Skip to main content
Dryad

Context-dependent effects of relative temperature extremes on bill morphology in a songbird

Data files

Apr 07, 2020 version files 944.91 KB

Abstract

Species increasingly face environmental extremes. While responses of morphological traits to changes in average environmental conditions are well-documented, responses to environmental extremes remain poorly understood. Bird bills contribute to thermoregulation, with considerable heat loss possible through the bill surface, and with bill morphology shaped by long-term thermal conditions. We used museum specimens to investigate the relationship of bill surface area (SA) in dark-eyed juncos Junco hyemalis to traditional measures of climate (temperature and precipitation) and to a novel measure of short-term relative temperature extremity, which quantifies the degree to which temperature maxima or minima have diverged from the recent five-year norm. We found that bill SA exhibits different patterns of association with relative extremity depending on the overall temperature regime and on precipitation. While thermoregulatory function predicts larger bill SA at higher relative temperature extremities, we found this to be the case only when the measure of temperature extremity existed in an environmental context that opposed it: relative minimum temperature in a warm climate, or relative maximum temperature in a cool climate. When, instead, environmental context amplified the relative temperature extremity, we found a negative relationship between bill SA and relative temperature extremity. We also found that the strength of associations between bill SA and relative temperature extremity increased as precipitation increased. Our results suggest that trait responses to environmental variation may qualitatively differ depending on the overall environmental context, and that environmental change that extremifies already-extreme environments should be of particular concern. Extreme-on-extreme environmental change may produce responses that cannot be predicted from observations in less extreme contexts, which should make it a priority for research on species' responses to climate change as well as trait evolution generally.Species increasingly face environmental extremes. Morphological responses to changes in average environmental conditions are well-documented, but responses to environmental extremes remain poorly understood. We used museum specimens to investigate relationships between a thermoregulatory morphological trait, bird bill surface area (SA), and a measure of short-term relative temperature extremity (RTE), which quantifies the degree that temperature maxima or minima diverge from the five-year norm. Using a widespread, generalist species, Junco hyemalis, we found that SA exhibited different patterns of association with RTE depending on the overall temperature regime and on precipitation. While thermoregulatory function predicts larger SA at higher RTE, we found this only when the RTE existed in an environmental context that opposed it: atypically cold minimum temperature in a warm climate, or atypically warm maximum temperature in a cool climate. When environmental context amplified the RTE, we found a negative relationship between SA and RTE. We also found that the strength of associations between SA and RTE increased with precipitation. Our results suggest that trait responses to environmental variation may qualitatively differ depending on the overall environmental context, and that environmental change that extremifies already-extreme environments may produce responses that cannot be predicted from observations in less extreme contexts.