Climate‐mediated population dynamics of a migratory songbird differ between the trailing edge and range core

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.

Clinging to the top: natal dispersal tracks climate gradient in a trailing-edge population of a migratory songbird

“…In contrast, directional dispersal can mitigate the effects of changing environmental conditions by allowing individuals to track optimal conditions. Our results, coupled with previous findings on demography [ 41 ], suggest that directional natal dispersal away from warmer and drier climate conditions may explain the local range shift towards higher elevations. Our study presents some of the first evidence of directional natal dispersal in a migratory species.…”

“…If these estimates represented true survival, the population would likely decline precipitously. While previous work predicts this species will be locally extirpated from our lower elevation intensive plot by 2030 [ 41 ], abundance at high elevation sites is predicted to remain stable. These results suggest that directional natal dispersal of individuals born at warmer and drier sites is maintaining abundance at colder and wetting locations.…”

“…In a long-term study of female black-throated blue warbler demography, Lewis et al [ 41 ] found that population declines were greater at the trailing edge of the range than at the range core. At both range positions, population density was highest, and trends were most stable, at higher elevations characterized by cooler and wetter climates.…”

“…Natal dispersal towards colder and wetter sites may only be possible when populations are below carrying capacity, as high conspecific abundance in the best habitat conditions may preclude young individuals from establishing territories [ 21 ]. The documented declines in this population [ 41 ] and the apparent infrequency of high elevation nestlings selecting lower elevation territories, suggest that this population is below carrying capacity and low elevation populations will continue to decline.…”

Genomics‐informed conservation units reveal spatial variation in climate vulnerability in a migratory bird

“…One possible explanation for this difference could be the lower number of routes associated with the South and Northwest, as fewer overall routes may lead to higher year to year variability adding uncertainty in our predictions. While the Eastern AU declines are the steepest, additional research into the vulnerability of the Southern AU may be important as it is located at the southernmost edge of the species distribution, where the effects of climate change are anticipated to be most severe (Lewis et al., 2023).…”