Cowbird parasitism, predation, and host selection in fragmented grassland of southwestern Manitoba

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Davis, Stephen Knight
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The frequency of parasitism by Brown-headed Cowbirds (Molothrus ater) and predation on ground-nesting passerines was quantified in three fragments of grassland habitat in southwestern Manitoba. All bird species studied were parasitized by cowbirds but at different parasitism frequencies. Cowbird parasitism was higher (69% of nests examined) in the smaller (22-ha) plot compared with two 64-ha plots (17% and 20%, respectively). Of the nests parasitized in all plots, 69% were parasitized multiply with 2 to 8 eggs laid per nest. There was no difference among the plots with respect to the frequency of multiple parasitism. Western Meadowlarks were parasitized at a frequency of 44%, with an average of 3.1 cowbird eggs laid in each parasitized nest. Chestnut-collared Longspurs and Sprague's Pipits were parazitized at frequencies of 18% and 14%, respectively. Parasitized nests fledged significantly fewer young (0.95) than unparasitized (1.45) nests. Only parasitized Grashopper Sparrow nests produced significantly fewer young than did unparasitized nests. Nesting success did not differ significantly between plots but nests in the smaller plot fledged significantly fewer young per nest than the other two plots. Exposed nests were more likely to be depredated than concealed nests. Cowbirds parasitized more frequently hosts that nested at highest density on each plot but parasitism frequency did not decrease with declining density of hosts. The laying season of all hosts overlapped with the cowbirds, but late nesting longspurs and Savannah Sparrows were not parasitized. Concealed nests were more likely to be parasitized than exposed nests. Parasitism frequencies were significantly lower for nests located more than 150 m from a perch and/or 100 m from the habitat edge.