Broad-scale fragmentation of Midwestern forests has increased avian nest predation and parasitism of nests by Brown-headed Cowbirds (Molothrus ater). Local disturbances may also attract cowbirds and elevate parasitism. Chapter One presents cowbird abundance and brood parasitism data from various forest contexts in south-central Indiana. Cowbird abundances were similar at forest edges and interior forest. Parasitism levels were high near external and internal edges. Host species varied in parasitism frequency and in patterns among contexts. Increasing forest core habitat and eliminating interior edges should benefit forest-breeding birds.

Ornithologists have studied edge effects in many systems but have devoted little attention to underlying mechanisms. It is thus unclear why edge effects occur in some but not all situations. Edge-elevated nest densities may attract predators and cowbirds, but density-dependent responses may not always increase proportions of nests depredated or parasitized. Chapter Two models interrelationships among edge proximity, cowbird density, host density, and parasitism level. These models suggest elevated nest densities are unlikely to increase parasitism unless cowbirds respond both functionally and numerically. I describe an empirical approach to study relationships among these variables.

Logging generates edges and may reduce nest success, but few studies have measured success before timber extraction. Chapter Three describes an experiment evaluating the effects of logging on survival and parasitism of Acadian Flycatcher (Empidonax virescens) nests in Yellowwood State Forest, Indiana. Four tracts were selectively logged between two monitored breeding seasons. Four tracts were controls. Nest success at control sites increased from 1995 to 1996, but was similar at treatment sites before and after logging. Cowbird parasitism at control sites decreased from 1995 to 1996, but was similar at treatment sites before and after logging. If factors responsible for between-year variation operate similarly at all sites, the interactions between year and condition may indicate logging decreases success. Forest managers should limit logging where productivity of late-successional birds is a priority until we better understand factors influencing variation in success. Longer-term studies before and after logging are necessary to understand how logging affects success. It is especially important to investigate how management practices affect species restricted to regenerating forest.