PREDATION OF A WINTERING MIGRATORY SONGBIRD BY INTRODUCED RATS: CAN NOCTURNAL ROOSTING BEHAVIOR SERVE AS PREDATOR AVOIDANCE?

Many studies have demonstrated the deleterious effects of introduced predators on resident populations of island birds, but few have quantified their effect on the survival and space-use behavior of migratory species. We used radio telemetry to investigate the winter survival and roosting patterns of Bicknell's Thrush (Catharus bicknelli) at two sites in the Dominican Republic. Depredation by introduced rats was the only cause of mortality among 53 radio-tagged individuals monitored between January and March over multiple years; five (9%) marked individuals were depredated. Predator trapping revealed the presence of both the black rat (Rattus rattus) and Norway rat (R. norvegicus) and that the density of rats was higher in broadleaf cloud forest than in nearby pine forest. Some thrushes that used cloud forest exclusively during the day roosted at night in adjacent pine habitat. We suggest that introduced rats exert predation pressure on wintering Bicknell's Thrush in the Dominican Republic and that nocturnal arboreal rat predation could influence the thrush's space-use strategies.