Habitat and Microhabitat Features Associated with Cowbird Parasitism in Two Forest Edge Cowbird Hosts

I examined the relationship of habitat and nest microhabitat features of Field Sparrows (Spizella pusilla) and Indigo Buntings (Passerina cyanea) to brood parasitism by the Brown-headed Cowbird (Molothrus ater) in central Missouri. In old field habitats, Indigo Buntings were more frequently parasitized than Field Sparrows, but Indigo Buntings nesting in forested habitat were parasitized at higher frequencies than buntings in old fields. Logistic regression models showed that nest concealment best explained parasitism for all Indigo Buntings and field-nesting Indigo Buntings, with poorly concealed nests more likely to be parasitized. However, side concealment was not related to parasitism for heavily-parasitized Indigo Buntings in forested habitats. Microhabitat variables did not explain parasitism at Field Sparrow nests, but their nests were lower and better concealed than field-nesting Indigo Buntings. Results suggest that nest microhabitat features may influence probability of parasitism, but species and habitat characteristics may override microhabitat in explaining frequency of parasitism.