Community-level patterns of parasitism: Use of three common hosts by a brood parasitic bird, the brown-headed cowbird

Abstract

:Patterns of host use by avian obligate brood parasites are geographically and temporally variable and have been linked to parasite density, host quality and abundance, and the availability of alternative hosts. We tested three non-exclusive hypotheses addressing the relationships between these factors and patterns of parasitism on the three most important host species in a community. We collected 5 y of data (1994–1998) on the number of host nests, proportions of host nests that were parasitized (frequency) by brown-headed cowbirds (Molothrus ater), and the number of cowbird eggs per parasitized nest (intensity) at Delta Marsh, Manitoba. Parasitism on yellow warblers and red-winged blackbirds was positively related to overall host density, whereas parasitism on song sparrows was most frequent and intense but did not appear to be related to any of the factors tested. We suggest that host quality may explain the higher than expected use of song sparrows. Our findings demonstrate that host use was significantly different among host species and non-random. Observed patterns suggest that factors that alter the relative availability of hosts, such as host breeding synchrony, may modify the parasitism levels experienced by different host species.