A Spatial and Genetic Analysis of Cowbird Host Selection

Molecular genetics makes it possible to measure basic, but long elusive parameters of the breeding biology of the Brown-headed Cowbird (Molothrus ater). Cowbird fecundity and host selection behavior were examined using a combination of molecular genetic techniques to link female cowbirds to the eggs they lay, radio-telemetry techniques to track female cowbirds' daily movements, and geographic information systems (GIS) to integrate these genetic and spatial data. The study site lies within a forested 1300 ha landscape in New York composed primarily of mature forest with adjacent old fields. It was found that female cowbirds used their home ranges as principal egg-laying areas. Individual females used characteristic individual home ranges throughout the breeding season, and they returned to the same home range every breeding season. Over one-half (54%) of females laid all their eggs in host nests inside or close to their home range. Proximity to a females' home range was the only significant ecological or biological feature affecting a cowbird's host.