The Auk 117(3):634-639, 2000 INFLUENCE OF ENVIRONMENTAL AND DENSITY-DEPENDENT FACTORS ON REPRODUCTION OF LITTLE EGRETS ROBERTE. BENNETTS,1,3 MAURO FASOLA,2 HEINZ HAFNER, 1 AND YVES KAYSER1 •StationBiologique de la Tourdu Valat,Le Sambuc,F-13200 Aries, France; and 2Dipartimentodi BiologiaAnimale, Universit& Piazza Botta 9, 1-27100Pavia, Italy ABSTRACT.--Weevaluated the influenceof environmentaland density-dependentfactors (intraspecificand interspecific) on clutchsize, brood size, and nesting success of Little Egrets (Egrettagarzetta) in the Camargueof southernFrance. We recordedthese reproductive pa- rametersin mostyears from 1970to 1998.We useda generalizedlinear modelingapproach (modelselection based on AIC) to examinethe environmentaleffects of springrainfall, win- ter temperature,and wind on theseparameters. We also examineddensity dependence of theseparameters based on the total number of Little Egretsand the total number of tree- nestingherons nesting in thesemixed-species colonies. Clutch size was positively associated with rainfall and negativelyassociated with the number of Little Egret nestsin the Ca- margue.Brood size was negatively associated with thenumber of Little Egretnests, although rainfall was only significantas an interactioneffect with thesetwo effects.Nesting success was negativelyassociated with the number of tree-nestingherons, the proportionof each colonyconsisting of Cattle Egrets(Bubulcus ibis), wind speed,and severalinteractions among thesevariables. Virtually all of the reproductiveparameters that we evaluatedwere nega- tively associatedwith the numberof Little Egretnests or the numberof tree-nestingherons. Anecdotalevidence suggests that CattleEgrets displace Little Egretsat somecentrally lo- catednest sites.Such sites are better protectedfrom strongwinds, which are a common causeof nesting failure. Received20 November1998, accepted23 November1999. DENSITY-DEPENDENTeffects on avian repro- nest building, Cattle Egrets frequently steal duction are well known, althoughlittle evi- sticksfrom nearby nests(Valentine 1958), and dencefor sucheffects exists in wading birds sometimesthey eject nest contents (Blaker (Butler 1994). In contrast,density-independent 1969,Siegfried 1972). Given (1) an advantageof environmentalfactors are commonlyreported selectingcertain nest sites,(2) increasednum- to influencereproduction in this group.For ex- bers of Little Egretsand Cattle Egrets,and (3) ample,Hafner et al. (1994)found that rainfall aggressivebehavior of Cattle Egrets,we hy- in winter and springis correlatedwith clutch pothesizedthat intra- and interspecificdensity size and broodsize of Little Egrets(Egretta gar- dependence related to nest-site acquisition zetta).Extended periods of temperaturesbelow could occur Here, we evaluate the influence of freezing are correlatedwith wintering and environmental and density-dependentfactors subsequentbreeding populationsize of Little on clutchsize, brood size, and nestingsuccess Egrets (Hafner et al. 1994), althoughit is not of Little Egrets. known if subsequenteffects influence repro- ductive success. METHODS Sincethe mid-1980s,numbers of Little Egrets in the Camargueregion of southernFrance Studyarea.--Our study area was the Camargue have increasedsteadily (Hafner and Fasola (delta of the RhoneRiver in southernFrance), which 1997).Cattle Egret numbers(Bubulcus ibis) also comprises180,000 ha and includes several colony have increased, from one nest in 1967 to a re- sites and associatedfeeding areas used by Little centhigh of 3,532nests in 1996(H. Hafnerun- Egrets(Hafner 1977,Hafner et al. 1982).The flood- publ. data). In the Camargue,anecdotal evi- plain of the RhoneRiver has been confinedwithin dencesuggests that Cattle Egretsoccupy the leveeson eitherside of the deltasince the early1850s centerof colonies,where reproductivesuccess (Benoit 1933), and the habitat outsidethe delta com- of Little Egretsis higher (Hafner 1977).During plex generallyis unsuitablefor breedingor foraging (Hafner 1977). Thus, the spatial extent of the Ca- margue population of Little Egrets has remained 3 E-mail: [email protected] largely the same for severaldecades (Hafner 1977, 634 July2000] ReproductiveSuccess ofLittle Egrets 635 1982).This populationalso is relativelyisolated; the TABLE 1. Annual estimates of mean clutch size, nearestmajor breeding areasare in Italy and Spain mean brood size, and proportion of nestssuccess- more than 400 km from the Camargue. ful for Little Egretsin southernFrance. Number of Breedingpopulation size.--Little Egretsin the Ca- nestsis in parentheses. marguenest in trees,usually in mixed colonieswith Nesting Cattle Egrets, Black-crownedNight-Herons (Nycti- Year Clutch size Brood size success coraxnycticorax), and SquaccoHerons (Ardeolarallo~ ides).Each year from 1967 to 1998, all colonieswere 1970 4.30 (156) -- 0.96 (142) locatedthroughout the study area usinga small air- 1971 4.39 (57) -- 0.90 (58) 1972 4.34 (215) -- 0.91 (115) craft(Hafner et al. 1994).From May to mid-July,each 1973 -- -- -- colonywas censusedweekly by the sameobserver 1974 -- -- -- (Hafner197•). Each colony was partitioned into sec- 1975 -- 3.02 (176) -- tors of approximately0.1 ha. Completenest counts 1976 -- 3.30 (152) -- were conductedwithin each sector,and any new 1977 -- 3.19 (117) -- nestswere added to the previouscount during each 1978 -- 3.10 (80) -- subsequentvisit. Based on repeated counts and 1979 -- 3.05 (150) -- marking of nestsin a sampleof sectors,we believe 1980 -- 3.06 (178) -- that overestimationwas extremelyunlikely and that 1981 -- 2.90 (221) -- the extent of underestimation was not substantial 1982 4.16 (58) 2.89 (189) 0.95 (57) 1983 -- 3.09 (144) -- (e.g. <10% in the largestcolonies). 1984 3.80 (51) 2.60 (279) 0.91 (54) Environmental variables.--We used the cumulative 1985 -- 3.31 (185) -- absolutevalue of daily minimum temperaturesbe- 1986 4.71 (135) 3.35 (155) 0.94 (156) low 0øC(i.e. the sumof the negativevalues) as an in- 1987 4.43 (30) 3.27 (137) 0.88 (49) dicationof the severityof cold winter weather(Haf- 1988 4.20 (97) 3.12 (136) 0.92 (122) ner et al. 1992, 1994). In accordancewith Hafner et 1989 3.83 (100) 2.98 (160) 0.88 (117) al. (1994), we used total cumulative rainfall between 1990 4.17 (106) 2.97 (207) 0.85 (148) Februaryand April as an indicationof spring rain- 1991 3.86 (117) 2.79 (163) 0.63 (173) fall. A strongwind, known as the Mistral, occursin 1992 4.02 (120) 2.69 (199) 0.81 (155) 1993 3.85 (121) 3.02 (325) 0.89 (132) the Camarguethroughout the yearand canbe a sub- 1994 4.03 (121) 2.78 (297) 0.92 (122) stantialsource of nestingfailure (Valverde1955, Haf- 1995 4.13 (104) 2.63 (238) 0.78 (147) ner 1978). Thus, we used the cumulative sum of the 1996 3.92 (79) 2.66 (277) 0.78 (151) maximumdaily wind speedduring the main nesting 1997 3.45 (60) 2.42 (358) 0.79 (73) period (15 April to 15 June)as a measureof wind ef- 1998 3.99 (116) 2.47 (283) 0.86 (121) fects.Climatic data were provided by the Tour du Valat meteorologicalstation, which is locatedin the centerof the Camargue. linear models to assess effects of environmental and Reproductiveparameters.--We assessed clutch size, density-dependentexplanatory variables on clutch brood size, and nestingsuccess for a sampleof col- size and brood size.We alsoincluded time (year) and onies during most years (Table 1). Nests were colonyeffects as potential explanatoryvariables. We markedalong transectsand their contentschecked usedlogistic regression to explorethe factorsinflu- once a week. To reduce disturbanceearly in the encingnesting success. Model selectionwas based on breedingcycle, we recordedclutch size at 10% or Akaike's Information Criterion (AIC; Akaike 1973, fewer of the number of nestsin large coloniesand Burnhamand Anderson1998). We checkedthe valid- 20% or fewer in small colonies. We defined brood ity of a Poissonassumption using the model devi- sizeas the numberof chicksalive at 20 to 25 daysof ance divided by its degreesof freedom (SAS 1997, age.After this age,chicks can of escapeby walking Burnham and Anderson 1998). We also checkedfor and may not be presentat nests(H. Hafner unpubl. temporal autocorrelationby testing the association data). We considereda nest to be occupiedwith the between the residuals of fitted models at times t and laying of the first egg and successfulif at least one t-1. youngreached the ageat whichit wasable to escape by walking. Becausecomplete censuseswere con- RESULTS ducted weekly and nestswere marked early during incubation,we did not expect a bias from finding Our selected model indicated that mean nestsin latterstages. Thus, we did not usethe May- field (1961)estimator. Rather, nesting success was es- clutchsize was positively associated with rain- timatedas the proportionof occupiednests that was fall (X 2 = 11.67, df = 1, P < 0.001) and nega- successful. tively associatedwith the total numberof Little Dataanalysis.--Data were analyzedwithin a gen- Egret nests(X 2 = 8.85, df = 1, P = 0.003; Fig. eralized linear modeling framework.We used log- 1). The model did not exhibit a substantial 636 BENNETTSET AL. [Auk, Vol. 117 4.8 3.6 3.4 4.6 3.2 4.4 ß 4.2 ß 2.8 4.0 ß 2.6 ß ß 3.8 2.4 3.6 2.2 50 100 150 200 250 300 350 0 1000 2000 3000 4000 5000 6000 Total rainfall(January-April) Total Number of Little Egret Nests 4.8 FIc. 2. Annual brood size relative to totalnumber of Little Egretnests in the studyarea. 4.6 N •.• 4.4 our selectedmodel becauseof its significance • 4.2 in an interaction term with the number of Little Egret nests (X2 = 8.31, df = 1, P = 0.004).