Search for: All records

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 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.