Migration and Winter Ecology of a Declining Forest Songbird

Fundamental gaps in understanding of the year-round biology of migratory songbirds are hampering conservation of dozens of at-risk species. Until recently, determining where and when during their annual cycle migratory songbirds are most limited was not possible, because linking an individual’s survival and condition across seasons and over thousands of kilometres was extremely difficult. With miniaturized tracking devices, I followed a declining migratory songbird, the Wood Thrush (Hylocichla mustelina) over its entire annual cycle, from well-studied breeding sites in temperate forests of eastern North America to its overwintering range in the Neotropics. As with most migratory birds, information on non-breeding ecology and habitat use is lacking for Wood Thrushes. In this dissertation, I examined demographic patterns in migration behaviour, and quantified effects of habitat occupancy in winter on condition of the birds and on spring migration performance. I found that Wood Thrush spring migration shows a distinct age-related switch from late timing and more stopovers as juveniles to earlier timing and fewer stopovers as adults. I also show, at a study site in Belize, that winter habitat quality (abundance of food and moisture) declines over the non-breeding season, and that Wood Thrushes are in the lowest body condition of the winter immediately prior to initiation of spring migration. Finally I show that despite variation in pre-migration body condition and habitat moisture, there were no carry-over effects of winter habitat occupancy to spring migration. Overall this work supports the hypothesis that Wood Thrush spring migration is primarily under genetic control, rather than condition-dependent, since winter conditions do not appear to limit migratory performance. Climate change models predict more droughts in Central America, which could decrease the condition of overwintering Wood Thrushes, and may result in detectable carry-over effects on migration in the future.