Post-Fledging Movement and Spatial Habitat-Use Patterns of Juvenile Swainson's Thrushes

Abstract

We used radio telemetry to study post-fledging dispersal patterns of juvenile Swainson's Thrushes (Catharus ustulatus) in central coastal California from 2000 to 2002. We followed 30 different broods during the dependent period and 35 juveniles after independence. Adults with dependent juveniles had two types of movements, drifting and stationary, similar to those described for Wood Thrush (Hylocichla mustelina). All juveniles established at least one post-fledging dispersal area (n = 29) after independence and most (62%) established a second; therefore, most home ranges were comprised of disjunct dispersal areas. Median home range (95% fixed-kernel) and core (50% fixed-kernel) areas were 2.0 ha (range = 0.2–9.0) and 0.41 ha (range = 0.05–1.6), respectively. Juveniles spent from 3 to 39 days in different post-fledging dispersal areas; they generally had a primary dispersal area where they spent the majority of their time. Juveniles spent an average of 22.7 ± 1.5 days in primary and 9.6 ± 0.9 days in secondary dispersal areas. The median post-fledging dispersal distance was 147 m (range = 28–1,040 m) for initial dispersal from a brood-rearing area and 418 m (range = 154–2,624 m) for subsequent dispersal movements. Initial dispersal was not directed; however, final dispersal (brood-rearing area to final location) was directed to the northwest for all plots combined and west for one plot. We suggest that post-fledging movements and spatial habitat-use patterns are affected by adult breeding strategies, and by spatially and temporally clumped resources.