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Effect of Cowbird Parasitism on American Goldfinch Nesting

A. L. A. Middleton
The Auk
Vol. 94, No. 2 (Apr., 1977), pp. 304-307
Stable URL: http://www.jstor.org/stable/4085093
Page Count: 4
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Effect of Cowbird Parasitism on American Goldfinch Nesting
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Abstract

In a breeding study of a marked goldfinch population, 22 nests (9.4%) were parasitized by the cowbird. Although 4 cowbird eggs hatched from the 26 laid, no cowbirds fledged from a goldfinch nest. The clutch size of parasitized goldfinches was significantly smaller than those of unparasitized birds, and the mean number of nestlings hatched and fledged was also smaller. By contrast the percentage of goldfinch eggs that hatched and fledged was higher in parasitized nests. Some 55% of the parasitized nests produced fledglings while 36% were lost to predation. Eleven of the parasitized nests belonged to adult female goldfinches, and it is suggested that adult birds are most likely to be affected by cowbird parasitism. The implications of cowbird parasitism on the goldfinch are discussed. Although the cowbird gained little advantage from the association its parasitism largely offset the advantages of early nesting by the adult goldfinches. The goldfinch has not developed any apparent defense against cowbird parasitism, but those individuals that nested in man-made habitats were largely free from its influence.

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