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Abstract
Climate change is leading to range shifts in many species, including range contractions at the low-latitude and low-elevation trailing edges. Little is known about the proximate mechanisms through which climate change causes spatial variation in demographic rates, population declines, and range contractions. I assessed the proximate mechanisms underlying trailing edge range contractions by analyzing long-term trends in an 18-year dataset on the demography of black-throated blue warblers (Setophaga caerulescens) breeding across an elevational climate gradient at the trailing edge of the range in North Carolina and the core of the range in New Hampshire. Populations remained stable at the range core; however, warming temperatures were correlated with population declines at the trailing edge. Trailing edge declines started at the lowest, warmest breeding elevations and gradually moved upslope. The demographic drivers of decline varied by study plot. Productivity, nestling mass, and late-season food abundance all declined at both the trailing edge and the range core; however, food abundance was negatively affected by warming temperatures and matched the elevational pattern of warbler population decline. Furthermore, food abundance was an important driver of population dynamics in areas with observed or projected population declines. I also deployed geolocators to determine migratory connectivity and the potential effect of non-breeding threats on breeding declines. Trailing edge populations overwintered in a restricted area of the Caribbean, which has experienced significant habitat loss over the past 2 decades. The results of my dissertation suggest that warming temperatures on the breeding grounds are a primary driver of trailing edge population declines, potentially exacerbated by overwintering habitat loss. The mechanistic pathways linking warming temperature and population declines, however, appear to be complex and involving multiple interacting pathways. Population declines at the trailing edge do not seem to be directly caused by declining productivity. Instead, declining food abundance, and its interactions with nestling mass, is a likely mechanism through which climate change is affecting trailing edge populations.