TRANSIENT SWAINSON’S THRUSH (CATHARUS USTULATUS) WINTERING IN THE ANDEAN FOOTHILLS OF ECUADOR ARE ALMOST EXCLUSIVELY YOUNG MALES

Abstract

We know relatively little about the non-breeding period of most migratory birds. Decades of research on Swainson’s Thrush (Catharus ustulatus) has focused on their breeding ecology, whereas their non-breeding ecology is limited to broad patterns of distribution and migratory stop-over ecology. For the nearly 50 years, the US Breeding Bird Survey recorded declines of 0.7% per year for the Russet-backed Swainson’s Thrush (C. u. swainsonii) subspecies, which spends the non-breeding period in South America. However, there is an insufficient understanding of the constraints across their annual cycle to determine reasons for the sustained decline. In 2013 and 2014, we examined their little studied non-breeding period on the Andean slopes of Ecuador where prior experience showed that they were seasonally abundant. Here rapid deforestation threatens primary forest. We used point counts and radio telemetry to evaluate whether primary forest was used preferentially to recently regenerated second-growth forest. From point counts, 76% of detections and almost all captured individuals occurred in secondary forest. Of 86 birds captured in mist nets, 85 were males and 83 young of the year, indicating a highly skewed ratio in favor of young males. Radio telemetry demonstrated preference for secondary forest, especially for fruiting Cecropia spp. trees, with no apparent territorial behavior by those tracked and with short residency times. A marked decrease in density over the course of the field season suggests an itinerant population possibly tracking the ephemeral use of fruit resources. Alternatively, these temporal and spatial patterns could suggest that this population undergoes a mid-winter intratropical migration or at least landscape-level movements. However, we observed thrushes throughout our extended sampling period suggesting transiency with high turnover. Together, this study documents demographic separation during the non-breeding period with preference for of secondary forest and high transiency, important findings in informing management across the annual cycle.