55 Mixed clutchesin shorebird nests:why are they so uncommon? Juan A. Amat Amat,J.A. 1998.Mixed clutchesin shorebirdsnests: why arethey so uncommon? Wader Study Group Bull. 85: 55-59. Mixedclutches, in whichtwo femalesdeposit their eggs in thesame nest, have been recorded in manyshorebird species, but their frequencyof occurrenceis low in comparisonwith thatof otherprecocial birds (e.g. ducks). This paper provides new observationsof mixedclutches both between and within species, for KentishPlover Charadrius alexandrinus, Black-winged Stilt Himantopus himantopusand Avocet Recurvirostra avosetta. I arguethat mixed clutches are relatively uncommon among shorebirds because of a limitationin theparental ability to carefor verysmall chicks when broods are large, rather than by a limitationin the abilityto incubate large clutches. JuanA. Amat, EstacirnBioldgica de Do•ana, C. S. I. C., Apartado1056, E-41080 Sevilla,Spain. E-mail; [email protected] INTRODUCTION There are many factorsaffecting the rate of nestparasitism, Facultativenest parasitism among birds may be eitherintra- or someof which includeconditions during nesting, such as the interspecific.Intraspecific nest parasitism has been reported in utilisationof similarnesting habitats by hostsand parasites, the many avian species,both altricial and precocial (Yom-Tov easewith which neststo be potentiallyparasitised are found, 1980;Rohwer & Freeman1989). Facultativeinterspecific nest laying synchronisationbetween hosts and parasites, etc. (e.g. parasitism,however, is mainly limited to precocialspecies, Weller 1959; Sayler 1992). Theseconditions should be met mainly belongingto the familiesAnatidae and Phasianidae particularlyin the caseof speciesnesting at high densities, (e.g. Weller 1959; Sayler 1992). In manyprecocial species, the suchas colonial nesting birds (Brown 1984;Emlen & Wrege chicksare not fed by their parents,thus the costsof nest 1986; Lyon & Everding 1996). parasitismshould not be as high for precocialas for altricial species. Severalhypotheses have been advancedto accountfor This notereports on the occurrenceof mixed clutchesin the the occurrenceof facultativenest parasitism among precocial nests of Kentish Plovers Charadrius alexandrinus, Black- birds,and these may be groupedinto a numberof categories wingedStilts Himantopus himantopus and Avocets (reviewedby Sayler1992). One categoryconcerns Recurvirostraavosetta in two wetlandsof southernSpain. competitionfor nestingsites, and suggeststhat whenthe Examplesof mixed clutchesin the literatureare reviewedto availabilityof nestingsites is limited,two femalesmay be examinethe reasonswhy mixed clutchesare relativelyrare forcedto lay in the samenest. Anothergroup of hypotheses amongshorebirds (Charadrii), even among colonially nesting addressesthe behaviouralstrategies adopted by parasitic species. nestingfemales; these strategies may be either alternativeor conditional.Individuals adopting alternative strategies usually METHODS initiatelaying in the nestsof otherindividuals, and after laying The observations were made in the marshes of the someeggs in suchnests continue laying, in somecases almost Guadalquivir(3707 ' N, 6ø 12' W) andFuente de Piedralake without interruption,in their own nests(Weller 1959; Clawson (3706' N, 4045' W). Descriptionsof thesesites may be foundin et al. 1979;Hori 1987;Sorenson 1991). Individualsadopting Amat (1985) and Fraga & Amat (1996). conditionalstrategies usually parasitise nests of otherswhen thereis no possibilityof initiatingnormal nesting; therefore Nests were considered to contain mixed clutches of suchindividuals would be makingthe 'bestof a bad situation' conspecificswhen (1) morethan one egg appearedduring a (Amat 1985; Lank et al. 1989;Briggs 1991; Sorenson1991; singleday in the nest,(2) therewere clear differencesin size, Lyon 1993). In the caseof shorebirds,it hasbeen suggested backgroundcolour and/or spotting pattern between some that,when females lose their nests to predatorsduring the egg(s)and the rest of the clutch(but seeMcRae 1997), and (3) layingperiod, they may depositthe remainderof the clutchin clutchsizes were largerthan the mode, i.e. >3 eggsfor Kentish nestsof other shorebirds(Colwell 1986). If so, such Plovers,and >4 eggsfor Black-wingedStilts andAvocets. In individualswould be adoptinga conditionalparasitic nesting all casesin which clutchsizes were largerthan the mode,it was strategy.In othercases, the occurrenceof superclutcheshas possibleto separatetwo groupsof eggsin the clutch,based on beenattributed to female-femalepairings or polygynous size,background colour and/or egg spottingpattern (usually by matings(Scott 1974; Walters& Walters 1980; Warrineret al. at leasttwo criteria). Assignmentof eggsto individualKentish 1986). Ploverfemales was possiblewhen females were capturedmore thanonce at nests,since egg sizecharacteristics (length, 56 breadthand volume) are highlyrepeatable between clutches in only threeeggs hatched from the nest. the KentishPlover (J. A. Amat, R. M. Fraga & G. M. Arroyo, unpubl.). In the caseof KentishPlovers, laying dates were Also at Fuente de Piedra, two stilt nestswere found each estimatedby an equationgiven in Fraga& Amat (1996), based containingeggs of otherspecies. One of thesenests had four on observedegg masses in relationto egg volumes,for those stilt eggsand an Avocetegg, andwas desertedwhen found. cases in which nests were found once the clutch was This was the singlestilt neston an islet where therewas a completed.In the Guadalquivirmarshes, nests were visitedat smallcolony of Avocets(four nests).About 1.5 m from the irregularintervals, while at Fuentede Piedra,nests were visited stilt nest,there was anAvocet nest with threeeggs very similar every 3-6 daysto determinetheir outcome. to the avocetegg foundin the stilt nest. In the marshesof the Guadalquivir,only informationon stilt The otherBlack-winged Stilt nestcontained two Kentish clutcheswas gathered,whereas at Fuentede Piedra Plovereggs and four stilt eggswhen discovered on 16 May; informationfor all threespecies was recorded. anotherplover egg wasfound about 1.5 m from the nest. On 20 May the two plovereggs were cracked,and two dayslater RESULTS oneplover egg had disappearedfrom the nest. On 5 June,the Kentish Plover remainingplover egg had disappeared and two recently During a studyon the breedingbiology of this speciesin 1991- hatchedstilt chickswere on the nest. Peepingvocalisations 1996, nine nests(1% of 883) were foundcontaining eggs of were heardfrom the othertwo stilt eggsand they hatchedlater morethan one female. Threeof the nestshad six eggs,one had in the day. Only the stiltswere recordedincubating in this nest. five eggs,three had four eggs,one had threeeggs, and one nest had two eggs. In two of the nestswith six eggs,three eggs All the nests with mixed clutches in Fuente de Piedra were were depositedby a secondfemale in desertednests that recordedin 1996, and represented5% of 57 Black-wingedStilt alreadycontained three eggs; these two nestswere nestsfound there that year. subsequentlyabandoned. In the othernest with six eggs,two femaleslaid on alternatedays; the first femalewas captured Avocet duringincubation. This female had beencaptured in another An Avocetnest was foundwith five eggsduring mid- nestin the samenesting season, as well as in previousand incubation.Three eggs were from one femaleand two from subsequentbreeding seasons. This last nestwith six eggswas anotherfemale. The first setof eggshatched about 48 hours predatedduring incubation. before the other. This nest was found in 1996 and was located on an islet wherethere was a mixed colonyof sevenAvocet In the nestwith five eggs,two femaleslaid simultaneously,but and nine stilt nests. It represented2% of 41 Avocetnests found the nest was deserted before incubation started. The outcome in Fuente de Piedra in 1996. of the remainingfive nestswith mixed clutcheswas variable but, in all the nests,incubation started, and only a pair of adults was capturedincubating at eachone. Two of the nestswith DISCUSSION four eggsand the nestwith threeeggs were successful,a nest Some of the cases of mixed clutches in the shorebird nests that with four eggswas desertedduring mid-incubation, and the were observedcould be accountedfor by the competitionfor nestwith two eggswas predated. This lastnest was discovered nestingsites hypothesis, but the ecologicalconditions duringincubation, and it couldhave suffered partial predation promotingthe occurrenceof suchcompetition varied between (see Fraga & Amat 1996). differentcases. For example,superclutches of Black-winged Stiltswere recordedin the marshesof the Guadalquivirin a Black-wingedStilt year of low waterlevels (1984), but not in the subsequentyear During studiesof otherwaterbird species, nests of this species whenwater levelswere higher. Under drier conditions,the were foundwith mixed clutches,either of conspecificsor of frequencyof two femaleslaying in the samenest possibly othershorebird species. In 1984, in a colonyof 24 nestsin the increaseddue to a limitationin the availability of nestingsites marshesof the Guadalquivir,two nestswith seveneggs were (Colwell 1986). found. Ten daysafter discovery,only oneegg remained in each of these nests. The fate of these nests could not be In contrast,at Fuente de Piedra, all casesof nestswith mixed determined,as the colonywas visited at irregularintervals. No clutchesof bothBlack-winged Stilt andAvocet were recorded nestswith a superclutch(i.e. >4 eggs)were recordedin 1985, duringa year of high water levels(1996)