Resource selection by juvenile Swainson's Thrushes during the postfledging period

Resource-selection studies of passerine birds during the breeding season have mainly been limited to understanding those factors important to nesting. However, little is known about what resources are selected by juveniles that are no longer dependent on their parents. The postfledging period may be a critical part of the breeding season for independent juveniles because they must avoid predators and learn to forage on a changing resource base. We used radio-telemetry to study postfledging habitat use and resource selection of juvenile Swainson's Thrushes (Catharus ustulatus) in coastal California from 2000 to 2002. We generated popul ation- level contours (50% and 95% fixed-kernel) to describe habitat use by independent juveniles, and we determined juvenile resource selection by comparing vegetation characteristics at sites used by juveniles versus random sites. Juvenile Swainson's Thrushes used mixed-hardwood forest and coastal scrub during the postfledging period as well as riparian vegetation used by nesting adults. The most parsimonious predictors of resource selection were fruit abundance variables, suggesting that postfledging habitat selection by the Swainson's Thrush is best explained by the optimal-foraging hypothesis. We suggest that juvenile thrushes can track food resources in a habitat mosaic and use vegetation types distinct from what is traditionally considered Swainson's Thrush breeding habitat.