Temporal and Spatial Patterns of Breeding Brown-Headed Cowbirds in the Midwestern United States

Abstract

Brown-headed Cowbirds (Molothrus ater) are an obligate brood parasite and a potential threat to some populations of migratory songbirds. I used radio-telemetry to study temporal patterns in behavior, habitat use, and sociality, as well as spatial patterns and movements among breeding, feeding, and roosting areas. I obtained a mean of 42 locations of 84 radio-tagged female cowbirds on three study sites in Illinois and Missouri. Radio-tagged females usually were located in forest and shrub-sapling habitats with a mean of 1.4 males during the morning breeding period. During midmorning to early afternoon, females commuted to short-grass, cropland, and feedlot habitats; they fed in small flocks. At dusk females roosted singly or in small groups near breeding or feeding areas, or commuted to a large communal roost. Behavior and time of day, behavior and habitat use, and habitat use and time of day were highly associated. For approximately 90% of the radio-tagged cowbirds, breeding, feeding, and roosting locations were distributed nonrandomly within home ranges, and came from distinct utilization distributions. Cowbirds moved an average of 3.6 km between roosting and breeding locations, 1.2 km between breeding and feeding locations, and 2.6 km between feeding and roosting locations. Midwestern cowbirds show the same pattern of commuting between disjunct breeding and feeding areas as elsewhere in their range.