Host activity and the risk of nest parasitism by brown-headed cowbirds

Proportions of nests parasitized by brown-headed cowbirds (Molothrus ater) vary greatly among host species, but factors underlying this variation remain poorly understood. Cowbirds are believed to find nests by watching host behavior. We tested the hypothesis that the activity of hosts during nest building correlates with the probability of parasitism among and within four sympatric hosts: dusky flycatchers (Empidonax oberholseri), warbling vireos (Vireo gilvus), yellow warblers (Dendroica petechia), and American redstarts (Setophaga ruticilla). Daily probability of parasitism varied substantially among these species, from 3% for dusky flycatchers to more than four times that for warbling vireos. The four species did not differ in the proportion of cowbirds fledged from their nests. Differences in nest placement did not explain differences in probability of parasitism among or within species. Parasitism frequencies increased among species that made longer nest-building visits, had a greater propensity to perch during nest approach, spent more time near their nests, and had males that vocalized more often near nests. Within species, females that visited their nests less often, spent more time on the nest per visit, and males that sang more and were active in a smaller area around their nests were more likely to be parasitized by cowbirds. These data support the hypothesis that cowbirds use the activity of hosts during nest building to locate nests.