Patterns of host nest use by Brown-headed Cowbirds parasitizing Song Sparrows and Yellow Warblers

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McLaren, Celia M.
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I investigated patterns of host nest use by Brown-headed Cowbirds (' Molothrus ater') parasitizing two common hosts, the Song Sparrow (' Melospiza melodia') and the Yellow Warbler ('Dendroica petechia '), at Delta Marsh, Manitoba. I examined two non-exclusive aspects of host nest use: susceptibility of nests to discovery by nest-searching cowbirds and factors influencing host nest selection. Patterns of parasitism and multiple parasitism on Song Sparrows were determined by following the nesting attempts of this host during two breeding seasons and using molecular genetic techniques to quantify the number of female cowbirds laying eggs in parasitized nests. I assessed the importance of both nest habitat and microhabitat as well as host activity in influencing the likelihood of parasitism of Song Sparrow and Yellow Warbler nests by measuring several vegetation and behavioural characteristics of nests that we unparasitized, parasitized once, and parasitized more than once by the same female and by more than one female (i.e. multiple parasitism). (Abstract shortened by UMI.)