Movement ecology of wood storks in the Southeastern U.S.

Abstract

Organisms move to colonize new environments, reproduce, and find resources. Migration is a specialized type of movement that allows animals to track resources that are heterogeneous in space and time. Migration can take different forms according to the temporal periodicity and predictability of resource variation; partial migration, where a population includes both migrant and resident individuals, emerges in environments where seasonality is paired with year-to-year unpredictability of resources. While most of the research on partial migration to date has focused on how it is controlled at the individual level, our understanding of its ecological implications and adaptive value is still limited. This dissertation contributes to furthering our understanding of partial migration by focusing on a population of wood storks (Mycteria americana) in the southeastern U.S. as a study model. The overall objective of my work was to investigate the adaptive value of behavioral heterogeneity within the wood stork population in terms of migratory behavior; if different individual migratory strategies are associated with different fitness advantages, behavioral heterogeneity might help wood storks persist in the face of resource unpredictability and environmental change. I addressed the following specific objectives:

1. Provide a quantitative description of individual migration patterns in the wood stork population;
2. Develop an analytical method to estimate reproductive outcome from movement data, to be used as a proxy for fitness;
3. Assess whether individuals that differ in their migratory behavior also differ in terms of resource selection and fitness.

My findings show that the wood stork population includes ~60% migrant and ~40% resident individuals, thus establishing its status as partially migratory. Residents and migrants differ in their foraging-site selection during breeding, with residents foraging closer to urban areas than migrants do; foraging sites located closer to urban areas are also associated with higher reproductive outcome, estimated from individual movement data using the method I developed. Overall, these results suggest that partial migration may be an adaptive strategy in the wood stork population because, when migrants fail to reproduce because of unpredictably bad foraging conditions, the contribution of residents to recruitment may provide a smaller but more reliable demographic reservoir.