Explanations for the infrequent cowbird parasitism on common grackles

Article Abstract:

A study was conducted to identify the variables influencing the absence of parasitism on common grackles by brown-headed cowbirds. Results found no proof of parasitism on the 401 grackle nests examined in east-central Illinois, with cowbirds refusing to parasitize gackles despite the presence of the opportunity to do so. The main reason behind this observation seems to be previous rejection behavior. Grackles may have avoided cowbird eggs at a higher frequency in the past and are therefore themselves avoided by cowbirds.

Parasitic birds

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Further evidence of parasitism of Chopi Blackbirds (Gnorimopsar chopi) by the specialized Screaming Cowbird (Molothrus rufoaxillaris)

Article Abstract:

The Screaming Cowbird (Molothrus rufoaxillaris) parasitizes the Chopi Blackbird (Gnorimopsar chopi) in Southeastern Brazil and Northeast Argentina. The Screaming Cowbird is an avian brood parasite that parasitizes Chopi Blackbirds in the absence of Bay-winged Cowbird (Molothrus badius) host. An analysis of research conducted in 1802 shows that the parasitism occurred in Eastern Paraguay during Spanish Colonial times. This indicates that the parasitism is not a recent phenomenon.

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Forest fragmentation affects the behavioral response of American Redstarts to the threat of cowbird parasitism

Article Abstract:

A study was conducted to examine the defensive behavior of American Redstarts based in a fragmented and contiguous boreal forest in western Canada against the threat of cowbird parasitism. The responses of American Redstarts to models of a female Cowbird and a control species was investigated at various locales using distraction displays during the egg laying or early incubation periods. Results indicated that the fixation of antiparasite behavior within bird populations.

Birds

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