The Auk 117(3):634-639, 2000
INFLUENCE
OF ENVIRONMENTAL
FACTORS
ON
AND
REPRODUCTION
DENSITY-DEPENDENT
OF LITTLE
EGRETS
ROBERTE. BENNETTS,1,3MAURO FASOLA,2 HEINZ HAFNER, 1 AND YVES KAYSER1
•StationBiologique
dela Tourdu Valat,Le Sambuc,F-13200Aries,France;and
2Dipartimento
di BiologiaAnimale,Universit&PiazzaBotta9, 1-27100Pavia,Italy
ABSTRACT.--We
evaluatedthe influenceof environmentaland density-dependent
factors
(intraspecific
andinterspecific)
onclutchsize,broodsize,andnestingsuccess
of LittleEgrets
(Egrettagarzetta)in the Camargueof southernFrance.We recordedthesereproductiveparametersin mostyearsfrom 1970to 1998.We useda generalizedlinearmodelingapproach
(modelselectionbasedon AIC) to examinethe environmentaleffectsof springrainfall,winter temperature,and wind on theseparameters.We alsoexamineddensitydependenceof
theseparametersbasedon the total numberof Little Egretsand the total numberof treenestingheronsnestingin thesemixed-species
colonies.Clutchsizewaspositivelyassociated
with rainfall and negativelyassociated
with the numberof Little Egret nestsin the Camargue.Broodsizewasnegativelyassociated
with thenumberof LittleEgretnests,although
rainfall wasonly significantas an interactioneffectwith thesetwo effects.Nestingsuccess
was negativelyassociatedwith the numberof tree-nestingherons,the proportionof each
colonyconsistingof Cattle Egrets(Bubulcus
ibis),wind speed,and severalinteractionsamong
thesevariables.Virtually all of the reproductiveparametersthat we evaluatedwere negatively associated
with the numberof Little Egretnestsor thenumberof tree-nesting
herons.
Anecdotalevidencesuggeststhat CattleEgretsdisplaceLittle Egretsat somecentrallylocatednest sites.Suchsitesare better protectedfrom strongwinds, which are a common
causeof nestingfailure. Received
20 November
1998, accepted
23 November
1999.
DENSITY-DEPENDENT
effectson avian repro- nest building, Cattle Egrets frequently steal
duction are well known, althoughlittle evi- sticksfrom nearby nests(Valentine1958), and
dencefor sucheffectsexistsin wading birds sometimesthey eject nest contents (Blaker
(Butler 1994).In contrast,density-independent 1969,Siegfried1972).Given(1) an advantageof
environmentalfactorsare commonlyreported selectingcertainnest sites,(2) increasednumto influencereproductionin this group.Forex- bers of Little Egretsand Cattle Egrets,and (3)
ample,Hafner et al. (1994)found that rainfall aggressivebehaviorof Cattle Egrets,we hyin winter and springis correlatedwith clutch pothesizedthat intra- and interspecific
density
size and broodsizeof Little Egrets(Egrettagar- dependence related to nest-site acquisition
zetta).Extendedperiodsof temperaturesbelow
freezing are correlatedwith wintering and
subsequentbreeding populationsize of Little
Egrets(Hafner et al. 1994), althoughit is not
known if subsequenteffects influence reproductive
success.
could occur Here, we evaluate the influence of
environmental and density-dependentfactors
on clutchsize,broodsize,and nestingsuccess
of Little Egrets.
METHODS
Sincethemid-1980s,numbersof LittleEgrets
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).CattleEgretnumbers(Bubulcus
ibis)also comprises180,000 ha and includes several colony
have increased, from one nest in 1967 to a re-
centhigh of 3,532nestsin 1996(H. Hafnerunpubl. data). In the Camargue,anecdotalevidencesuggeststhat Cattle Egretsoccupythe
centerof colonies,where reproductivesuccess
of Little Egretsis higher(Hafner 1977).During
3 E-mail:
[email protected]
634
sites and associatedfeeding areas used by Little
Egrets(Hafner 1977,Hafner et al. 1982).The floodplain of the RhoneRiver hasbeen confinedwithin
leveeson eithersideof thedeltasincetheearly1850s
(Benoit 1933), and the habitat outsidethe delta complex generallyis unsuitablefor breedingor foraging
(Hafner 1977). Thus, the spatial extent of the Camargue population of Little Egrets has remained
largely the samefor severaldecades(Hafner 1977,
July2000]
Reproductive
Success
ofLittleEgrets
635
1982).This populationalsois relativelyisolated;the TABLE 1. Annual estimates of mean clutch size,
meanbrood size,and proportionof nestssuccessnearestmajor breeding areasare in Italy and Spain
ful for Little Egretsin southernFrance.Numberof
more than 400 km from the Camargue.
nestsis in parentheses.
Breeding
population
size.--LittleEgretsin the Camarguenestin trees,usuallyin mixedcolonieswith
Nesting
Cattle Egrets,Black-crownedNight-Herons(NyctiYear
Clutch size
Brood size
success
coraxnycticorax),and SquaccoHerons (Ardeolarallo~
1970
4.30 (156)
-0.96 (142)
ides).Eachyear from 1967 to 1998,all colonieswere
1971
4.39 (57)
-0.90 (58)
locatedthroughoutthe study areausinga smallair1972
4.34 (215)
-0.91 (115)
craft(Hafneret al. 1994).FromMay to mid-July,each
1973
---colonywas censusedweekly by the sameobserver
1974
----
(Hafner
197•).Eachcolony
waspartitioned
intosec-
torsof approximately0.1 ha. Completenestcounts
were conductedwithin each sector,and any new
nestswere added to the previouscountduring each
subsequentvisit. Based on repeated counts and
marking of nestsin a sampleof sectors,we believe
that overestimationwasextremelyunlikelyand that
the extent
of underestimation
was not substantial
(e.g.<10% in the largestcolonies).
Environmental variables.--We
used the cumulative
absolutevalue of daily minimum temperaturesbe-
low 0øC(i.e. the sumof thenegativevalues)asan indicationof the severityof cold winter weather(Hafner et al. 1992, 1994). In accordancewith Hafner et
al. (1994), we used total cumulative rainfall between
Februaryand April as an indicationof spring rain-
fall. A strongwind, knownas the Mistral, occursin
the Camarguethroughouttheyearand canbe a substantialsourceof nestingfailure (Valverde1955,Hafner 1978). Thus, we used the cumulativesum of the
maximumdaily wind speedduringthemainnesting
period (15 April to 15 June)as a measureof wind effects.Climatic data were provided by the Tour du
Valatmeteorologicalstation,which is locatedin the
centerof the Camargue.
Reproductive
parameters.--We
assessed
clutchsize,
brood size, and nestingsuccessfor a sampleof colonies during most years (Table 1). Nests were
markedalong transectsand their contentschecked
once a week. To reduce disturbanceearly in the
breedingcycle,we recordedclutchsize at 10% or
fewer of the number of nestsin large coloniesand
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
-------4.16 (58)
-3.80 (51)
-4.71 (135)
4.43 (30)
4.20 (97)
3.83 (100)
4.17 (106)
3.86 (117)
4.02 (120)
3.85 (121)
4.03 (121)
4.13 (104)
3.92 (79)
3.45 (60)
3.99 (116)
3.02 (176)
3.30 (152)
3.19 (117)
3.10 (80)
3.05 (150)
3.06 (178)
2.90 (221)
2.89 (189)
3.09 (144)
2.60 (279)
3.31 (185)
3.35 (155)
3.27 (137)
3.12 (136)
2.98 (160)
2.97 (207)
2.79 (163)
2.69 (199)
3.02 (325)
2.78 (297)
2.63 (238)
2.66 (277)
2.42 (358)
2.47 (283)
-------0.95 (57)
-0.91 (54)
-0.94 (156)
0.88 (49)
0.92 (122)
0.88 (117)
0.85 (148)
0.63 (173)
0.81 (155)
0.89 (132)
0.92 (122)
0.78 (147)
0.78 (151)
0.79 (73)
0.86 (121)
linear models to assess effects of environmental
and
density-dependentexplanatoryvariableson clutch
sizeand brood size.We alsoincludedtime (year)and
colonyeffectsas potentialexplanatoryvariables.We
usedlogisticregressionto explorethe factorsinfluencingnestingsuccess.
Model selectionwasbasedon
Akaike's Information Criterion (AIC; Akaike 1973,
Burnhamand Anderson1998).We checkedthe valid-
20% or fewer
in small colonies. We defined brood
ity of a Poissonassumptionusing the model devisizeas the numberof chicksalive at 20 to 25 daysof ance divided by its degreesof freedom (SAS 1997,
age.After this age,chickscanof escapeby walking Burnham and Anderson 1998). We also checkedfor
and may not be presentat nests(H. Hafner unpubl. temporal autocorrelationby testingthe association
data).We considereda nestto be occupiedwith the between the residuals of fitted models at times t and
laying of the first egg and successful
if at leastone t-1.
youngreachedthe ageat whichit wasableto escape
by walking. Becausecompletecensuseswere conRESULTS
ducted weekly and nestswere marked early during
incubation,we did not expect a bias from finding
nestsin latterstages.Thus,we did not usetheMayfield (1961)estimator.Rather,nestingsuccess
wasestimatedasthe proportionof occupiedneststhatwas
Our
selected
model
indicated
that
mean
clutchsizewaspositivelyassociated
with rain-
fall (X 2 = 11.67, df = 1, P < 0.001) and negasuccessful.
tively associated
with thetotal numberof Little
Dataanalysis.--Datawere analyzedwithin a gen- Egret nests(X 2 = 8.85, df = 1, P = 0.003;Fig.
eralized linear modelingframework.We used log- 1). The model did not exhibit a substantial
636
BENNETTS
ETAL.
[Auk, Vol. 117
3.6
4.8
3.4
4.6
3.2
4.4
ß
4.2
ß
2.8
4.0
2.6
ß
ß
ß
3.8
2.4
2.2
3.6
50
100
150
200
250
300
350
0
1000
2000
3000
4000
5000
6000
Total Numberof Little Egret Nests
Total rainfall(January-April)
FIc. 2.
4.8
Annual
brood size relative to totalnumber
of Little Egretnestsin the studyarea.
4.6
N
•.• 4.4
our selectedmodel becauseof its significance
•
in an interaction
4.2
term with the number of Little
Egret nests (X2 = 8.31, df = 1, P = 0.004). Our
final model indicateda small degreeof over-
e) 4.0
dispersionwith respectto a Poissonassumption (deviance/df = 2.3); however,adjusting
for overdispersionusing a quasi-likelihood
3.8
3.6
0
1000
2000
3000
4000
5000
6000
framework (SAS 1997,Burnham and Anderson
Total Numberof LittleEgret Nests
1998)did not changewhich modelwas selected, and the significanceof explanatoryeffects
(upper)andtotalnumberof Little Egretnestsin the changedonly slightly.As with clutchsize,we
found no evidenceof temporalautocorrelation
study area (lower).
FIG. 1.
Annual clutch size relative to total rainfall
of the residuals.
Our final modelof nestingsuccess
indicated
amount of overdispersionwith respect to a an effect of the total number of tree-nesting
Poissonassumption(deviance/df = 1.3), nor herons,the proportionof the colonyconsisting
was there evidenceof temporalautocorrelation of Cattle Egrets,wind speed,and severalinterof the residuals.
actionsamongthesevariables(Table2). The toBrood size also was negatively associated tal numberof tree-nestingheronsand the prowith the total numberof Little Egretnests(X2 portion of Cattle Egrets in colonieswere neg= 18.36, df = 1, P < 0.001;Fig. 2). Cumulative ativelyassociated
with nestingsuccess
(Fig.3).
rainfallwasnot significantasa maineffect(X2 Wind speedwas not significantas a main ef= 0.25, df = 1, P = 0.617) but was retained in fect, but it contributedstrongly to two interTABLE
2. Maximum-likelihood
analysisof variancetablefor final logisticregression
modelof nestingsuccessof Little Egretsin southernFrance.
Source
Year
Totalpopulationsizeof tree-nesting
herons(TPS)b
X2a
df
8.48
I
P
< 0.001
2.26
I
Proportionof colonyconsistingof Cattle Egrets(PCE)
18.29
1
<0.001
Wind
TPS
PCE
TPS
0.87
33.15
29.56
41.86
1
1
1
1
0.350
<0.001
<0.001
<0.001
b
x PCE
x wind
x PCE x wind
Basedon type3 likelihood-ratio
statistics
andthusnot dependent
on orderwithin model(SAS1987).
Not significantat ct = 0.05 asmain effect,but retainedbecauseof significancein interactions.
0.133
July2000]
Reproductive
Success
ofLittleEgrets
1
637
ing populationsize of Little Egretsin the Camargue(Hafner et al. 1994).However,our analysis did not indicatea subsequentinfluenceof
winter temperature on reproductive parameters.An associationbetweenrainfall preceding
the breedingseasonand reproductivesuccess
also has been reportedfor severalspeciesof
herons(e.g. Ogden et al. 1980,Bancroftet al.
1988, Bildstein et al. 1990, Maddock and Baxter
ß
0.6
2000
4000
6000
8000
1 oooo
1991),includingthe Little Egret (Hafner et al.
1994).Rainfall was the primary environmental
influence we observed on clutch size and brood
Number of Tree-Nesting Heron Nests
size.Yearsof higherrainfallcorrespond
with a
greater surface area of Little Egret foraging
habitat (Hafner et al. 1994),and prolongedperiods
without
surface
water
in
freshwater
marshescan result in prey die offs (Frederick
0.9
andCollopy1989).Althoughrainfallpreceding
the breedingseasonmay havepositivebenefits
relatedto foraginghabitat,rainfall duringthe
breeding seasonhas been known to result in
colonyabandonmentin someNorth American
wetlands (e.g. Frederick and Collopy 1989).
Such an effect was not apparent in the Ca-
0.8
0.7
0.6
0
0.2
0.4
0.6
0.8
margue.
Proportionof Cattle Egrets
Wind had a significanteffecton nestingsuc-
F1G.3. Annualproportionof neststhatweresuccess,but only as an interactionterm with dencessfulrelative to total number of tree-nestingherons(upper)and proportionof eachcolonyconsisting sity- dependenteffects.High winds canresult
in collapsedneststructures,particularlywhen
of Cattle Egrets(lower).
action effects. In contrast to clutch and brood
size,theresidualsof ourfitted modelof nesting
success exhibited temporal autocorrelation
without the inclusionof an additionalyear effect, which was also supportedby AIC and
therefore
included
in
our
final
model.
An
nestsare locatedon the edgeof a colonywhere
they havea more directexposurewithout the
sheltering effects of adjacent trees. Mistral
windsin the Camarguetypicallyare outof the
north. Thus, nests on the north ends of colonies
also are more likely to be susceptibleto wind
damage.During someyears(e.g. 1991),high
winds can cause a substantial
decrease in over-
anomalousevent (seen only once since 1967)
all nestingsuccess.
occurred during 1986 where Eurasian JackDensity-dependent
effects.--Density-dependent
daws (Corvusmonedula)
nestingin proximity to
reproductionhas been observedin many anione small colony destroyedour entire sample
mal populations (Sinclair 1989, Woiwod and
of nestsfor that colony.Becausethe jackdaws
Hanski 1992, Holyoak and Baillie 1996). Deprobablyused the disturbanceof our visit to
spite evidencefor density-dependenteffectson
gainaccess
to nestswhile the parentswereoff
their nests,we excludedthis colonyfrom our
analysis.Removalof this colonyfrom analysis
did notalterourgeneralresultsbutwouldhave
introducedan additionalcolonyeffectand col-
reproductionin otherwaterbirds(Gastonet al.
1983, Birkhead and Furness 1985, Hunt et al.
1986),evidencegenerallyhas beenlackingfor
sucheffectson ardeids.Recently,Butler(1994)
noted the generallack of evidencefor density
ony x year interaction.
dependence in the reproduction of wading
birds. Although our data are correlative,and
DISCUSSION
inferencesregarding causalityare therefore
Environmental
effects.--Severely
cold winters limited, they are in contrastto Butler'sassessreportedlyaffectwinter and subsequentbreed- ment in that they indicate density-dependent
638
BENNETTS
ETAL.
effectson reproductionof Little Egretsin the
Camargue.Virtually all of thereproductive
parameterswe evaluatedwere negativelyassociated with the numberof nestingLittle Egrets
and/or othertree-nestingherons.In eachcase,
the association
was explainedbetter by numbersof heronsthan by a linear trend overtime
(i.e. clutchand brood size), or was in addition
to a linear trend (i.e. nestingsuccess).
Competitionfor nestsitesin the Camargue
may occurbecausesuitableundisturbedstands
of trees are quite limited (Hafner 1982). Given
[Auk,Vol. 117
ACKNOWLEDGMENTS
We appreciatethe helpful commentsof Ga•tan Lefebvre,DouglasMock,BrigittePoulin,andtwo anonymousreviewers.This work was carriedout under
licenseof the FrenchMinistry of Environmentand
theCRBPO(FrenchNationalRingingCenter).Weare
gratefulfor the supportprovidedby StationBiologique de la Tour du Valat.
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Editor:J. S. Sedinger