Movement ecology of a migratory songbird (Tree Swallow, Tachycineta bicolor)
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Description
In this dissertation I examine the non-breeding-season movement ecology of a migratory songbird, the Tree Swallow (Tachycineta bicolor). Specifically, I focus on both seasonal migration (group-level movements) and the formation of communal roosts (individual-level movements). Using a combination of remote and direct tracking methods, I show first that southeastern Louisiana is an important and unique stopover site for this species during autumn migration. Tree Swallows tracked with geolocators from three separate breeding areas spent on average 30 days at this site before migrating to their main overwintering areas, a much longer period of time than traditional songbird migratory stopovers. Next, using historical Doppler weather radar data, I compare the annual dynamics of occupancy patterns, roost-site consistency, and autumn migration phenology between Louisiana- and Florida-arriving swallows, another important nonbreeding area for this species. Arrival to Louisiana occurs over a much shorter time window than in Florida, and relative abundance decreases throughout the middle winter months in Louisiana before an increase again during spring migration. In Florida, swallows arrive much more gradually, and relative abundance remains high throughout the winter, more akin to a traditional winter site. For both locations the variation in autumn arrival phenology can be partly explained by the amount of precipitation along the respective flyways: in general, higher rainfall along the Mississippi and Atlantic flyways is associated with later arrival to both Louisiana and Florida, respectively. Having shown that the roosts are generally in the same location each night, I next focus on the causes and consequences of individual movements. Using radio-telemetry, I show that swallows have high, but not perfect, roost site fidelity from night to night, but do occasionally switch between roosts. Roosts thus form a network connected by the movements of individuals between them. I develop an individual-based model to show that this pattern of roost dynamics can be explained by individuals showing a high level of roost-site fidelity combined with a some amount of conspecific attraction to their nearest neighbors. Other real-world roost dynamics emerge from the model when these two parameters (roost fidelity and conspecific attraction) are independently adjusted. By extending the model to include the transmission of infectious diseases, I show theoretically that these roost dynamics can affect the spread of a disease throughout the population when the disease is spread via density-dependent transmission mode, but if the disease is spread via frequency-dependent transmission mode, infection rate is not affected by roost dynamics and spreads evenly across the parameter space. This can have consequences for species that form communal roosts or other types of social networks, especially in a world with increasing emerging infectious diseases.