortality varied seasonally. Mortality was primariIy from starvation during the period of low breeding activity and poor environmental conditions, and primarily from conspecific interference during the period of high breeding activity and better environmental conditions. Fledging success was not highest when breeding activity was highest. This suggests that optimal breeding time is a trade-off between low food availability but low conspecific interference, and high food availability but high conspecific interference. The optimal breeding time May therefore differ for diff('rent individuals, depending on their conspecific competitive ability. III Chapter 3, the function of breeding plumes in cattle egrets was studied by investigating whether plumage was a 130 criterion of mate choice, and whether it was correlated with paternal care, maternaI care or offspring viability. Plumage scores were positively correlated in breeding pairs, largely because well plumed females bred only wiLh weIl plumed males. The plumage scores of mates varied more for males. Neither male ne st attendance, incubation, nor nestling attendance were correlated with male plumage. WeIl plumed males copulated more often than poorly plumed males. This is unlikely to be the basis of the female preference, since sperm is unlikely to Iimit hatching success. WeIl plumed males fed chicks more often than poorly plumed males, and had the higher fledging success in periods when food was limiting. This may be the basis of the female preference. It suggests that the preference, and therefore the evolution of male plumes, are not based sol~ly on the production of attractive sons (the Fisherian hypothesis). 'l'he pairing pattern is consistent with both the good genes and good parental hypotheses for sexual selection. Neither copulation rate nor any aspects of maternaI behaviour were correlated with female plumage. Males therefore, would not obtain better maternaI care fo~ their chicks if the y paired with weIl plumed females. The correlation between plumes and paternal but not maternaI care may ~uggest that plumes do not signal good genes for parental care. 'rhe nest and nestling attendance of males were positively Gorrelated with the plumage scores of their mates. This may suggest that, 137

although males do not obtain more maternaI care for their offspring from weIl plumeà females, they value offspring with these females more highly. This may indicate that plumes signal high genetic quality. The parental behaviour of females was not influenced by the plumage scores of their mates. In Chapter 4, effects of breeding plumes on the

di~tribution of extra-pair copulations (EPCs) in cattle egrets was investigated. Seventeen percent of aIl copulations observed were EPCs. The males who attempted EPCs were better plumed than the mean for the male population. Females accepted EPCs selectively, allowing

them only with males of equivalent or better plumage q~ality than their mates. The plumage scores of females with whom males attempted EPCs did not differ from the mean for the female population, suggesting that males attempt EPCs opportunistically. Males whose mates had accepted EPCS tended to attend nestling less and may have fed them less. Fledging success for these nests tended to be lower than the

mean for the population. This suggests that ~ales reduce parental care in response to their mates accepting EPCs. Despite this potential cost, the fa ct that females selectively accepted EPCs suggests a net benefit for females, providing that the EPC is with a weIl plumed male. The benefit will increase if the EPC is undetected. Females { receive no parental care from EPC partners. Selective 138 acceptance of EPCs with weIl plumed males supports the suggestion that plumes signal high genetic quality. Mating preferences of cattle egrets are therefore most consistent with the good gene hypothesis for sexual selection. Plumes may signal genetic quality, as well as potential capability

for parental care by signalling the age of thE' bearer.