Warming in the land of the midnight sun: breeding birds may suffer greater heat stress at high- versus low-Arctic sites

Abstract

Rising global temperatures are expected to increase reproductive costs for wildlife as greater thermoregulatory demands interfere with reproductive activities. However, predicting the temperatures at which reproductive performance is negatively impacted remains a significant hurdle. Using a thermoregulatory polygon approach, we derived a reproductive threshold temperature for an Arctic songbird—the snow bunting ( Plectrophenax nivalis ). We defined this threshold as the temperature at which individuals must reduce activity to suboptimal levels (i.e. less than four-time basal metabolic rate) to sustain nestling provisioning and avoid overheating. We then compared this threshold to operative temperatures recorded at high (82° N) and low (64° N) Arctic sites to estimate how heat constraints translate into site-specific impacts on sustained activity level. We predict buntings would become behaviourally constrained at operative temperatures above 11.7°C, whereupon they must reduce provisioning rates to avoid overheating. Low-Arctic sites had larger fluctuations in solar radiation, consistently producing daily periods when operative temperatures exceeded 11.7°C. However, high-latitude birds faced entire, consecutive days when parents would be unable to sustain required provisioning rates. These data indicate that Arctic warming is probably already disrupting the breeding performance of cold-specialist birds and suggests counterintuitive and severe negative impacts of warming at higher latitude breeding locations.

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

“…These results show that even moderate increases in air temperature can significantly increase a snow bunting’s body temperature when they are active, which in turn increases heat dissipation requirements. Problematically, snow buntings posesses extremely poor evapaorative cooling capacities (O’Connor et al, 2021) and therefore will likely rely heavily on reducing their activity to thermoregulate (O’Connor et al, 2022). As a result, our findings strongly indicate that as the Arctic continues to warm, snow buntings will increasingly rely on behavioural adjustments in activity to avoid lethal body temperatures during energetically expensive stages such as the nestling-provisioning period.…”

“…Collectively, these results underscore the likelihood that as the Arctic continues to warm, nestling-provisioning buntings performing at high sustained metabolic rates will increasingly experience periods where they are unable to physiologically limit their body temperature from approaching lethal levels, requiring buntings to behaviourally allocate more time towards dissipating heat. As has recently been demonstrated for hot, arid bird species (Conradie et al, 2019; Cunningham et al, 2021; Smit et al, 2016), behavioural adjustments are likely to result in a cost to essential parental activities (e.g., nestling feeding) and therefore fitness (O’Connor et al, 2022).…”

“…Moreover, the capacity for physiological thermoregulation through increases in evaporative water loss among Arctic birds appears significantly constrained (Choy et al, 2021; O’Connor et al, 2021). Indeed, recent thermal modelling work has predicted that under increasing air temperatures, cold-adapted Arctic birds will have to allocate more time toward regulating body temperature through behavioral adjustments to avoid lethal hyperthermia, but at the expense of interfering with important life-history stages (e.g., reproduction) (O’Connor et al, 2022).…”

“…In contrast, O’Connor et al, (2021) recently showed that snow buntings are considerably constrained in their capacity to dissipate heat through evaporative pathways and become heat stressed at relatively moderate air temperatures. Consequently, under increasing heat loads buntings must rely on behavioural thermoregulatory strategies to maintin non-lethal body temperatures (O’Connor et al, 2022). To investigate body temperature patterns and heat dissipation behaviours in buntings, we worked with an outdoor captive population of snow buntings and took advantage of the natural seasonal variation in air temperatures from -15 °C (winter) to 36 °C (summer).…”

“…We expected maximum body temperatures in actively flying buntings to increase with air temperature, with buntings showing signs of heat stress via panting at comparatively low air temperatures. We were particularly interested in determining at which T a birds would begin to increase T b while active, as breeding buntings may increasingly have to rely on behavioural trade-offs under a warming Arctic to avoid high body temperatures (O'Connor et al, 2022).…”