Mate and Nestling Desertion in Colonial Little Egrets

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 diedafter uniparental desertion. Deserters reacquired courtship coloration on their loresand repeatedlyvisited particular sites away from their nestsbefore desertion, which suggests that theypaired with 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 tall coastal forest. The forest consists simultaneouslyfrom the blinds.Data were recorded mainly of Japaneseblack pines (Pinus thunbergii) and on a portablecomputer (see Hensler et al. 1986)and an evergreenoak (Quercusphilyraeoides). in notebooks. 294 MASAHIROFulIOKa [Auk,Vol. 106

TABLE1. Parental care and desertion patterns in relation to sex and brood size in Little Egretsat the Suzuka (1982) and Hamajima (1986) coloniesin Japan.

Year Brood size b Season c Nesting statusa Difference in (male-female) 1982 1986 1-2 3-4 Early Late care daysa Desert-Desert -- 3 3 -- 2 1 0.7 + 1.2 Desert-Color -- 1 1 -- 1 1 9.0 Color-Desert -- 4 2 2 3 1 14.6 _+ 4.0 Stay-Desert 1 6 4 3 1 5 15.0 + 5.2 Stay-Color -- 5 -- 5 4 1 Stay-Stay 3 2 1 4 -- 2 Total 4 21 11 14 11 10

• Desert= desertion;Color = courtshipcoloration but no desertion;Stay • neithercourtship coloration nor desertion.There were no instances of the other three possiblecombinations (i.e. Color-Color,Color-Stay, and Desert-Stay). bBoth yearscombined. cEarly = before or on 15 May; Late = after 15 May. Data from 1986 only. dThe mean difference(male minus female in days + SD) in duration of parentalcare betweenthe partners.

The chronologyfor each brood was counted from h and 429 nest-h)between 4 Juneand 6 August,and the day the first chick hatched(day 0). Becausepa- 21 nestsin 1986,observed on 33 days(333 h and 1,408 rental activitieschanged with time, I divided the nest- nest-h)between 17 April and 2 July. In addition, 1- ling period into two stagesfor further analyses.The 6 of the 1986 focal nestswere monitored with a por- firsthalf was definedas the period from day 0 until table video camerasystem on 17 days (183 h and 249 one parent desertedor, for neststhat were not de- nest-h). Becauseparental activitiesof one individual serted,the mean desertionperiod of 22 days (seeRe- may dependon thoseof its partner,the measurement suits).The secondhalf was defined as the periodafter of pairs (or nests),not individuals, was regardedas desertion(i.e. uniparental period) or after the mean an independent variable except when comparisons desertionday (including daysafter the chicksreached were madewithin pairs.When no significantdiffer- flying age). Data from the two stageswere not com- enceswere found between years/colonies,the data bined. In comparisonsbetween deserted nests and were combined. In comparisonsbetween categories, nests that were not deserted or between the sexes in I excluded data based on total observations of <20 all nests,only data from the first half were used. h/nest (or per individual). When samplesdid not A parent was defined as having desertedits mate alwayshave equalvariances and the samplesizes were and young if it showedcourtship coloration and no small,I usednonparametric statistical tests (Sokal and longer fed the primary brood of dependentyoung. Rohlf 1981).All mean valuesare given with one stan- One exceptionalfemale, which firststopped and then dard deviation (_+SD). (20 days later) provided care, was not countedas a deserter. Those nests that were not deserted (i.e. in RESULTS which both parentscontinued to feed young through- out the observationperiod) were called biparentalcare Occurrenceof desertion.--Iobserved a total of nests.Furthermore, parents that did not desert the nests,but showed courtshipcoloration at more than 15 mate desertionsby 18 parents:1 female de- one observationwere consideredto be attempting to sertion(25.0%) among 4 nestsat Suzukain 1982 renest.The durationof parentalcare was calculatedfor and 10 female(47.6%), 3 biparental(14.3%), and eachnesting adult asthe periodfrom hatching(brood 1 (4.8%) male desertionamong 21 nestsat Ha- age 0) to the end of the observationfor each non- majimain 1986(Table 1). In 2 of the 3 biparental deserter,and to the midpont betweenthe last feeding desertions, the male deserted the brood on the day and the first nonfeedingday for deserters.This sameday astheir matesand 2 daysafter female definition makes the difference between deserters and desertions, the males deserted the brood on the nondeserterssmall. Even though I did not recordpa- plete nest failure. At only one 1986 nest,the rental behaviorquantitatively, all nondeserterscon- male parent desertedthe brood,while the fe- tinued to feed chicks until at least day 40 in all but male continued to feed the brood. Overall, fe- the last four nests in 1986. I restrictedall analysesto neststhat were monitored males desertedthe young and mates signifi- for at least one month post-hatchingor until both cantly more frequently than males (14 females parentsdeserted the young. Data from observation vs. 4 malesin 25 pairs;Fisher's exact test, P = periodsof <2 h were excluded.The resultingsample 0.007). was 4 nestsin 1982,observed on 26 days(totaling 371 Additional "remating attempts," inferred April 1989] MateDesertion in Egrets 295

from the coloration of lores, comprised10 of 50 nested with his original mate, which had also total parents (Table 1). In the last two nestsin exhibited courtship coloration. Both flew re- 1986, neither parent regained courtship color- peatedlyto the samerestricted area unusually ation. The proportion of parents that showed close (ca. 15 m) to their previous nest, where courtship coloration, including deserters,was they repeatedlyperformed "greeting ceremo- significantly greater at Hamajima (64.3%of 42) nies" (Blaker1969b). Renesting by one male (a than at Suzuka (12.5% of 8; P = 0.015, Fisher's nondeserter) was confirmed. The new nest, exact test). Females reacquired courtship col- about 90 m from the previous one, contained orationsignificantly more often than males(20 three eggson day 30 of the first brood. He con- vs. 8 in 25 pairs; G = 10.196, P < 0.005). At 4 tinued to provide solocare to the original brood nestsdeserted by females,the males also reas- of two young, which had been desertedby the sumed courtship coloration and flew to sites female. The fate of the second nest was not different from where their mates had flown; the monitored. males continued to feed their first broods. In ! Parental care.--Male Little Egrets attended of 10 biparental care nests, the female reac- chicksfor slightly longer periodsthan females quired courtship coloration, disappeared from in the daytime,both in desertedand biparental her first nestfor 20 days(days 17.5-38), but then care nests,but the frequency of nest visits did resumedfeeding her original chicks. not differ between the sexes in either deserted Uniparental desertion or first desertion in bi- or biparental care nests (Table 2). The mean parental desertionsusually occurredca. 3 weeks duration of male parental carewas about a week after first hatching (œ= day 21.9 + 4.1, range longer than that of females (Table 2). This es- = 16.5-29.5, n = 15). Small broods were deserted timate is conservative because most nests were more frequently than large broods (Table 1; P not monitoredthrough the end of parental care. = 0.012, Fisher's exact test). Brood size had no effect on either nest atten- Becausethe mean date of first hatching in 4 danceor visitation rates,among either deserted nestsof 1982 (14 June + 12 days) differed sig- nests or all focal nests (Table 3). Also, no dif- nificantlyfrom that in 21 nestsof 1986 (17 May ference was found in the timing of desertion + 11 days; Mann-Whitney test, U = 82, P < as a function of brood size. Individuals that sub- 0.01), I examined the effect of season for 1986 sequentlydeserted their broodsvisited nestsat only (Table 1). Probabilityof desertionwas not the same rate as their mates (0.17 + 0.04 visits/h relatedto hatchingdate: 7 of 1! early nests(the vs. 0.18 + 0.05 visits/h, n = 12 pairs, Wilcoxon first chick hatched on or before the median signed-rankstest, T = 27.5). hatching date of 15 May) were desertedand Prior to uniparental desertions,future desert- desertion occurred at 7 of the other 10 nests (P erstended to spendless time at their nests(47.3 = 1.0, Fisher'sexact test). Similarly, there was + 10.6%, n = 12) than their mates (58.2 + 9.3%: no difference in the mean hatching date be- Wilcoxon signed-ranks test, T = 2, P < 0.01). tween deserters(14 May + 13 days,n = 17) and However, because all but one of these deserters nondeserters(19 May + 9 days,n = 25; Mann- were females,this resultmay be autocorrelated Whitney test, U = 263.5). However, the mean with the basic gender-bias(see Table 2). Like- hatching date of parentsthat developed court- lihood of desertiondid not appearto be related ship coloration(14 May + 11 days,n = 27) was to particularly high levels of parental care:total significantly earlier than that of the others (22 attendancerates during the first half of the nest- May + 10 days,n = 15; Mann-Whitney test, U ling period were similar for nests eventually = 299.5, P < 0.05). deserted (54.0 + 8.4%, n = 15) compared with Deserting parents and other birds that had those not deserted (55.6 + 4.5%, n = 10: Mann- courtshipcoloration were observedrepeatedly Whitney test, U = 79.5). These two classesof visiting particular colony areas.Because of the nests also received comparable visitation rates restrictedview from the blinds, renesting by from parents (0.17 + 0.04 visits/h vs. 0.18 + deserters could not be confirmed. However, such 0.05 visits/h, U = 93.5). Furthermore, males females were often seen intently watching whose mates had deserted them did not differ courtingmales. Similarly, such males were seen from malesin biparental carenests with respect defendingtheir courtshipterritories. In the lone to both attendance rates (15.0 + 12.8%, n = 8 caseof male desertion,the male probably re- vs. 11.2 + 10.4%,n = 6; Mann-Whitney test, U 296 MASAHIROFUJIOKA [Auk,Vol. 106

TABLE2. Comparisonsof parental care between sexesin Little Egretsat the Suzuka(1982) and Hamajima (1986) coloniesin Japan (œ+ SD).

Wilcoxon test b Desertioncategories a Male Female n T P Nest attendance (%) Deserted (Ist-half) 58.5 + 8.1 49.4 + 11.1 15 8 <0.01 Biparental(lst-half) 59.4 + 6.3 51.8 + 7.5 10 9 <0.05 Biparental(2nd-half) 11.2 + 10.4 6.6 + 7.0 6 0 <0.05 Nest visits/h Deserted (lst-half) 0.17 + 0.05 0.17 + 0.04 15 40 NS Biparental(lst-half) 0.18 + 0.05 0.18 + 0.07 10 20.5 NS Biparental(2nd-half) 0.33 + 0.06 0.24 + 0.09 6 2 NS Duration of care c 34.7 + 8.6 28.1 + 8.6 25 5 <0.01

aThe "1sthalf" of the nestlingperiod is the durationfrom the first hatchingto the time one parentdeserted the broodfor desertednests or to the mean desertionday (21.9)for biparentalcare nests. The "2nd half" is all daysthereafter. bWilcoxoh's signed-ranks test. ANOVAs yielded the sameresults for all but nestattendance in biparentalcare nests during the secondhalf. cThe periodfrom the first hatchingto the desertionor the end of observation(see Methods for details).

= 29) and visitation rates (0.32 _+ 0.08 visits/h lings. The chicks died during days of heavy vs. 0.33 + 0.06 visits/h, U = 27.5). rain, apparently becauseof starvation. In 1986, Brood reduction.--Brood reduction could be three died from the observer's disturbance; one related to parental desertionin three ways: (1) fell from its nestand disappeared;another died chicksmay die as a result of desertion, (2) risk of totally unknown causes;and the other 13 of chick starvationmay reducethe incentive for mortalities (72%) were apparently due to star- parentsto deserttheir chicks,and (3) parents vation(Table 4). Of 7 chicksthat died afterbeing may desert their brood as a result of brood re- measured,3 fourth-hatchedchicks weighed only duction (becauseof the brood's lower repro- 34.6 _+3.9% of their first-hatchedsibling in each ductive value). nest,significantly lighter on a relativescale than Initial brood sizes (number of chicks hatch- the 6 fourth-hatched chicks that survived (56.9 ing) were 4.3 _+ 1.0 in 4 nestsof 1982 and 3.5 _+13.3%; Mann-Whitney test, U = 21, P < 0.01). +_ 1.6 in 21 nests of 1986. Three of 17 chicks in The other 3 fifth-hatched chicks also were se- 1982 and 18 of 74 chicks in 1986 died during verely underweight (averaging 25.2 _+ 5.5%). the first half of the nestling period. All three Comparativedata on survival chicks were not in 1982were siblingsof a nestwhere the female available because all fifth-hatched chicks died. parent eventually desertedtwo remaining nest- Brood reduction did not result in a higher

TABLE3. Comparisonsof parental care in relation to brood size in Little Egrets at the Suzuka (1982) and Hamajima(1986) colonies(• _+SD, number of nestsin parentheses).No U value was significant.

Desertioncategories a Small broodsb Largebroods b Uc Nest attendance (%) Deserted (Ist-half) 53.4 + 10.1 (10) 55.0 + 4.1 (5) 26 Deserted (2nd-half) 16.0 + 18.1 (4) 14.1 + 7.3 (4) 9 Pooled (lst-half) 53.4 + 9.6 (11) 55.6 + 4.3 (14) 83 Nest visits/h Desertion (1st-half) 0.17 + 0.04 (10) 0.17 + 0.03 (5) 27 Desertion (2nd-half) 0.31 + 0.12 (4) 0.34 + 0.04 (4) 12 Pooled (lst-half) 0.17 + 0.04 (11) 0.17 + 0.04 (14) 84 Desertiontiming a 21.4 + 4.1 (10) 22.9 + 4.3 (5) 32 Sameas Table 2. Comparisonwas not made between biparentalcare nestsalone becausethere was only one small brood. Small broodshad I or 2 chicks;large broods3 or more chicks. Mann-Whitney U-test. Daysfrom the firsthatching to the uniparentaldesertion (see Methods). April 1989] MateDesertion in Egrets 297

T^nLE 4. Brood reduction in relation to initial brood size in Little Egretsat the Hamajima colony (1986). 73 The numbersof nestsare given.

Initial brood size a z Nestlings dead (n) 1 2 3 4 5 6 Total

0 2 5 1 ------8 1 -- 1 -- 3 5 -- 9 10[•1• 8APRIL 2 1 1 1 3 z Mortality (%) 10 20 30 9 19 29 9 19 29 8 0 8.3 0 31.3 23.3 33.3 21.7

MAR. APR. MAY JUN. a One brood of 4 chicks was excluded because 3 died after nest disturbance. Fig. 1. The distributionof laying datesof firsteggs by Little Egretsat the Hamajimacolony in Japan(1986).

have been overlooked in previous studies of frequency of desertion. One or both parents colonialwading birds becauseof the difficulties deserted 6 (46.2%) of 13 nests where brood re- associatedwith individuallymarking these large duction had occurred; 8 (72.7%) of 11 nests were birds and becausefull biparental care was pre- deserted where all chicks survived (P = 0.240, sumedobligate. Three species-specificfeatures, Fisher'sexact test). This may be becausebrood along with environmentalfactors, probably af- reduction occurred more frequently in large fect the frequencyof desertionin a population. broods,where desertionwas lessfrequent (see The effectof uniparental careon reproductive above) than in small broods. successrelative to thoseof biparentalcare varies During the secondhalf of the nestlingperiod, from speciesto specieseven among monoga- 4 chicks died as a direct result of biparental mous animals (Wittenberger and Tilson 1980). desertion. However, no chicks died either after It depends on offspring demand for care (a uniparental desertionor at biparental carenests. function of brood size and brood age) and, in Two males each reared broods of 4 chicks uni- a particular seasonor area, on variation in food parentally, the maximum number that survived availability.Beissinger and Snyder(1987) found in Hamajima broods. that no desertionoccurred in Snail Kites during Breedingseason and sex ratio.--At Hamajima, drought years, whereas uniparental desertion laying occurredover at least 88 days, from 15 occurredat nearly every nest during favorable March to 10 June(Fig. 1). Most early nestswere years. destroyedby strong winds in late March, cre- The breeding seasonmust be long enough ating two peaksin laying. Becauseseveral nestfor the parents (or at least for the deserters)to lings were observedin September,laying prob- renest(Beissinger 1987a). ! found that there were ably continued until the end of June. unmatedindividuals of both sexesduring a rel- The number of courting males and courting atively long period (Fig. 2). Similarly, asyn- females (the operational sex ratio, sensuEmlen chronousbreeding offers a greater chance for and Oring 1977) varied with time (Fig. 2). renesting (Maynard Smith 1977). Egrets have Courting females tended to outnumber court- relatively long periods during which eggs are ing males for the first month of the study. laid (e.g. 67-78 days for Cattle Egrets,Blaker Thereafter,unmated males were almostalways 1969a;at least88 daysin this study,Fig. 1) com- more common than females. No unmated males pared with other gregariousbirds (e.g. 67% of establisheda territory within the study areaaf- BankSwallow [Ripariariparia] nests may hatch ter 27 May, but many malesstill displayedin during a single 6-day period, Emlen and De- more distant parts of the colony. mong 1975). Anotherimportant factor may be the amount DISCUSSION of resourcesavailable to breeding pairs.If there are some resourcesfor renesting in the terri- Variationin theoccurrence of desertion.--Itis not tories,intrasexual conflict could physically pre- clear why desertionwas common at the study vent renesting with new mates by either sex colonies,especially at Hamajima. Desertion may (e.g. Freed 1987).A parallel argument has been 298 MASAHIROFUIIOKA [Auk, Vol. 106

10 1.0

•z 0.5•

0 10 20 30 9 19 29 9 NtAR. APR. NtAY

Fig. 2. The number of courting Little Egret males (black bars) and females (white bars) with 1 SD, and the operational sex ratio (sensuEmlen and Oring 1977: line) around the blind at the Hamajima colony in Japan(1986). The maximum number of courting egretsper day that were individually distinguishedwas averagedacross 5 days (3 or more observationdays per 5-day stage).The last courting bachelorwas seen in the observationarea on 27 May and the last courting female on 11 May, but severalapparently unmated egretswere seen later in other areasof the colony.

madefor male polyterritorialism(Slagsvoid and tle Egret nestand one Cattle Egret nest (Fujioka Lifjeld 1988). unpubl. data). Decisionmaking.--Several facts suggest that Potential desertersprobably look for new Little Egret desertersmay have engaged in mates while reducing care to current broods, courtship (or pair formation) activities before then desertonly if they acquirea new mateand deserting. Desertersshowed a resurgenceof if their young seemlikely to be reared by the courtshipcoloration while still taking care of current partner. In such a protracted decision- the young.These color changesmay accurately making process,the individual can adopt or predictrernating potential. Male egretsthat lose give up an alternativetactic accordingto rela- their matesare known to ternatesoon after they tively accurateprediction (i.e. few mistakes).In reacquire courtship coloration (Fujioka 1986). this sense,the processcontrasts with situations Those egretsthat showed courtshipcoloration, in which an individual must forsake alterna- including future deserters,flew repeatedlyto tivesin order to adopta tacticthat may be more particular areasin the colony while tending a profitable on average. nest.Finally, sixfemales and four malesshowed Although many factorsare involved in de- courtship coloration but did not desert. Some sertion patterns (Winkler 1987), decision mak- of them might have tried to rernatebut failed. ing dependsfundamentally on two basicvari- Beissinger(1987b) reported reducedparental ables:the probabilityof renestingand the impact care in Snail Kite deserters before desertion and that uniparental care of the current brood will suggestedthat suchbirds might be testingtheir have on reproductive success.In general, de- mates'commitment and capabilityof caringfor sertion should be favored when the young without help. I found no reduced care,probably because I samplednest visitation, V• + pV2 > (1 + p')V2 (1) a less sensitive measure than amount of food delivered. When such estimates were made in where Vt and V2 are the expectedreproductive 1982, however, the female parents did reduce successof uniparental and biparental care, re- their contributionsbefore desertingat one Lit- spectively,and p and p' are the chancesof re- April1989] MateDesertion inEgrets 299 nesting after desertion and nondesertion, re- be sufficientfor small but not for large broods. spectively (Maynard Smith 1977, 1982). In Second,the expectedbrood size in the next addition, there are the probabilities,q, that the breeding attempt may differ from that of the partner will desert the brood first, and r that presentbrood. Therefore, small broodsmay be the partner also will desert, thus not provide desertedmore frequently than large broods (this the necessaryuniparental care. Taking these study,S. R. BeissingerMS, Lazarusand Inglis variables in account,Equation 1 becomes 1986, Winklet 1987 for theoretical models). Beissingerand Snyder(1987) also reported that (1 - r)V• + pV2 > qV• + (1 - q)V2 + p'V2. (2) singlechick broods tended to be desertedear- lief than larger broods.Similarly, Mock and The assumptionsin this formula are that av- Parker (1986) found high mortality amongsin- eragereproductive success does not changewith gletonGreat Egrets (Casmerodius albus) and Great time during the season,and that chickscannot Blue Herons (Ardeaherodias), suggesting a high survive without care from at least one parent. biparental desertion rate. If a parent hasalready found a new mate (i.e. Sex-skewedinvestment.--Female Little Egrets p = 1) and "knows" that the presentbrood can desertedmates and nestlingsmore often than be raisedfully by its former partner alone (i.e. V• = V2), then desertion is favored whenever males. Equation 3 explains neither this fact nor why parents did not simply renest with the p' < 1 - rorr < 1 - p', (3) samepartners ("clutch overlap," Burley 1980). In colonial birds, the reproductiveeffort may that is, whenever the probability of renesting be larger in males than in females simply be- with the same mate or risk of biparental deser- causeforaging and mate guarding are incom- tion is low. For Little EgretsI calculatedr as patible activities (Beissinger 1987a). Male co- 0.12 (3 of 25 pairs) and p' as 0.04 (1 of 25 pairs, lonial birds have to defend courtship territories, assumingthat the solecase of "male desertion" producegametes, guard matesagainst extrapair resultedin renestingby the samepair). There- copulations,and provide food to females,while fore, desertion would be favored after the crit- the primary costfor femalesis egg production ical point of biparental care (see below) when- (Hunt 1980, Roskaft 1983, Roskaft et al. 1983, ever a new mate has been found. Roskaftand Slagsvoid1985, Beissinger 1987a). In the careperiods of many biparentalspecies, This early disparity does not reverse because there may be a critical point after which uniparental duties after laying or hatching are parental care becomes feasible (i.e. after the shared equally (or slightly male-biased)be- phasewhen offspringrequire the highestlevels tween both parents (e.g. Hunt 1980, Monte- of parental care). I found desertersprovided vecchi and Porter 1980, Burger 1981, Leffelaar carelong enough to ensurechick survival,but and Robertson 1986, Beissinger 1987b, this for only about 40% of the normal nestling pe- study). Unlike males nesting in dispersedter- riod. Although severalchicks died of starvation ritories, which can have more than one nest in during the early nestling period with biparen- the same territory, colonial males must repeat tal care, no chicks died after uniparental deser- all these investments in order to reinate. Thus, tion. At least one desertedmale egret managed regardlessof the amount each parent has in- to take care of the young and still renestwith vestedin the presentoffspring, renesting costs another female. Male terns (Sterna hirundo) can may be lower for colonial females.This asym- raise chicks alone after mate loss in the middle roetry may make it possiblefor femalesto be of the nestling period (Nisbet et al. 1978), ready to reinateearlier than males.There is no whereasmate loss before the criticaltiming may reason that females should wait for mates to be result in nest failure in gulls (Larusspp.) (Con- ready to tenest. over 1984b) or infanticide in egrets (Fujioka A desertingfemale needs a new partner (in- 1986).With mate removal experiments,Sasv•ri deed, a monogamousone) becausebiparental (1986) demonstrated that lone Great Tit (Parus care is obligate during the early breeding stage major)and Blue Tit (P. caeruleus)parents were in colonial birds. Over an entire population, unable to rear chicks less than about a week old therefore, males theoretically have an equal although they raisedolder chicks. chance to reinate unless the sex ratio of poten- Brood size can influence the decision to de- tial breeders is skewed toward males. In fact, sertin two ways.First, uniparental feeding may someLittle Egret malesdeserted or remated af- 300 MASAHIROFUJIOKA [Auk,Vol. 106 ter desertionby their mates.Little Egretfemales BLUMER,L.S. 1979. Male parental care in the bony often fight among themselves over accessto fishes. Q. Rev. Biol. 54: 149-161. courting maleswhen the operationalsex ratio 1985. Reproductive natural history of the Brown Bullhead Ictalurusnebulosus in Michigan. is skewed toward females (Fujioka 1987). The Amer. Midi. Nat. 114: 318-330. observedsexual difference in desertionmay be 1986. Parental care sex differences in the due in part to the temporarily male-biasedop- Brown Bullhead, Ictalurusnebulosus (Pisces, Ictalu- erationalsex ratio (Fig. 2). ridae). Behav. Ecol. Sociobiol. 19: 97-104. BURGERßJ. 1981. Sexual differencesin parental ac- ACKNOWLEDGMENTS tivities of breeding Black Skimmers. Am. Nat. I thank S. Yamagishifor adviceand encouragement 117: 975-984. throughoutthis studyas part of my Ph.D. dissertation BURLEY,N. 1980. Clutch overlap and clutch size: at the Departmentof Biology,Osaka City University. alternativeand complementaryreproductive tac- This paper was completedwhile I was a visiting re- tics. Am. Nat. 115: 223-246. searcherat the Departmentof ZoologyßUniversity of CH^s•, I.D. 1980. Cooperativeand noncooperative Oklahoma,and the Departmentof Psychology,Uni- behavior in animals. Am. Nat. 115: 827-857. versityof Washington.I am particularlygrateful to CONOVeR,M.R. 1984a. Occurrence of supernormal D. W. Mock. I am also grateful to M. I. Beilharz, R. clutches in the Laridae. Wilson Bull. 96: 249-267. Kishimoto, M. Kohda, J. Quinn, K. Ueda, E. Urano, 1984b. Consequencesof mate lossto incu- and especiallyS. R. Beissinger,T. C. Lamey, D. W. bating Ring-billed and California gulls. Wilson Bull. 96: 714-716. MockßJ. A. RodgersJr., P. L. Schwagmeyer,and an anonymousreferee for many usefulcomments on var- CRAMP,S., & K. E. L. SIMMONS(Eds.). 1977. Hand- iousversions of the manuscript.This studycould not book of the birds of Europe,the Middle Eastand have been completed without the long-term assis- North Africa. Vol. 1, Ostrich to ducks. Oxford, Oxford Univ. Press. tancewith the field observationby N. Yabe and as- sistancein other aspectsof the fieldwork by M. A. DAVIES,N. B., & A. I. HOUSTON. 1986. Reproductive Fitch, D. Hashiguchi, Y. Higuchi, M. Hotta, Y. Inoue, success of Dunnocks, Prunella modularis, in a vari- R. Kishimoto,and S. Komeda.Instruction in computer ablemating system: II. Conflictsof interestamong programsand statisticalanalyses was from T. C. La- breeding adults. J. Anim. Ecol. 55: 139-154. mey and H. Momose. I received unpublished data DOWSETT-LEMAIRE,F. 1979. The sexual bond in the from S. R. Beissingerand Y. Inoue. Many references Marsh Warbler, Acrocephaluspalustris. Le Gerfaut 69: 3-12. were searchedand managedby a cooperativedatabase system,the Data Bank of Ornithological Literatureß EMLEN,S. T., & N.J. DEMONG. 1975. Adaptive sig- Japan.This researchwas supported in partby a Grant- nificanceof synchronizedbreeding in a colonial in-Aid for Encouragementof Young Scientists(No. bird: a new hypothesis.Science 188: 1029-1031. 60790034)from the JapaneseMinistry of Educationß , & L. W. OraNG. 1977. Ecology,sexual selec- Scienceand Culture, a Fellowship for Young Scien- tion, and the evolution of mating systems.Sci- ence 197: 215-223. tists and a Fellowshipfor ResearchAbroad by the JapanSociety for the Promotion of Science. FITCH,M. A., •: G. W. 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