ECOLOGICAL CAUSES AND CONSEQUENCES OF NON-BREEDING MOVEMENTS IN A DECLINING MIGRATORY SONGBIRD, WOOD THRUSH (HYLOCICHLA MUSTELINA)

Abstract

In this dissertation I evaluate the ecological correlates of non-breeding space-use strategies and how these drive within and between season movement dynamics in a declining migratory songbird, wood thrush (Hylocichla mustelina). In Chapter 1, I deployed high-resolution GPS transmitters across 5 breeding populations to quantify habitat selection as wood thrush moved across the annual cycle. I found seasonal variation in habitat selection at the regional, landscape and local scales which suggests the factors driving the evolution of habitat selection preferences vary across seasons and environmental conditions. In Chapter 2, I combined radio telemetry and GPS tracking to examine how environmental conditions drove space-use strategies during the non-breeding stationary period. I found evidence that both small- and large-scale movement dynamics were dependent on moisture levels on tropical non-breeding grounds. At small spatial scales, dry conditions drove low food availability, reduced individual body condition and these individuals had larger home ranges. In this same chapter I integrated archival GPS tag data to demonstrate that wood thrush from across the breeding range engaged in permanent large-scale mid-winter shifts in home ranges and, similar to radio-tagged birds, that individuals from wetter, higher quality habitats were more likely to use this strategy. I suggest that the facultative movements are a condition-dependent strategy allowing wood thrush to find alternative habitats as conditions deteriorate across the dry season in their non-breeding grounds. Finally, to determine how food availability may influence timing of spring migration, I performed a food manipulation experiment with captive wood thrush in Chapter 3. I found that food availability and body condition modulated the intensity, but not onset, of migratory restlessness (Zugunruhe), an index of migratory disposition in captive birds. These results suggest that non-breeding food limitations could constrain migration preparation in wood thrush. Low food availability also advanced the onset of migratory fattening. I suggest that advancing migratory fattening when food availability is low may provide a mechanism to flexibly adjust migration timing under poor environmental conditions. Together these findings suggest that wood thrush exhibit a diversity of behavioural mechanisms to handle environmental heterogeneity across the annual cycle.