Nest Predation and Population Declines in Illinois Songbirds: A Case for Mesopredator Effects

Mesopredator release has typically been examined at small spatial scales; however, processes such as habitat fragmentation, suppression of top predators, and changes in land use that lead to higher mesopredator densities typically occur at large spatial scales. In Illinois, raccoon (Procyon lotor) numbers have increased since the early 1980s, with unknown consequences for breeding songbirds. I examined population trends between 1979 and 2001 for songbirds inhabiting woodland and scrub-successional habitats within Illinois by using data from the North American Breeding Bird Survey (BBS). I assigned species a priori into two groups: (1) raccoon-vulnerable species that nest low and thus are highly vulnerable to nest predation by raccoons and (2) raccoon- invulnerable species that nest high and largely escape nest predation by raccoons. The mean number of raccoon-vulnerable species per Illinois BBS route declined by approximately 10%, whereas the number of raccoon-invulnerable species increased approximately 15% through the early 1990s. Population trends of the two groups also diverged. Raccoon-vulnerable species had predominantly negative population trends from 1980 to 2001, whereas raccoon-invulnerable species show roughly equal numbers of positive and negative trends. In contrast, population trends prior to 1980 did not show this divergence. A survey of studies from Illinois that report nesting success connects these divergent patterns between the two groups to substantially higher nest-predation rates for raccoon-vulnerable species. Taken together, results of these studies and mine suggest that large-scale changes in the abundance of raccoons have had demonstrable consequences for songbird populations and communities in Illinois.