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ABSTRACT The abiotic range limitation hypothesis states that species distributions are shaped by physiological constraints imposed by temperature and precipitation. To test this hypothesis, we assessed the impacts of climate on hatch rates by reciprocally translocating complete clutches of both Setophaga caerulescens (Black-throated Blue Warbler) and S. citrina (Hooded Warblers) across a local range boundary of S. caerulescens in the southern Appalachian Mountains. The S. caerulescens population occurs at the trailing edge of its breeding range, whereas the S. citrina population occurs near the core of its range. The hatching probability of S. caerulescens eggs declined from 0.93 ± 0.02 to 0.60 ± 0.07 when moved to S. citrina nests in warmer conditions. Translocation, however, had little effect on hatching probability of S. citrina eggs when moved to S. caerulescens nests in cooler environments. Thirteen reciprocal clutch translocations were performed; 17 clutches were moved as controls; and 49 nests were not manipulated. We monitored species-specific incubation behavior, measured microclimate conditions inside and outside nests using hygrochron iButtons, and examined the effects of temperature and humidity on nestling growth rates. Higher ambient temperatures had a greater effect on hatching probability than did humidity, but we were unable to determine if reduced hatching was caused by changes in temperature, humidity, or their interaction. We suggest that, in warmer conditions, S. caerulescens eggs in S. citrina nests may have been unable to cool sufficiently to avoid excessive water loss due to higher ambient temperatures but not a difference in relative humidity. Our finding that hatch rates of S. caerulescens declined when translocated to warmer conditions supports the hypothesis that distributions of trailing-edge populations are limited in part by climate effects on reproductive rates. 

ABSTRACT Many populations near receding low-latitude range: margins are declining in response to climate change, but most studies of trailing-edge populations have focused on single species. Using 10 years (2014–2023) of avian survey data from a high-elevation trailing-edge population hotspot in the Appalachian Mountains, USA, we tested the hypothesis that high-elevation communities would experience turnover through thermophilization, as warm-adapted species near the center of their geographic ranges expand into regions formerly dominated by peripheral populations of cool-adapted species. Three of the nine cool-adapted, peripheral populations decreased in abundance, and whereas 6 species exhibited little change. For warm-adapted populations near the core of their range, 1 of 16 decreased in abundance, 11 increased, and 4 exhibited no change. Within the most abundant species in this community, our results indicate that warm-adapted species are expanding their ranges faster than the rate at which ranges of cool-adapted species are contracting. Avoiding future community turnover may require conservation strategies that maintain microclimates for cool-adapted species facing novel abiotic and biotic conditions at high elevations. 

Abstract AimAccounting for biotic interactions in species distribution models is complicated by the fact that interactions occur at the individual‐level at unknown spatial scales. Standard approaches that ignore individual‐level interactions and focus on aggregate scales are subject to the modifiable aerial unit problem (MAUP) in which incorrect inferences may arise about the sign and magnitude of interspecific effects. LocationGlobal (simulation) and North Carolina, United States (case study). TaxonNone (simulation) and Aves (case study). MethodsWe present a hierarchical species distribution model that includes a Markov point process in which the locations of individuals of one species are modelled as a function of both abiotic variables and the locations of individuals of another species. We applied the model to spatial capture‐recapture (SCR) data on two ecologically similar songbird species—hooded warbler (Setophaga citrina) and black‐throated blue warbler (Setophaga caerulescens)—that segregate over a climate gradient in the southern Appalachian Mountains, USA. ResultsA simulation study indicated that the model can identify the effects of environmental variation and biotic interactions on co‐occurring species distributions. In the case study, there were strong and opposing effects of climate on spatial variation in population densities, but spatial competition did not influence the two species' distributions. Main ConclusionsUnlike existing species distribution models, the framework proposed here overcomes the MAUP and can be used to investigate how population‐level patterns emerge from individual‐level processes, while also allowing for inference on the spatial scale of biotic interactions. Our finding of minimal spatial competition between black‐throated blue warbler and hooded warbler adds to the growing body of literature suggesting that abiotic factors may be more important than competition at low‐latitude range margins. The model can be extended to accommodate count data and binary data in addition to SCR data. 

Abstract PurposeTrailing-edge populations at the low-latitude, receding edge of a shifting range face high extinction risk from climate change unless they are able to track optimal environmental conditions through dispersal. MethodsWe fit dispersal models to the locations of 3165 individually-marked black-throated blue warblers (Setophaga caerulescens) in the southern Appalachian Mountains in North Carolina, USA from 2002 to 2023. Black-throated blue warbler breeding abundance in this population has remained relatively stable at colder and wetter areas at higher elevations but has declined at warmer and drier areas at lower elevations. ResultsMedian dispersal distance of young warblers was 917 m (range 23–3200 m), and dispersal tended to be directed away from warm and dry locations. In contrast, adults exhibited strong site fidelity between breeding seasons and rarely dispersed more than 100 m (range 10–1300 m). Consequently, adult dispersal kernels were much more compact and symmetric than natal dispersal kernels, suggesting adult dispersal is unlikely a driving force of declines in this population. ConclusionOur findings suggest that directional natal dispersal may mitigate fitness costs for trailing-edge populations by allowing individuals to track changing climate and avoid warming conditions at warm-edge range boundaries. 

Abstract Identifying genetic conservation units (CUs) in threatened species is critical for the preservation of adaptive capacity and evolutionary potential in the face of climate change. However, delineating CUs in highly mobile species remains a challenge due to high rates of gene flow and genetic signatures of isolation by distance. Even when CUs are delineated in highly mobile species, the CUs often lack key biological information about what populations have the most conservation need to guide management decisions. Here we implement a framework for CU identification in the Canada Warbler (Cardellina canadensis), a migratory bird species of conservation concern, and then integrate demographic modelling and genomic offset to guide conservation decisions. We find that patterns of whole genome genetic variation in this highly mobile species are primarily driven by putative adaptive variation. Identification of CUs across the breeding range revealed that Canada Warblers fall into two evolutionarily significant units (ESU), and three putative adaptive units (AUs) in the South, East, and Northwest. Quantification of genomic offset, a metric of genetic changes necessary to maintain current gene–environment relationships, revealed significant spatial variation in climate vulnerability, with the Northwestern AU being identified as the most vulnerable to future climate change. Alternatively, quantification of past population trends within each AU revealed the steepest population declines have occurred within the Eastern AU. Overall, we illustrate that genomics‐informed CUs provide a strong foundation for identifying current and future regional threats that can be used to inform management strategies for a highly mobile species in a rapidly changing world.