ENDOGENOUS AND EXOGENOUS FACTORS INFLUENCING THE MOVEMENT AND MIGRATORY BEHAVIORS OF A WINTERING SONGBIRD

159 pagesAs migratory organisms decline globally at unprecedented rates, there is a growing imperative to understand ecological and evolutionary responses to climate change – especially those that affect the availability and phenology of resources. Understanding how climate affects the behavior and population dynamics of migratory birds is challenging because migrants have complex annual cycles that extend across geographically disparate locations. How and when migrants transition between different stages of the annual cycle – in other words, their movements – can have substantial individual and population-level consequences (e.g., seasonal interactions). To date, research on movements has been constrained by our inability to track organisms at fine enough spatiotemporal resolutions, resulting in biases towards larger organisms, easily observed strategies (e.g., territoriality), and breeding-centric studies. Ultimately, a more holistic understanding of the impacts of global climate change on migratory populations requires a full annual cycle approach. My doctoral research investigated (1) how migratory birds adjusted within-season (space use) and between-season (departure phenology and migratory) movements in response to climate-associated changes in resources and phenology during the nonbreeding season and (2) the extent to which those behaviors were shaped by the interplay between endogenous states (condition and dominance status) and exogenous conditions (resource availability). We combined new tracking technologies (2015-2020) with long-term demographic data (1995-2020) from an overwintering population of the migratory songbird, American redstart (Setophaga ruticilla), in Jamaica, to study individual and population-level drivers and consequences of movement behaviors. Tracking data revealed several distinct space use behaviors that include territoriality, floating, and transience and identified the variability in space use behaviors that often blur the lines between distinct strategies that include the prevalence of secretive forays that ranged from 0 up to 8 forays per day per individual. In resource-rich years (i.e., high NDVI), birds used less space as compared to resource-poor years. Likewise, within a season, variation in weekly resource availability (i.e., weekly NDVI) had a negative influence on the prevalence of forays and the area of space utilized. Lastly, dominance status (i.e., age and sex) significantly influenced the type and plasticity of space use behaviors, with dominant classes (i.e., adult males) most likely to occupy territories and respond strongly to changing seasonal resource availability. As one of the few studies to investigate the sources of variability in space use behavior in terms of both internal and external states for overwintering migrants, this work provides evidence of the importance of the interplay between environmental conditions and individual behavior in ultimately scaling to influence the distribution of space use behaviors at the population-level. These results highlight the rich variability in space use behaviors that are typically not captured in most conventional population studies and have gone largely overlooked and underappreciated. Timely arrival on the breeding grounds is vitally important for individual fitness and can be delayed by poor overwintering conditions. To date, no studies have assessed how the phenology of spring departure has changed over time and in response to environmental change. In the second component of my dissertation, I assessed how the phenology of spring departure of American redstarts changed over the past 24 years (1995-2019), identified the endogenous and exogenous drivers of departure timing, and described associations among spring departure schedules, breeding origins and winter rainfall. The timing of an individual’s migratory departure was shaped by both breeding latitude and winter rainfall, in addition to age and sex. Spring departure schedules were significantly earlier in wet vs. dry years and for southern vs. northern breeders. At the population level, spring departure schedules advanced at a rate of 0.21 days per year, which translated to 5 days over the past 24 years. This long-term shift in departure was better explained by the > 300-kilometer southward shift in the breeding origins of our wintering population than by rainfall, given the absence of long-term trends in precipitation at our study site. Our study is among the first to provide a nonbreeding perspective on the drivers of phenological changes in the annual cycles of migratory organisms. Given the costs of delayed arrival on the breeding grounds, a fundamental question remains— can individuals mitigate delayed spring departure schedules by migrating at a relatively faster rate? For the final chapter of my dissertation, I focused on individual American redstarts (Setophaga ruticilla) that were tracked after departing their wintering grounds in Southwest Jamaica through Florida and selected sites further north using the Motus Wildlife Tracking System. Individuals that departed relatively late migrated at a faster rate than those departing comparatively early. Importantly, delayed individuals were significantly less likely to return the following season. These results support the hypothesis that individual migrants attempt to compensate for delays in the start of spring migration and possibly incur survival costs because of these behaviors—a potential mechanism underlying differential survival during spring migration. Through the implementation of novel technological approaches and long-term demographic data, we shed light on many subtle and difficult to observe behaviors by peering deeper into some of the commonly held assumptions on how individuals utilize space, how they respond to seasonally variable conditions, and how migratory birds mediate climate-induced changes in resource availability. Ultimately, this dissertation highlights how the endogenous states of individuals interact with exogenous conditions to shape the movement of migratory birds that has important implications on the reproductive performance of individuals breeding thousands of miles away