REJECTION BEHAVIOR BY COMMON CUCKOO HOSTS TOWARDS ARTIFICIAL BROOD PARASITE EGGS ARNE MOKSNES, EIVIN ROSKAFT, AND ANDERS T. BRAA Departmentof Zoology,University of Trondheim,N-7055 Dragvoll,Norway ABSTRACT.--Westudied the rejectionbehavior shown by differentNorwegian cuckoo hosts towardsartificial CommonCuckoo (Cuculus canorus) eggs. The hostswith the largestbills were graspejectors, those with medium-sizedbills were mostlypuncture ejectors, while those with the smallestbills generally desertedtheir nestswhen parasitizedexperimentally with an artificial egg. There were a few exceptionsto this general rule. Becausethe Common Cuckooand Brown-headedCowbird (Molothrus ater) lay eggsthat aresimilar in shape,volume, and eggshellthickness, and they parasitizenests of similarly sizedhost species,we support the punctureresistance hypothesis proposed to explain the adaptivevalue (or evolution)of strengthin cowbirdeggs. The primary assumptionand predictionof this hypothesisare that somehosts have bills too small to graspparasitic eggs and thereforemust puncture-eject them,and that smallerhosts do notadopt ejection behavior because of the heavycost involved in puncture-ejectingthe thick-shelledparasitic egg. We comparedour resultswith thosefor North AmericanBrown-headed Cowbird hosts and we found a significantlyhigher propor- tion of rejectersamong CommonCuckoo hosts with graspindices (i.e. bill length x bill breadth)of <200 mm2. Cuckoo hosts ejected parasitic eggs rather than acceptthem as cowbird hostsdid. Amongthe CommonCuckoo hosts, the costof acceptinga parasiticegg probably alwaysexceeds that of rejectionbecause cuckoo nestlings typically eject all hosteggs or nestlingsshortly after they hatch.Received 25 February1990, accepted 23 October1990. THEEGGS of many brood parasiteshave thick- nestseither by grasping the eggs or by punc- er shells than the eggs of other bird speciesof turing the eggs before removal. Rohwer and similar size (Lack 1968,Spaw and Rohwer 1987). Spaw (1988) used this distinction in ejection Severalhypotheses have been developed to ex- type when they considered the possibility of plain this phenomenon,the mostrecent being physicalconstraints to ejectionfor thosespecies that of Spawand Rohwer (1987).Spaw and Roh- parasitizedby Brown-headedCowbirds. They wer (1987)and Rohwerand Spaw(1988) argued compared the characteristictype of ejection that the thick eggshellof the parasiticAmerican (graspor puncture)or lack of ejectionresponse cowbird (Molothrus)species has evolved so as (acceptance)with the bill size for each of 40 to resistpuncture ejection by small host species. parasitizedpasserine species. They suggestthat They tested an assumptionof this hypothesis small bill size constrains some species from by measuring the length and the width of the grasping the cowbird eggs for ejections,and bill (the product of these two measurements that the strength of the cowbird eggslimits suc- they termed the "graspindex") of Brown-head- cessfulpuncture ejections for mostof thesespe- ed Cowbird (M. ater) hosts which had been clas- cies. Rohwer and Spaw (1988) propose that the sified as acceptorsor rejecters.They concluded costs associated with these constraints have se- that some small-sized hosts are more or less lected for acceptance.It is not clear from their forced to be acceptorsbecause of heavy cost indirect testwhich acceptorspecies are capable involved in getting rid of the thick-shelled of successfullypuncturing cowbird eggs for Brown-headedCowbird egg. This hypothesis ejections(and would do so, given sufficientse- has received support from Picman (1989) and lective pressure)and which acceptorspecies Rohwer et al. (1989). cannot puncture the cowbird egg becausethe Rothstein (1975, 1976, 1977) showed that, al- eggshell is too strong. though many parasitizedspecies accepted (or So far only one host speciesof the North did not remove) introduced nonmimetic arti- American Brown-headed Cowbird, the North- ficial eggs of Brown-headed Cowbirds, some ern Oriole (Icterusgalbula), has been shown to speciesejected them. He observedand inferred be a true puncture ejector (Rothstein 1977). To that thesepotential hosts ejected eggs from their test whether Northern Orioles experienceany 348 The Auk 108: 348-354. April 1991 April 1991] Parasite-EggRejection in Cuckoos 349 cost in puncture-ejecting the thick-shelled tance should therefore strongly select for an Brown-headed Cowbird egg, Rohwer et al. ejection responsein those speciescapable of (1989) added Brown-headed Cowbird and con- ejectingcuckoo eggs (Davies and Brooke 1989). trol eggs into oriole nests,and found that the One prediction for Common Cuckoo hosts is host speciesoccasionally damaged some of its that large-billed hosts,which can graspthe par- own eggsin the processof ejectingthe cowbird asitic egg and eject it, would be expectedto do egg. so; intermediate-billed hosts, which cannot The Common Cuckoo (Cuculuscanorus) is the graspthe parasiticegg, should puncture-eject; most abundant brood parasite in Europe, and and hosts with the smallest bills, which cannot eggsfrom this specieshave been reported from eject the parasiticegg, will desertthe nest. The nestsof > 100 different host species,but cuckoo thresholdfor initiating ejectionbehavior should chicks have not been observed in nests of all be lower for cuckoo hosts than for cowbird hosts. these potential hosts (Baker 1942, Lack 1968, Wyllie 1981). Unlike the Brown-headed Cow- MATERIAL AND METHODS bird, the Common Cuckoois regardedas a host specialist,laying eggsthat normally mimic those The fieldwork of this study was carried out in both of the hosts.Because successful parasitism by mountain and lowland areas in Central Norway the cuckooreduces the host'sbreeding success (Moksnes and Roskaft 1987, 1988, 1989; Moksnes et al. 1991). dramatically,natural selectionwill be expected We introducedartificial cuckooeggs into the nests to favor host defense mechanisms that reduce of 19 species.The eggswere made of araldite (a hard the probabilityof being parasitized(Davies and plastic)to which a small amount of fiberglasspowder Brooke 1989, Moksnes et al. 1991). as well as ground color, matching that of normal Davies and Brooke (1989) and Moksnes et al. cuckooeggs, had been added. The eggswere castin (1991) have shown that many speciesparasit- lead molds lined with a layer of silicone rubvet. A ized by the Common Cuckoo discriminateand mixture of glycerol and albumen was injected into rejectnonmimetic artificial cuckooeggs exper- the eggs.Afterwards they were painted to resemble imentally introduced into their nests. Further- cuckooeggs. They were of the samesize and weight more, cuckoo hosts with shorter bills were more asnatural CommonCuckoo eggs (for a more detailed description,see Moksnes and Roskaft1988, 1989). The likely to rejectby desertion,while specieswith plasticeggs were, however,more resistantto destruc- longer bills ejected cuckoo eggs (Davies and tion than natural ones;very few of the host species Brooke 1989). were ableto puncturethese artificial eggs. The species We parasitized experimentally 19 Common were parasitizedwith eggs painted to resemble dif- Cuckoohosts with artificial cuckooeggs and 3 ferent host species,and could therefore be mimetic hostswith artificial or natural conspecificeggs. or nonmimeticcompared with thoseof the hosteggs. We recordedthe rejectionbehavior of the hosts. We report only those caseswhere rejection occurred. From the results of these experiments, we eval- We carried out the experiments during the egg- uated, with comparative analysis, possible laying and incubation periods in 1986-1990. During physical constraintsin rejection behavior of the egg-layingperiod the eggswere exchangedafter the hosthad laid its fourth egg.Because of difficulties smallbill size.Finally, we comparedour results in locating nests during the laying period, some of with those reportedfor cowbird hosts. the artificial parasitismexperiments had to be made If we assumethat cuckooeggs are similar in during the incubationperiod also(see Moksnes and size, shape, and eggshell thickness(strength) Roskaft [1989] and Moksnes et al. [1991] for the dis- to those of cowbirds, and that the most com- tribution of theseexperiments according to the laying monly parasitizedspecies are small passetines, and incubationperiods of the hosts).There was no it is reasonableto expect that potential cuckoo differencein the rejectionbehavior according to stage hostsexperience similar constraintsin ejection in the incubation period, but some speciestended to behavior. However, unlike cowbird hosts, which accept at a higher rate during the last days before may successfullyraise some of their own off- hatching (Moksnes et al. 1991;but seealso Davies and Brooke 1989). When nests were first visited, we spring along with the cowbird nestling (e.g. cordedthe number of hosteggs. The eggswere float- Mayfield 1961, Rothstein 1975, Clark and Rob- ed (Hays and Lecroy 1971)to determineif they were ertson 1981), there is little reproductive success freshly laid or had been incubated. By floating the to a hostthat acceptsa cuckooegg becausecuck- eggsor by examiningthe embryos,we were able to oo nestlingsnormally ejectall hosteggs or nest- estimatethe laying datesfor each of the nestsin our lings shortly after they hatch. Costs of accep- sample. 350 MOKSNES,ROSY, AFT, AND BR•,• [Auk,Vol. 108 Rejectionbehavior towards artificial and conspecific dinavian museums.Egg volume was estimatedby the eggs.--We removed one of the host eggs and added formula(Hoyt