Cost reduction in the cold: heat generated by terrestrial locomotion partly substitutes for thermoregulation costs in Knot Calidris canutus

To test whether heat generated during locomotion substitutes for the thermoregulation cost, oxygen consumption of four post-absorptive temperate-wintering Knot Calidris canutus was measured at air temperatures of 25̀C (thermoneutral) and 10̀C (c. 10̀ below the lower critical temperature) when the birds were at rest at night and during running on a treadmill. After allowing for body mass, the thermoregulation cost at 10̀C was significantly lower in active birds compared with birds at rest. At rest, the birds spent, on average, 0.50 watt (W; range, 0.47-0.57 W) on thermoregulation. During exercise, this cost factor averaged 0.33 W (range, 0.25-0.42 W). The average difference in thermoregulation cost was 35% (ranging from 26% to 49% between individuals) and provides an estimate of the amount of substituted heat. A review of nine studies, all restricted to small birds, showed that substitution is a widespread phenomenon. The consequences of such partial substitution for the annual energetics of Knot wintering in the temperate Wadden Sea v tropical west Africa are examined. Compared with a previous additive model, the model which includes substitution (i.e. the use of heat produced during activity) reduces the differences in maintenance metabolism between the two wintering strategies by 17%, from 1.19 W to 0.99 W.