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  1. Black-throated blue warbler (Setophaga caerulescens) populations have been declining at the southern edge of the breeding range in North Carolina over the past two decades. Determining the causes of population declines in migratory species requires knowledge of the threats faced throughout the entire annual cycle, necessitating accurate information about the migratory routes and non-breeding areas used by birds. We used light-level geolocators to identify the fall migratory routes and non-breeding distributions of adults breeding at the southern edge of the range in North Carolina (n = 5), where populations are declining, and at the core of the range in New Hampshire (n = 8), where populations are stable. The strength of migratory connectivity was moderate (mean = 0.42). New Hampshire birds used non-breeding areas broadly distributed across the Caribbean, whereas North Carolina birds used a restricted non-breeding area largely in the Dominican Republic. Suitable forest cover declined at a higher rate from 2000 to 2019 in the Dominican Republic than in other Caribbean countries (8.4% vs. 2–4% loss), exposing birds from the trailing edge to significantly higher suitable habitat loss on the non-breeding grounds compared with range-core birds. Birds from the two study populations also exhibited differing migratory routes, with North Carolina birds migrating south through Florida and many New Hampshire birds performing an overwater flight from the Carolinas to the Caribbean. Our results suggest the possibility that, at least for this species, forest loss on the island of Hispaniola could be exacerbating population declines at the southern edge of the breeding range in North Carolina. 
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  2. Abstract

    Understanding the demographic drivers of range contractions is important for predicting species' responses to climate change; however, few studies have examined the effects of climate change on survival and recruitment across species' ranges. We show that climate change can drive trailing edge range contractions through the effects on apparent survival, and potentially recruitment, in a migratory songbird. We assessed the demographic drivers of trailing edge range contractions using a long‐term demography dataset for the black‐throated blue warbler (Setophaga caerulescens) collected across elevational climate gradients at the trailing edge and core of the breeding range. We used a Bayesian hierarchical model to estimate the effect of climate change on apparent survival and recruitment and to forecast population viability at study plots through 2040. The trailing edge population at the low‐elevation plot became locally extinct by 2017. The local population at the mid‐elevation plot at the trailing edge gradually declined and is predicted to become extirpated by 2040. Population declines were associated with warming temperatures at the mid‐elevation plot, although results were more equivocal at the low‐elevation plot where we had fewer years of data. Population density was stable or increasing at the range core, although warming temperatures are predicted to cause population declines by 2040 at the low‐elevation plot. This result suggests that even populations within the geographic core of the range are vulnerable to climate change. The demographic drivers of local population declines varied between study plots, but warming temperatures were frequently associated with declining rates of population growth and apparent survival. Declining apparent survival in our study system is likely to be associated with increased adult emigration away from poor‐quality habitats. Our results suggest that demographic responses to warming temperatures are complex and dependent on local conditions and geographic range position, but spatial variation in population declines is consistent with the climate‐mediated range shift hypothesis. Local populations of black‐throated blue warblers near the warm‐edge range boundary at low latitudes and low elevations are likely to be the most vulnerable to climate change, potentially leading to local extirpation and range contractions.

     
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