Postfledging survival and movements of juvenile Gray-backed Thrushes (Turdus hortulorum)

Abstract

The postfledging period is a critical phase of avian life history. In the initial few weeks after fledging, juveniles face various hazards and develop necessary life skills. Knowledge of the postfledging period is important for understanding the fate of juvenile birds. From May to July of 2015, we tracked 22 Gray-backed Thrush (Turdus hortulorum) juveniles from 20 nests in a population in northeastern China using radiotelemetry. Juveniles dispersed from their natal sites for a mean (± SE) of 21.0 ± 2.4 d after fledging. The initial dispersal distance was 560.9 ± 109.4 m (the distance between the natal area and the first postdispersal area). The natal range size of the juveniles had a mean of 1.7 ± 0.5 ha (95% minimum convex polygons). Ten of 22 (45.5%) of the monitored fledglings died within the first 4 weeks after fledging, and predation was the primary cause of mortality. The Cox proportional-hazards regression model showed that there was a positive correlation between tarsus length and the survival rate of fledglings, while the influences of fledgling weight, date of fledging, and brood size on survival were not significant. Our results indicate that mammalian predators were the main threat to juveniles in the first few days after fledging when the juveniles had limited mobility. After the first week postfledging, raptors became the most important source of juvenile mortality.