Genetic and temporal patterns of multiple parasitism by brown-headed cowbirds (Molothrus ater) on song sparrows (Melospiza melodia)

Identifying the factors used by an avian brood parasite to select host nests is important in understanding the determinates of individual reproductive success, yet such factors are poorly known for most parasitic species. Insights into these factors may come from understanding the conditions under which female parasites lay more than one egg per host nest (multiple parasitism). Using genetic techniques we examined patterns of multiple parasitism on a preferred host, the song sparrow (Melospiza melodia), to determine some basic patterns of multiple parasitism. Multiple parasitism involved equal frequencies of the same female parasitizing the same nest again and two or more females parasitizing the same nest. The frequency of multiple parasitism increased as the season progressed. We also documented a high frequency of parasitism that was not synchronized with host laying. These laying patterns may be the result of cowbirds “making the best of a bad situation” or of suboptimal host choice by inexperienced, nonselective fe-