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Cowbird Parasitism of Dickcissels in Different Habitats and at Different Nest Densities

John L. Zimmerman
The Wilson Bulletin
Vol. 95, No. 1 (Mar., 1983), pp. 7-22
Stable URL: http://www.jstor.org/stable/4161714
Page Count: 16
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Cowbird Parasitism of Dickcissels in Different Habitats and at Different Nest Densities
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Abstract

Nest histories from Dickcissel populations in oldfields and prairies of Kansas were analyzed to ascertain the impact of cowbird parasitism on Dickcissel productivity, assess the productivity of Brown-headed Cowbirds in Dickcissel nests, and test Fretwell's (1977) hypothesis that the frequency and intensity of cowbird parasitism were inversely related to host nest density. Both the frequency and the intensity of cowbird parasitism were significantly greater in prairie populations than in oldfield populations of the Dickcissel. The fledging success of the greater number of cowbird eggs laid per prairie nest was also higher so that almost twice as many cowbirds were fledged from each parasitized prairie nest than were produced from each parasitized oldfield nest. In both habitats less than 10% of the nests were abandoned in response to cowbird activity. The major effect of parasitism on Dickcissel productivity was the removal of host eggs by the cowbird that resulted in a significant reduction in the numbers of Dickcissels fledged from parasitized nests in both habitats. Early in the nesting season, almost two eggs were removed from each parasitized nest, but this loss decreased to about one egg by mid-July. The reduction in recruitment (Payne 1977) is 26.9% in oldfields and 29.1% in prairies. Both the frequency of cowbird parasitism and intensity as measured by the "cowbird pressure" (McGeen 1972) were inversely associated with the density of available nests, supporting Fretwell's (1977) hypothesis. This relationship also explains the habitat difference in parasitism since Dickcissel densities were lower in the prairie.

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