An arctic breeding songbird overheats during intense activity even at low air temperatures

Abstract

Birds maintain some of the highest body temperatures among endothermic animals. Often deemed a selective advantage for heat tolerance, high body temperatures also limits birds' thermal safety margin before reaching lethal levels. Recent modelling suggests that sustained effort in Arctic birds might be restricted at mild air temperatures, which may require reductions in activity to avoid overheating, with expected negative impacts on reproductive performance. We measured within-individual changes in body temperature in calm birds and then in response to an experimental increase in activity in an outdoor captive population of Arctic, cold-specialised snow buntings (Plectrophenax nivalis), exposed to naturally varying air temperatures (- 15 to 36 °C). Calm buntings exhibited a modal body temperature range from 39.9 to 42.6 °C. However, we detected a significant increase in body temperature within minutes of shifting calm birds to active flight, with strong evidence for a positive effect of air temperature on body temperature (slope = 0.04 °C/ °C). Importantly, by an ambient temperature of 9 °C, flying buntings were already generating body temperatures ≥ 45 °C, approaching the upper thermal limits of organismal performance (45-47 °C). With known limited evaporative heat dissipation capacities in these birds, our results support the recent prediction that free-living buntings operating at maximal sustainable rates will increasingly need to rely on behavioural thermoregulatory strategies to regulate body temperature, to the detriment of nestling growth and survival.

Keywords: Arctic breeding species; Climate change; Evaporative cooling; Flight; Heat tolerance; Hyperthermia; Thermoregulation.

Figure 1

Individual mean body temperatures for calm (non-active) snow buntings (Plectrophenax nivalis) implanted intraperitoneally or subcutaneously with temperature sensitive transponder tags. Error bars represent standard deviation. The horizontal dashed lines are the respective mean maximum body temperatures for intraperitoneal and subcutaneous birds when actively flying. All values represent data recorded between sunrise and sunset.

Figure 2

Mean body temperature as a function of air temperature measured in a captive population of calm (non-active) snow buntings (Plectrophenax nivalis). Horizontal lines represent the respective average modal body temperatures for intraperitoneal and subcutaneous birds. Error bars were removed for clarity. All values represent data recorded between sunrise and sunset.

Figure 3

Body temperature patterns in active snow buntings (Plectrophenax nivalis) as a function of a time since entering the aviary and b air temperature. In panel a, negative minutes represent the time span prior to entering the aviary and positive values represent the time span after entering the aviary. The vertical dashed line in panel a represents the start time for each experiment (i.e., Minutes = 0). In panel b, the regression line represents the conditional effect of air temperature on body temperature using the model-averaged regression estimates from the 95% confidence set of the best-ranked models (see Table 4).

Figure 4

The impacts of air temperature on a the percent of actively flying snow buntings (Plectrophenax nivalis) with a body temperature (T_b) greater than or equal to 45 °C and b the percent of buntings exhibiting panting behavior. Note the y-axes are on different scales. The horizontal dashed line in b represents the threshold above which 50% of the study population was dissipating heat through panting behavior. There are fewer data points in b as panting was only recorded during 2020.

Figure 5

Mean hyperthermic scope (i.e., maximum flight body temperature—modal body temperature) as a function of modal body temperature (T_b-mod) among actively flying snow buntings (Plectrophenax nivalis).