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The Brown-headed Cowbird (Molothrus ater has long been associated with bison (Bos bison) in North America on the Great Plains (Friedmann 1929). As a result, I anticipated that cowbirds would parasitize host nests more frequently in the presence of bison than in their absence. I predicted that several common ground-nesting avian species, Dickcissels (Spiza americana), Grasshopper Sparrows (Ammodramus savannarum), and Eastern Meadowlarks (Sturnella magna), would suffer higher frequencies of brood parasitism in bison-grazed habitat than in ungrazed habitat on the tallgrass Konza Prairie Research Natural Area, located in northeastern Kansas. The frequency of cowbird parasitism for all species combined was significantly higher in bison-grazed (0.69) than in ungrazed habitat (0.44) (p =0.044, X2 = 4.061, df =1). These results are consistent with the hypothesis and suggestion that bison-grazed habitat may be a more optimal site for cowbird brood parasitism than ungrazed habitat. I pose two principal explanations for the higher frequency of parasitism observed in the bison-grazed area. First, cowbirds may be able to forage more efficiently in the bison-grazed area, indirectly inflating parasitism frequencies by conferring a variety of energetic and nutritional advantages upon the females. Second, the cowbirds' abilities to find and parasitize nests may be enhanced by the shorter, less dense grass characteristic of grazed habitat. Further studies investigating the conservation implications of this phenomenon are merited.



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