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Cowbird Parasitism and the Population of the Kirtland's Warbler

Harold Mayfield
Evolution
Vol. 15, No. 2 (Jun., 1961), pp. 174-179
DOI: 10.2307/2406078
Stable URL: http://www.jstor.org/stable/2406078
Page Count: 6
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Cowbird Parasitism and the Population of the Kirtland's Warbler
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Abstract

The Kirtland's Warbler, a rare species of bird with a small and special habitat, is in a precarious position for survival. For habitat in its nesting range, it is dependent on forest fires, which are no longer as extensive as in former times and therefore perhaps do not provide many areas that are optimal for this bird. Now, with a reproductive potential that is marginal, the Kirtland's Warbler is subjected to a new and unrelenting pressure from the social parasite, the Brown-headed Cowbird. Against this threat it has developed no defense. The cowbird takes a toll of the warbler at every stage of the nesting process and reduces the production of warbler fledglings about 36 per cent below the level achieved in the absence of cowbirds. This toll might not be serious for a more prolific species but may prove to be crucial to the Kirtland's Warbler, since there is no promise that the pressure will be relieved even though the warbler approaches the brink of extinction.

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