Figure 2.

(a) Snow bunting (Plectrophenax nivalis) thermoregulatory polygon bounded by BMR (0.564 W), minimum wet thermal conductance (0.023 W/°C), maximum dry thermal conductance (0.073 W/°C) and maximal sustained metabolic rate set at 6 × BMR. (b) Sustainable performance (expressed as a multiple of BMR) is possible for buntings under thermal balance. At operative temperatures below 11.7°C, buntings can maintain thermal balance and sustain optimal performance (i.e. performance ≥ 4 × BMR). As operative temperatures increase, buntings must reduce activity, and concomitantly metabolic rate, to maintain thermal balance, resulting in a suboptimal performance (i.e. performance less than 4 x BMR). Optimal performance is defined as the sustained level of work required by adults to sufficiently rear nestlings. The continuous colour scheme signifies the transition into more detrimental impacts on sustainable performance at higher temperatures (i.e. dark red zone). The black dashed lines are the extrapolation of the minimum and maximum thermal conductance slopes to the average body temperature recorded during laboratory measurements. (Online version in colour.)