Failure of Brown-Headed Cowbird Parasitism in Nests of the American Goldfinch

j. Field Ornithol., 62(2):200-203

FAILURE OF BROWN-HEADED COWBIRD PARASITISM IN NESTS OF THE AMERICAN GOLDFINCH

ALEX L. A. MIDDLETON Departmentof Zoology Universityof Guelph Guelph, OntarioNIG 2WI, Canada Abstract.--Brown-headed Cowbirds (Molothrusater) parasitized47 of 802 American Gold- finch (Carduelistristis) nests at Guelph, Ontario, between1968 and 1989, but cowbirdeggs hatchedin only 15 of them. In 12 of thesenests the cowbirdsdied within 4 d of hatching. In the remainingnest the cowbirdsurvived for 12 d, but at deathwas poorlydeveloped and still incapableof nestdeparture. The mostlikely causeof the cowbirddeaths, as suggested by observationalevidence, was dietary inadequacy.

FRACA$O DE MOLOTHRUS ATER EN PARA$ITAR NIDO$ DE CARDUELIS TRISTIS Sinopsis.--De un total de 802 nidos de Carduelistristis, estudiadosen Guelph, Ontario, entreel 1968 y 1989,el tordo(Molothrus ater) parasit647, delos cuales en s61o15 eclosionaron huevosdel par•tsito.En 12 de estosnidos los polluelosde tordo murieron dentro de los primeros4 d•as de edad. En el nido restanteel tordo sobrevivi6hasta la edad de 12 d•as, peroal momentode sumuerte se encontraba pobremente desarrollado e incapazde abandonar el nido. Las observacionessugieren ofrecer como explicaci6n que la causam•ts probable de la muerte de estos tordos fue la alimentaci6n inadecuada. The diet of nestlingcardueline finches is mainly composedof seeds, which is rare among birds (Newton 1967, 1972) becauseseed diets are relatively low in protein (Newton 1972:179; O'Connor 1984). Even in timesof foodshortage such nestlings, though underweight, still canmain- tain feather growth at a rate which is nearly normal (Newton 1972). By contrast,low protein diets in other groups of birds may significantly restrictall aspectsof development(O'Connor 1984:141).This reality may have implicationsfor successfulbrood parasitismbecause, not only must the parasite selecthosts that can incubateits eggsbut also thosethat can provide adequatenutrition for its nestlings.In this context,the relation- shipbetween the AmericanGoldfinch (Carduelis tristis) and Brown-headed Cowbird (Molothrus ater) is of interest. The AmericanGoldfinch is regularlyparasitized by the Brown-headed Cowbird (Friedmann and Kiff 1985, Middleton 1977). However, the successof the relationship, as defined by the production of fledgling cowbirds,is equivocal(Friedmann and Kiff 1985:266). Berger (1961, 1968) contendedthat it is unlikely that goldfinchescan rear cowbirdsand further suggestedthat the failure of cowbirdsto survivethe nestlingperiod resultsfrom their inability to thrive on the granivorousdiet of the goldfinch (Berger 1961:271). Although I have not rigorouslytested this hypothesis, field data from my long-term study (Middleton 1988) supportit.

METHODS AND RESULTS Between 1968 and 1989 inclusive, 802 American Goldfinch nestswere found at Guelph, Ontario, of which 47 were parasitizedby the Brown-

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TABLEl. Body characteristicsof Brown-headedCowbird and American Goldfinch nestlings from a successfuland an unsuccessfulnest.

Brown-headed Cowbird (12 d) American Goldfinch (13 d) Unsuccessful Unsuccessful Body characteristics Successfulnest • nest Successfulnest 2 nest Body weight (g) 29 _ 1 26.5 11.6 _ 0.2 9.75-10.5 Body length (mm) 108 _ 2 90.1 85 _ 1.0 54.8-58.2 Wing length (mm) 64 _ 1 20.8 340 (Extrapo- 34.0-37.8 lated) Tarsus length (mm) 24 _ 0 20.2 13 13.9-13.6 Plumage Teleoptiles No teleop- Teleoptiles Teleoptilesin fringing tiles fringingand early fring- and well present. well devel- ing. developed. oped.

From Scott(1979). Age 11 d. From Holcomb (1969). headed Cowbird. Thirteen of these nestsproduced cowbird hatchlings, but in 12, the cowbirdsdied by the fourth day (Mean = 2.1 _ 1.03 d SD). In one nest, the cowbird survivedfor 12 d. At the remaining nests 12 eggsfailed to hatch, 8 were taken by predators,7 disappearedduring incubation and 7 were left in abandoned nests. The nest in which the cowbirdsurvived was found on 2 July 1981, 3 m high in an ornamentalmaple (Acer sp.) on the Universityof Guelph campus.The nest containedfour goldfincheggs and one cowbird egg. The cowbirdegg hatched on 15 July and the two goldfincheggs to hatch did so on 16 July. Both goldfinchparents were color-bandedand daily observationswere made at the nest. Incubation, brooding and feeding behaviorsappeared normal (Nickell 1951, Stokes1950). On 27 July the cowbirdwas found dead in the nest and was collected.On 28 July there was little parental activity at the nest and on 29 July the two goldfinch nestlingswere found dead. At collectionthe carcasseswere weighed to the nearest0.25 g on a Pesolascale, then preservedin formalin.Subsequently, total bodylength, wing length, and tarsuslength were measuredto the nearest0.1 mm, for all three specimens,using a Vernier caliper and accordingto the methods usedby Scott(1979) for the cowbirdand Holcomb (1969) for the gold- finches. Compared to other nestlingsof their speciesat the same age (Holcomb 1969, Scott 1979), developmentof all three nestlingswas re- tarded, especiallythe developmentof the cowbird'splumage (Table 1). The goldfinchnestlings were emaciatedand there was no signof foodin the gut.

DISCUSSION Although the cowbird nestlingsurvived beyond the 10 d usually as- sociatedwith nest departure (Harrison 1978; Middleton, unpub. data) and survivedlonger than any other in my study, it was far from ready 202] ,•. L. A. Middleton J.Field Ornithol. Spring 1991 to leavethe nest.Despite being fed regularly,it had not thrived.Therefore, death was unlikely caused by undernourishment,but rather through malnourishmentresulting from the inappropriatequality of the diet pro- videdby its fosterparents. The missingingredient was presumablyprotein or its precursors(Newton 1972:196). The goldfinchesapparently starvedafter the nest was abandonedby their parents. Abandonment may have been causedby the death of the cowbirdand/or, in their weakenedcondition, the youngfinches may have been unable to retain parental attentionfollowing death of the cowbird. As previously suggested by Berger( 1961, 1968) and M iddleton( 1977), my observationsprovide concrete evidence that cowbirdsare unlikely to survivein goldfinchnests. In successfulnests many cowbird eggsfail to hatch, which may be a result of incubationproblems (Payne 1977). If the eggsdo hatch most nestlingsdie within a few days, probably when the residualyolk reservesare finally depleted.In thesenests the surviving goldfinchesare not seriouslyaffected and becomefledglings (Middleton 1977). My resultsindicate that even if cowbirdssurvive beyond the first few days, they are still unlikely to becomefledglings. Furthermore, in this rare situationthe presenceof the cowbirdsis likely to result in total nestfailure, as observedat Guelph. Clearly, the American Goldfinch is an unsuitablehost speciesfor the Brown-headedCowbird. My data stronglysuggest the impossibilityof successfulfostering and cast further doubt on the validity of the early reports of such success(Friedmann and Kiff 1985).

ACKNOWLEDGMENTS

This study was supportedthrough grant number A6495 from the Natural Scienceand Engineering Research Council of Canada. I gratefully acknowledgethe discussionwith, and commentsof, my colleaguesEd Bailey and Tom Nudds on the original manuscript. CharlesBlem, Lloyd Kiff and Mary Sydlik providedadditional suggestions that strengthened the paper.

LITERATURE CITED

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NICKELL,W. P. 1951. Studiesof habitats,territory and nestsof the Eastern Goldfinch. Auk 68:447-470. O'CONNOR,R.J. 1984. The growthand developmentof birds.John Wiley and SonsLtd., Chichester.315 pp. PAYNE,R. B. 1977. The ecologyof brood parasitismin birds. Ann. Rev. Syst. Ecol. 8:1-28. SCOTT,T.W. 1979. Growth and age determinationof nestlingBrown-headed cowbirds. Wilson Bull. 91:464-466. STOKES,A.W. 1950. Breeding behavior of the Goldfinch. Wilson Bull. 2:107-127. Received2 Dec. 1989; accepted6 Aug. 1990.