Environmental predictors of nestling condition, postfledging movement, and postfledging survival in a migratory songbird, the Wood Thrush (Hylocichla mustelina)
Given that population dynamics of birds are known to be sensitive to high fledgling mortality, a comprehensive understanding of the environmental factors that drive variation in fledgling survival is essential to avian conservation. We quantified multiple aspects of the Wood Thrush postfledging period using breeding and radio-telemetry data collected over the course of 4 breeding seasons in southern Indiana, USA. First, we examined how drought, forest cover, and brood parasitism affected nestling body condition and brood size. Second, after controlling for the age-specific increase in survival, we used multimodel inference to examine how brood parasitism, drought, forest cover, nestling body condition, and nest vegetation structure influenced postfledging survival. Finally, we measured the relationship between these covariates and fledgling movements. Drought decreased cumulative survival probabilities, with the youngest age group (<4 days postfledging) being most affected; however, this relationship was dependent on the amount of mature forest cover. During non-drought years, fledgling survival was lower in study plots with a high proportion of mature forest cover. By contrast, postfledging survival during a drought year was higher in study plots with a high proportion of forest cover. This drought year was also associated with lower nestling body condition and brood size, and with delayed postfledging dispersal from natal territories. Our results suggest that while Wood Thrush postfledging survival is relatively insensitive to both nest vegetation structure and brood parasitism, forest cover and breeding-season precipitation interact to affect multiple aspects of the species' postfledging period.
