MATE AND NESTLING DESERTION IN COLONIAL LITTLE EGRETS MASAHIRO FUIIOKA• Departmentof Biology,Faculty of Science,Osaka City University,Sumooshi , Osaka 558, Japan ABSTRACT.--Iobserved 25 Little Egret (Egrettagarzetta) nests during the nestling period. Eleven(44%) were desertedby the femaleparent, one (4%)by the male,and three (12%)by both parents.Deserters of both sexesleft nestssoon after singleparent care became feasible (• = 22 + 4 daysafter hatching,ca. 40%of the time from hatchingto the independenceof young).Small broods were desertedsignificantly more frequently than largeones. No chicks died after uniparentaldesertion. Deserters reacquired courtship coloration on their loresand repeatedlyvisited particular sites away from their nestsbefore desertion, which suggests that they pairedwith new matesbefore leaving their primarynests. During biparental care periods, I foundno sexualdifference in feedingfrequency, though males attended their younglonger than femalesin the daytime.The tendencyfor femaleegrets to becomeready to remateearlier than malesmay be becausethey investless prezygotic effort in secondbreeding attempts than males.Males have to establishnew territoriesand guardtheir new matesagainst extrapair copulations,although male-biasedoperational sex ratio also might favor female desertion. Received22 September1988, accepted 16 December1988. BIPARENTALcare has typically been regarded spring are capableof independent living. De- asa manifestationof cooperativebehavior, but sertionby males,which mayresult in polygyny, recent discussionshave emphasizedthe poten- occursoccasionally among birds, but desertion tial for evolutionary conflictbetween the sexes by females(potential polyandry or multi-clutch (Trivers 1972, Chase 1980, Davies and Houston systems)is far lesscommon (Jenni 1974; Ridley 1986, Winkler 1987). Biparentalcare and "ap- 1978;Oring 1982, 1986).This sexualdifference parent" monogamy (sensuGowaty 1983) are in mate desertion is more extreme in mammals prevalent in birds, presumablybecause their where there are few maternal desertions (Kiel- external eggsand undeveloped chicksrequire man 1977), but is occasionallyreversed in am- substantialparental care before independence phibians (Ridley 1978, Wells 1981) and espe- and both males and females can care for off- cially in fishes, where paternal care is more spring (Lack 1968, Oring 1982). However, "... prevalentthan maternalcare (Blumer 1979, Bay- it is clearthat monogamyis not a mating system, lis 1981, Gross and Shine 1981, Kuwamura 1987, but a diversearray of reproductivestrategies" and references therein). (Mock 1985). Monogamy commonly involves Recent studies have documented facultative extrapair copulation(Ford 1983, McKinney et uniparental desertionby both sexes(ambisex- al. 1984), occasionalor facultative polygyny ual desertion:Beissinger 1986) within popula- (Ford 1983), and female-female associations tions of biparental birds and fishes(Dowsett- (Conover 1984a) within the same population. Lemaire 1979;Solheim 1983;Blumer 1985, 1986). Desertion is another general variation on mo- For the Snail Kite (Rostrhamussociabilis), esti- nogamy: either parent may try to reduce its matesof relative reproductiveinvestments have shareof parentalcare at the expenseof its mate, allowed predictionof which parent will desert while still ensuring the brood is reared suc- (Beissinger 1987a, b; Beissinger and Snyder cessfully(Trivers 1972, Maynard Smith 1977, 1987).Among colonial wading birds,where bi- Ridley 1978,Winkler 1987). parentalcare appears to be favoredstrongly and Mate desertion can be defined as the termi- breeding patternsare relatively homogeneous nation of careby one (uniparentaldesertion) or (Wittenbergerand Hunt 1985),uniparental de- both parents(biparental desertion)of the off- sertion has not been reported. Biparental de- spring in a breeding attempt before the off- sertion, however, was recently suggestedas a responseto reducedbrood size (Mock and Par- • Present address:Department of Psychology,NI- ker 1986). 25, University of Washington, Seattle, Washington I focusedon the division of parental duties, 98195 USA. mateand nestlingdesertion, and nestling mot- 292 The Auk 106: 292-302. April 1989 April 1989] MateDesertion in Egrets 293 tality in the Little Egret(Egretta garzetta). About Observationand analysis.--One 5.3-m high blind was one-half of 25 nestswere deserted.Strong cir- built at the edgeof the colonyduring the breeding cumstantialevidence suggests that the deserters season in 1982. In 1986 two elevated blinds (7.0 m remated elsewhere in the same colony before and 5.3 m), which afforded wide views over the tree crowns,were built on the top of the hill before the they had abandonedtheir primary nests.I dis- breedingseason with above-groundtunnels (see Shu- cusspossible reasons that desertionwas so com- gart et al. 1981) for accessto the blinds with little mon, that some individuals deserted whereas disturbance. others did not, and that females deserted more In 1982newly hatchedchicks in focalnests (see frequentlythan males(comparing the desertion below)were markedindividually with felt-tip pens option with the alternative of renesting with during daily nestinspections, and later markedwith the same mate). featherdye and coloredleg bands.Body massand tarsuslength were measuredevery 2-5 daysuntil the METHODS chickswere ca. 14 daysold. In 1986contents of focal nests(see below) were checked almost every day from Studyspecies.--The Little Egret is a colonial wading the blinds. To reduce disturbance, chicks were mea- bird of the Old World tropicaland temperateregions. suredonly onceand bandedwhen they were about Little Egrets feed on fish, insects,amphibians, and 12 daysold; then they were assignedhatching order aquaticcrustaceans in a wide variety of habitats(Haf- accordingto relative tarsuslengths. Because of large ner et al. 1982, Fujioka unpubl. data). size asymmetriesamong siblings (see Inoue 1981, The color of the lores changesdramatically to ma- 1985), chicks that hatched third or later (often dead genta before pair formation (calledcourtship coloration before measuring) were distinguishablefrom the in this paper, see below), then it fades gradually to blinds without marking. Brood size was defined as the noncourtingappearance of straw yellow by the the number of surviving chicksat the time of deser- time the last egg is laid (seeBlaker 1969b,Cramp and tion or day 22 (seeResults for meanage of deserted Simmons1977). Both parentsshare nesting duties af- broods) for nests not deserted. Broods of 1-2 chicks ter egg laying, but brood reductiondue to starvation were designatedas "small";those of ->3chicks were is common(Inoue 1981, 1985).Parents take ca. 1 month termed "large." to hatch eggs after completion of pair-formation. Individual adult Little Egrets were easily distin- Nestlings require full-time brooding for only 2-3 guishedby idiosyncraticcolor patterns on their legs weeksafter hatching(Inoue 1980)and probablybe- where yellow-blackboundaries were variable, clear, come independent at 8-9 weeks of age (Fujioka un- and stable. In 1986 I used a paint gun (Fitch and publ. data). Thus, a complete breeding cycle (from Shugart1983) to mark15 male and 5 femaleparents pair formation to the independenceof all chicks)re- of focal nestswithout capturing them. This made it quiresat least 3 months.The frequencyof parental possibleto easilyrecognize those individuals, even feedingsseems to decreaseafter the middle of the when they were away from their nests.Because the nestlingperiod, as reportedfor the Cattle Egret (Bu- sexesare identical, gender of adults was determined bulcusibis; Fujioka 1985). by courtshipdisplays, egg laying, or positionsduring Studysites.--I studiedtwo breeding coloniesin two copulations. years:the Suzukacolony from early May to early Behavioralobservation usually began just after sun- September1982 and the Hamajimacolony from early rise and continued until ca. 30 rain after sunset. In March to early July 1986. The Suzukacolony was bothyears, I recordednest visitation rate (parental visits located15 km northof TsuCity, Mie Prefecture,Japan per hour that includedfood deliveries)and nestat- (34ø50'N, 135ø35'E),on a small island in a pond sur- tendance(percentage of time a parent was at the nest rounded mostlyby rice fields,and was destroyedin or within ca.5 m of the nest)every 2-6 daysfor each 1985.Five ardeidspecies, including ca. 200 Little Egret nest. The color of lores, which was divided into five pairs, nested in an area of ca. 0.28 ha in 2-m to 7-m grades(straw yellow, powder white, pink, rose,and tall pine forest.Additional information on thiscolony magenta;after Palmer1962), also was recorded at the can be found in Fujiokaand Yamagishi(1981). daily observation.The latter three gradeswere con- The Hamajimacolony is located65 km southof the sideredcourtship coloration. In 1986I countedthe un- Suzukacolony (34ø18'N, 136ø46'E). This colonyprob- mated males that established courtship territories ably startedin ca. 1976,when live sardineswere first within ca. 20 m of the blinds and the unmated females stockedin fish reservesaround the site. Little Egrets with courtshipcoloration that approachedthe un- foragedin the reservesand along natural coastsat a matedmales in a manner unique to courtingfemales nearby bay. Five or six ardeid species,including ca. (seeBlaker 1969a). Up to 11 nestswithin a singlerange 300 Little Egret pairs, nestedin an area of ca. 0.75 ha of view were observed unaided or with binoculars in a 4-m to 15-m