sign in sign up
<<
Home , Bluethroat, Brambling, Brood parasite, Common cuckoo, Cowbird, Fieldfare

REJECTION BEHAVIOR BY COMMON HOSTS TOWARDS ARTIFICIAL

ARNE MOKSNES, EIVIN ROSKAFT, AND ANDERS T. BRAA Departmentof Zoology,University of Trondheim,N-7055 Dragvoll,

ABSTRACT.--Westudied the rejectionbehavior shown by differentNorwegian cuckoo hosts towardsartificial CommonCuckoo ( 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 . 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 ,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 hosts and we found a significantlyhigher propor- tion of rejectersamong 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 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 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 , 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 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 ,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 , and and hosts with the smallest bills, which cannot eggsfrom this specieshave been reported from eject the parasiticegg, will desertthe . 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 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 thosereported for 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 1979),V = 0.51.length. breadth 2.1,000 -•. either one mimetic or one nonmimetic plastic Com- In addition egg shapewas estimatedaccording to the mon Cuckooegg. In the ( montifrin- formula, egg shape= length/breadth (Picman 1989). gilla) and the (F. coelebs),we per- Eggshellthickness was measured to the closest0.001 formed additional experiments with plastic eggs mm with a Model 35 FederalBench Comparator thick- similarto conspecificeggs in sizeand colorpatterns. nessgauge (see Spaw and Rohwer 1987, for a closer Each nest was then visited every secondday for at descriptionof this method). least 6 days after the artificial egg was introduced. If no rejection behavior was observed by the sixth day, the parasiticegg was consideredaccepted (Moksnes et al. 1991). RESULTS Becausethe plasticeggs were difficult for the host to puncture-eject, we observed only three different Eggrejection.--We tested the rejection behav- patterns of rejection behavior: (1) The artificial egg ior of 19 speciestoward the artificial Common was removed from the nest, and all the host's eggs Cuckoo egg. Of these, 6 specieswere selective remained unharmed. Such behavior was defined as ejectors,7 were unselectiveejectors, and 6 were selectiveejection. (2) The artificial egg was either re- deserters (Table 1). moved from the nest or left in the nest. In both cases (n = 20), chaffinches(n = 13), and however, one, several,or all of the host'seggs were reed-buntings (n = 3) were also testedwith con- destroyed or removed. In the majority of such cases, the artificial egg was left in the nest. Such rejection specificeggs of normal size and eggshellthick- behavior was defined as unselectiveejection, irrespec- ness.All 36 were rejected,23 by selectiveejec- tive of whether or not the hostsubsequently deserted tions. After ejection,in one nestof eachspecies its nest. (3) The nest was abandoned, but the nest one of the host'seggs was missingalong with contentswere left unharmed. Suchrejection behavior the parasitic egg. In another Brambling nest, was defined as desertion. one of the host'sown eggswas missing.These Rothstein (1975) found that rejection or acceptance four rejectionswere classifiedas unselective behavior by Brown-headed Cowbird hosts was nor- ejections.The remaining nests were deserted. mally an all-or-none response. However, in some The Brambling and the Common Chaffinch Common Cuckoo hosts,this pattern is not so clear were also tested with artificial plastic eggs that (Davies and Brooke 1989, Moksnes et al. 1991). We classifieda speciesas an acceptorwhen a nonmimetic mimicked conspecificeggs. In the Brambling, artificial cuckoo egg was acceptedin >50% of the rejections occurred in 7 of 10 experiments. In experiments(Moksnes et al. 1991).Furthermore, clas- the Common Chaffinch, 8 of 11 experiments siftcation as a selective/unselective ejector,or desert- produced rejection,all by unselectiveejection. er, dependedon how the majority(see Table 1) of the Eggmorphology.--Brown-headed Cowbird and rejectingindividuals behaved,regardless of whether Common Cuckoo eggs are similar in many re- the speciesas a whole was an acceptoror not. spects. The eggshell thickness of Common For three species--the Brambling, the Common Cuckoo eggs is 0.108 mm (SD = 0.005, n = 10) Chaffinch, and the Common Reed-(Emberiza or slightly thinner than that of the Brown-head- schoeniclus)--somefurther experimentswere carried ed Cowbird (0.110 mm, Spaw and Rohwer 1987; out using natural conspecificeggs. The samedefini- tions in rejectionbehavior were usedas for artificial 0.125 mm, Picman 1989). The eggshellsof both the cuckoo and the cowbird are thicker than eggs. Bill sizeand eggmorphology.--We measured the fe- comparablespecies (cf. Spaw and Rohwer 1987). males of the host species(taken from museum col- On average, cuckoo egg volume (3.078 cm3;SD lections) by Rohwer and Spaw's (1988) method, to = 0.467, n = 424) is a little larger than that of record their grasp-index values. Tomial length was the Brown-headed Cowbird (2.838 cm3; Picman determined from the commissuralpoint at the corner 1989). The general shapeof the Common Cuck- of the mouth, diagonallyto the tip of the upper man- oo and Brown-headed Cowbird egg is similar, dible (to _+0.1 mm). Bill breadth was the distance although the cuckoo egg is slightly longer between the commissuralpoints (to _+0.1mm, Roh- (Common Cuckoo: 1.337 + 0.072 cm, n = 424; wer and Spaw 1988).Grasp index is the tomial length Brown-headed Cowbird: 1.306 cm, Bent 1958; multiplied by the commissuralbreadth. Except for two species,five females of each specieswere mea- 1.296 cm, Picman 1989). sured. Graspindex.--The smallestgrasp ejector (the We measuredegg length and breadth on 424 Com- Cedar Waxwing, Bombycillacedrorum) among the mon Cuckooeggs in the collectionsof different Scan- North American Brown-headed Cowbird hosts April 1991] Parasite-EggRejection in Cuckoos 351

TABLE1. Rejectionbehavior of different host speciestowards artificial Common Cuckooeggs. Abbreviations: n = number of rejectionsobserved, S = selectiveejection, U = unselectiveejection, D = desertion,RS = reactionstatus of the species,% A = percentageof acceptanceof nonmimeticcuckoo eggs as reportedby Moksneset al. (1991),A = acceptor,R = rejecter(for termssee Material and Methods).

Species n S U D RS (% A) Selectiveejectors (Turduspilaris) 3 2 -- 1 A (91) Song (T. philomelos) 8 7 -- 1 R (20) Blackbird (T. rnerula) 2 2 -- -- R (0) (T. iliacus) 25 22 -- 3 A (65) ( striata) • 6 4 1 1 R (56) (Lusciniasvecica) 14 14 -- -- R (47) Unselectiveejectors (Sylviaborin) 3 -- 3 -- R (33) Blackcap(S. atricapilla) 15 -- 11 4 R (23) (Hippolaisicterina) 5 1 2 2 R (33) CommonChaffinch (Fringilla coelebs) 24 2 15 7 R (31) Brambling(F. rnontifringilla) 40 4 28 8 R (10) (Ernberizacitrinella) 8 1 4 3 R (0) Common Reed-Bunting(E. schoeniclus) 21 1 16 4 R (9) Deserters (Anthuspratensis) 20 -- 1 19 A (92) Yellow Wagtail (Motacillafiava) 4 -- 1 3 R (20) (M. alba) 3 -- -- 3 R (0) (Carduelischloris) 7 -- 2 5 A (59) Warbler (Phylloscopustrochilus) 20 -- 1 19 R (10) Chiff Chaff (P. collybita) 6 -- I 5 R (0) Uncertain status (Prunella rnodularis)2 0 ------A (100) Lapland (Calcariuslapponicus) 0 ------A (100) The SpottedFlycatcher is regardedas a rejecterspecies because Davies and Brooke(1989) reported90% rejectionin this species. The Dunnockhas an uncertain-rejecterstatus in our sample,but Daviesand Brooke(1989) reported two casesof desertionin the species. has a grasp index ca. 230 mm2 (Rohwer and largest-billed speciesand selective ejectorswas Spaw 1988).All four speciesin our samplewith statisticallysignificant (Mann Whitney U-test; a grasp index of >230 mm 2 were selective ejec- U = 74, n = 6, 13, P < 0.001; Tables 1, 2). Sim- tors (Table 2). The Bluethroat ( svecica; ilarly, the finding that the smallest-billed hosts index = 131.1 mm 2) and the Spotted Flycatcher were deserterswas almost statisticallysignifi- (Muscicapastriata; index = 179.3 mm2) were se- cant (U = 61, n = 6, 13, P < 0.07). However, lective ejectors.These grasp-index values are far when we comparedthe unselectiveejectors with below those of the North American grasp ejec- the deserters, we found that the unselective tors. The grasp-indexvalue for the Bluethroat ejectorshad grasp indices larger than that of is also lower than for some of the unselective the deserters, but this difference was not statis- ejectorspecies we identified. tically significant (U = 26, n = 6, 7, NS). On the All 7 unselectiveejectors (Table 1) had grasp other hand, the findings that the unselective indices between 78.8 and 147.9 mm 2 (Table 2). ejectorshad grasp indices smaller than that of These values are lower than those of the North the selectiveejectors was statisticallysignificant American puncture-ejector species (Northern (U = 39, n = 6, 7, P < 0.01). Oriole, 176.0 mm2;Rohwer and Spaw 1988). Rejectionbehavior of cuckooand cowbird hosts.- The 5 specieswith the largest grasp indices We regarded the species--all of which are po- selectivelyejected the artificial Common Cuck- tential Common Cuckoo hosts (see Moksnes et oo egg, while the two smallest-billed speciesin al. 1991 for definitions of potential hosts)--as the sample were both deserters (Tables 1, 2). small species if grasp indices fell below 200 When ranked according to their grasp index mm2. Thirteen were classifiedas rejectersand and rejection status,the correlationbetween the only four as acceptors(European Greenfinch, 352 MOKSi•IES,ROSlCA•r, A•D BPO.A [Auk,Vol. 108

TABLE2. Graspindices and bill-size measurements(2 + SD) of different CommonCuckoo hosts. Grasp index is the product of the diagonal length and commisuralbreadth, n = number of measured.

Grasp index Diagonal length Commisuralbreadth Species n (mm 2) (mm) (mm) Selective ejectors Fieldfare 5 348.0 25.4 + 0.7 13.7 + 0.3 5 321.3 25.7 + 0.6 12.5 + 0.6 Blackbird 5 411.4 29.6 + 1.3 13.9 + 0.7 Redwing 5 283.2 24.0 + 0.6 11.8 + 0.3 Spotted Flycatcher 5 179.3 18.3 + 0.9 9.8 + 0.6 Bluethroat 5 131.1 16.6 + 0.5 7.9 + 0.2 Unselectiveejectors Garden Warbler 5 147.9 15.9 + 0.4 9.3 + 0.7 Blackcap 5 135.5 15.4 + 0.6 8.8 + 1.1 Icterine Warbler 4 142.7 17.4 + 0.9 8.2 + 0.1 Common Chaffinch 5 111.8 13.8 + 0.5 8.1 + 0.4 Brambling 5 106.9 13.2 + 0.5 8.1 + 0.4 Yellowhammer 5 105.4 12.4 + 0.4 8.5 + 0.4 Common Reed-Bunting 5 78.8 10.8 _+0.3 7.3 _+0.7 Deserters Meadow Pipit 5 118.6 15.4 _+0.3 7.7 _+0.3 Yellow Wagtail 5 112.7 16.1 + 0.6 7.0 + 0.3 White Wagtail 3 117.9 16.6 + 0.8 7.1 + 0.4 European Greenfinch 5 134.4 14.3 + 0.7 9.4 + 0.5 5 70.9 13.9 + 0.7 5.1 + 0.4 Chiff Chaff 5 66.6 12.1 + 0.5 5.5 + 0.4

Uncertain status Dunnock 5 108.6 15.3 + 0.4 7.1 + 0.3 5 88.2 11.6 + 0.3 7.6 _+0.2

Carduelischloris; Meadow Pipit, Anthuspratensis; ejections.(In Northern Orioles [Rothstein 1977], Lapland Longspur, Calcariuslapponicus; Dun- however, some host's eggs were damaged or nock, Prunella modularis;Table 1). In Rohwer removed.) Western Kingbirds (Tyrannusverti- and Spaw's (1988) sample, 2 of the potential calis)and American Robins (Turdusrnigratorius) Brown-headedCowbird hosts with grasp in- did not damage any of their own eggs when dices of <200 mm2 were rejecters (i.e. if the ejecting introduced real Brown-headed Cow- Marsh Wren, Cistothoruspalustris, is also classi- bird eggs, whereas Northern Orioles did fre- fied as a puncture ejector),and 25 were accep- quently (Rohwer et al. 1989).In our study,when tors. The differencein frequencyof rejecters artificial eggswere rejectedfrom the nest, and amongsmall-billed European Common Cuckoo all the host'seggs remained unharmed (selec- hosts(13/17) and North American Brown-head- tive ejection), we interpreted the behavior as ed Cowbird hosts(2/27) is statisticallysignifi- grasp ejection, even though there was a possi- cant (Fisher'sexact probabilities test; P < 0.001). bility that in naturethey might havebeen eject- ed by puncture ejection.These speciesmay be regarded as grasp ejectors.On the other hand, DISCUSSION when attempts to reject artificial eggs resulted in one, several,or all of the host'seggs being Rejectionbehavior.--The use of plastic eggs destroyed or removed, this destruction was makesit possibleto distinguishbetween grasp probably the result of repeated attempts to re- and puncture ejection becausethe plastic eggs move the parasitic egg by puncture ejection. are so resistantto puncture that host'seggs are This observationis validated by Reed Warblers usuallydamaged in the process.Rothstein (1975) (Acrocephalusscirpaceus) observed pecking on inferred that most cowbird egg-rejectersused artificial cuckooeggs (Davies and Brooke 1988). grasp ejection becausewhole plaster eggs were Plausibly,species that unselectivelyejected the removed and all host eggswere left intact after artificial eggscan be regardedas puncture ejec- April 1991] Parasite-EggRejection in Cuckoos 353

tors. We will use the term grasp or puncture none of the remaininghost eggs in the nestof ejectorsfor those species. thesetwo speciesshowed any signsof damage Rejectionof conspecific eggs. --Bramblings, chaf- due to puncture ejection.The Common Cuckoo ,and reed-buntings,which could punc- eggwas selectively ejected. However, both these ture-eject the artificial egg, normally ejected specieshave bills among the longestof all the conspecificeggs with only minor damage to specieswith grasp indicesof <200 mm2. Bill their own eggs(selective ejection). This could length (and strength) may be more important be consideredto representgrasp ejection. Based for grasp ejection than the grasp-indexvalue. on the artificial egg experiments,these three The Meadow Pipit acceptsCommon Cuckoo speciesprobably alsopuncture-ejected the con- eggs (Moksnes and Roskaft 1989, Davies and specificeggs, despite the fact that the smaller Brooke 1989). The occasionalrejection of Com- conspecificeggs could have been grasp-ejected. mon Cuckooeggs by this specieswould, based However, ejectionsof artificial conspecificeggs on bill length, presumably take the form of of sizessimilar to natural ones were always by puncture ejection. Our results do not support unselectiveejection. Of 36 ejectionsof natural thisprediction. The high rateof desertioncould conspecificeggs, 4 ejectionsresulted in an ad- therefore indicate a lack of ability to puncture- ditional ejection of the hosts'own eggs.This eject.However, Davies and Brooke(1989) have supportsthe idea that small-billed speciessuch made several observationsof Meadow Pipits asthe Brambling,Common Chaffinch, or Com- ejecting a Common Cuckoo egg. On the other mon Reed-Buntingmay experiencesome costs hand, finchesand sparrowsmight have stron- even in puncture-ejectingconspecific eggs. Be- ger bills than pipits and wagtails.These stron- causeCommon Cuckoo eggs are both largerand ger bills may make thesespecies more likely to thicker-shelledthan the eggsof thesethree spe- puncture-eject.Our data do not allow any fur- cies,the ejectioncost is most probably higher ther speculationsregarding this point. when genuine CommonCuckoo eggs are eject- We support the prediction that Common ed in . Northern Orioles, probably the Cuckoo hosts with medium-sized bills punc- only North American puncture ejectors,dam- ture-ejectCommon Cuckooeggs from the nest. aged someof their own eggsin 13 of 33 ob- The largestpotential hostsgrasp-ejected, while servedejections of Brown-headedCowbird eggs the smallest hosts deserted their nests. Only a (Rohwer et al. 1989). Such damagewas not as few speciesdeviated from this generalrule. Our dramatic as that we observed, perhaps because resultsverify reportsthat the percentageof re- Northern Orioles have greater grasp indices jection done by ejection increasedwith bill than any of these three species.On the other length of the host (Davies and Brooke 1989).At hand, our observationsmay support the idea present we cannot estimate the true cost of (Daviesand Brooke1988) that somerecognition puncture ejection,but our observationssupport costis involved in the rejectionof a foreign the general idea that the greater eggshell-thick- egg.These three speciesmight thus mistakenly ness of the North American Brown-headed have ejected some of their own eggs simply Cowbirdsmay have evolved to resistpuncture becauseof recognition problems. ejection by its smaller-sizedhost species.Al- Rejectionbehavior of cuckooand cowbirdhosts.- though the thicker eggshellof CommonCuck- More Common Cuckoo than Brown-headed oos may have evolved for other reasons(Lack Cowbird hostswere rejectors.As predicted,the 1968), the effect on the host specieswill be sim- frequency of puncture ejectorsamong cuckoo ilar to that for Brown-headed Cowbird hosts. hostswith graspindices <200 mm2 (7/17) was That is, Common Cuckooeggs laid in European higher than that for cowbirdhosts (2/27, x2 = host nests should produce reactions similar to 7.31, P < 0.01). The number of species even- those of Brown-headedCowbird eggs laid in tually found to be punctureejectors will prob- the nests of North American cowbird hosts, if ably increaseas the number of experimentsin- the costsof rejection or acceptanceare compa- creases(e.g. Reed Warblers;Davies and Brook rable.For thesetwo groupsof hosts,the costof 1988, 1989). ejectingthe parasiticegg should be similar,rel- The Bluethroat and the Spotted Flycatcher, ativeto respectiveegg size, shapes, and strength. both with grasp indicessimilar to the Brown- Nonetheless, the cost to a Common Cuckoo host headed Cowbird, must at present be regarded that acceptsa parasiticegg is much higher than as grasp ejectors.Except for one observation, for a Brown-headedCowbird host that accepts 354 MOKSNES,ROSKAFT, • BP, AA [Auk, Vol. 108 a cowbird egg. A Common Cuckoohost should termining incubationstage in eggsof the Com- therefore be able to tolerate a higher cost of mon . Wilson Bull. 83: 425-429. rejection than a Brown-headed Cowbird host. HOYT, D. F. 1979. Practical methods of estimating Furthermore, the cost of puncture ejection volume and fresh weight of bird eggs.Auk 96: 73-77. should increase as bill size decreases. Selection LACK,D. 1968. Ecologicaladaptations for breeding should favor those Common Cuckoo hosts that in birdsß London, Methuen. puncture-eject the parasitic egg while possess- MAYFIELD,H. 1961. Vestigesof a proprietaryinterest ing bills much smaller than thoseof the Amer- in nestsby the Brown-headedCowbird parasit- ican Brown-headed Cowbird hosts. izing the Kirtland'sWarblerß Auk 78: 162-166. MOKSNES,A., & E. ROSKAIrr. 1987. Cuckoo host in- ACICI•OwI•DGMP•NTS teractionsin Norwegian mountain areas.Ornis Scandinavica 18: 168-172. We are indebtedto F. Falkenberg,P. Fiske,D. Karl- --, & . 1988. Responsesof Tur- sen, L. Korsnes,J. A. Kroke, V. Kroke, H. E. Lerkelund, duspilaris and BramblingsFringilla montifringilla C. Meland, M. Meland, P. Olsen, A. C. Othman, C. to experimentalparasitism by the CuckooCuculus Pedersen,H. C. Pedersen,T. H. Ringsby,S. Svartaas, canorus. Ibis 130: 535-539ß and O. Vie for their assistance in the field. A. Olsen, --, & . 1989. of Meadow Pip- P. Olsen, and K. Sommervoldmade the plasticCom- its to parasitismby the Common CuckooßBehav. mon Cuckooeggs. S. Sumida and L. Kiff measured Ecol. Sociobiol. 24: 25-30ß the eggshell thicknessof cuckooeggs. The natural --, A. T. BRAA, L. KORSNES,H. M. LAMPE, history museums of Helsinki, Gothenburg, Stock- &'H. C. PEDERSEN. 1991.Behavioral responses holm, Lund, Copenhagen,Oslo, and Trondheim al- of potential hoststowards artificial Cuckoo eggs lowed us accessto their skin and egg collections.P. and dummiesß Behaviour 116: 64-89ß Tallantire improved the English in an early version. PICMAN,J. 1989. Mechanism of increasedpuncture C. D. Spaw and two anonymousreviewers improved resistanceof eggs of Brown-headedCowbirdsß the quality of the paper. This projectwas supported Auk 106: 577-583. by the TrondheimElectricity Board and by the Nedal RoI-IWER,S., & C. D. SPAw. 1988. Evolutionary lag and the Nansen Foundations. versus bill-size constraints:a comparative study of the acceptanceof cowbird eggsby old hostsß Evol. Ecol. 2: 27-36. LITERATURE CITED --, --, & E. ROSKAFr. 1989. Costs tO North- BAKER,E. C. S. 1942ß Cuckoo problemsßLondon, ern Orioles of puncture-ejectingparasitic cow- Witherby. bird eggsfrom their nestsßAuk 106:734-738. BENT, A.C. 1958ß Life histories of North American ROTHSTEIN,S. I. 1975. An experimental and teleo- blackbirds,orioles, tanagers, and alliesßUßSß Nat. nomic investigationof avian brood parasitism. Mus. Bull. 211ß Condor 77: 250-271. CLARK,K. Lß,& R. J. ROBERTSON.1981ß Cowbird par- ß 1976. Experimentsof defensesCedar Wax- asitism and evolution of anti-parasitestrategies wings use againstcowbird parasitismßAuk 93: in the Yellow Warblerß Wilson Bullß 93: 249-258ß 675-691. DAVIES, N. B., & M. DE L. BROOKE. 1988ß Cuckoos 1977. Cowbird parasitism and egg recog- versus Reed Warblers: adaptations and counter- nition of the Northern Orioleß Wilson Bull. 89: adaptationsßAnim. Behav. 36: 262-284ß 21-32. --, & --. 1989ß An experimental study of SPAW,C. D., & S. RO•. 1987. A comparativestudy co-evolution between the Cuckoo, Cuculuscano- of eggshellthickness in cowbirdsand other pas- rus, and its hostsßI. Host egg discriminationßJ. serines. Condor 89: 307-318. Anim. Ecol. 58: 207-224. WYI, I,IE, I. 1981. The cuckooß London, Batsfordß HAYS, H., & M. LECROY. 1971. Field criteria for de-