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Abstract
Trailing-edge populations, near the receding edge of a shifting range, are predicted to be at high risk of climate-induced extinction, but lack of information on the factors limiting trailing-edge populations hinders conservation efforts. Additionally, little is known about the global distributions of these populations, which are often genetically unique and important to ecosystem function. I identified global hotspots of trailing-edge population diversity using BirdLife International’s database of the ranges of all extant terrestrial bird species. At one hotspot, the southern Appalachian Mountains, I assessed the role of abiotic and biotic factors in limiting the distributions of two songbird species. Results from a playback experiment and four years of observational data demonstrated that Allee effects are not present at the range boundary of the Canada warbler (Cardellina canadensis), rather, climate conditions limit population growth rates. I assessed two mechanisms by which climate may limit the distributions of trailing-edge populations: via spatial variation in hemoparasite prevalence and via physiological constraints on egg development. Consistent with the hypothesis that pathogens contribute to climate-induced range shifts in host species, I found that the distribution of a cool-adapted bird species near its trailing-edge limit was negatively associated with pathogen prevalence. Hemoparasite prevalence did not affect occupancy of a warm-adapted species near the core of its range. I also found evidence that physiological constraints can limit the distribution of trailing-edge populations. Results from a reciprocal egg translocation experiment between two ecologically similar species across their local range boundaries revealed that hatch rates of the cool-adapted species declined when moved to warmer, drier conditions. There was little effect of translocation on eggs of the warm-adapted species. Overall my results demonstrate that both abiotic factors and biotic interactions can limit the distributions of trailing-edge populations. However, additional mechanisms remain to be explored, and future work should assess the influence of climate change on biotic interactions between avian predators and arthropod prey on species range limits. The multitude of limiting factors affecting trailing-edge populations poses challenges to conservation efforts, which will need to account for physiological constraints and pathogen prevalence when attempting to mitigate the impacts of climate change.