Snow Buntings, an arctic cold-specialist passerine, risk overheating under intense activity even at low air temperatures

Birds maintain some of the highest body temperatures (Tb) among endothermic animals. Often deemed a selective advantage for heat tolerance, high Tb also limits the capacity to increase Tb before reaching lethal levels. Recent thermal modelling suggests that sustained effort in Arctic birds might be restricted at mild air temperatures (Ta) during energetically demanding life history stages, which may force reductions in activity to avoid overheating, with expected negative impacts on reproductive performance. Consequently, understanding how Arctic birds will cope with increasing Ta has become an urgent concern. We examined within-individual changes in Tb in response to an experimental increase in activity in outdoor captive Arctic cold-specialised snow buntings ( Plectrophenax nivalis ), exposed to naturally varying Ta from -15 to 36 °C. Calm buntings exhibited a modal Tb range from 39.9 – 42.6 °C. However, we detected a dramatic increase in Tb within minutes of shifting birds to active flight, with strong evidence for a positive effect of Ta on Tb (slope = 0.04 °C/°C). Importantly, by Ta of 9 °C, flying buntings were already generating Tb ≥ 45°C, approaching the upper thermal limits of organismal performance (i.e., Tb = 45 - 47 °C). Under scenarios of elevated Tb, buntings must increase rates of evaporative water loss and/or reduce activity to avoid overheating. With known limited evaporative heat dissipation capacities, we argue buntings operating at peak energy levels will increasingly rely on behavioral thermoregulatory strategies (i.e., reducing activity) to regulate Tb, at the potential detriment to nestling growth and survival. ### Competing Interest Statement The authors have declared no competing interest.