COST REDUCTION IN THE COLD - HEAT GENERATED BY TERRESTRIAL LOCOMOTIONPARTLY SUBSTITUTES FOR THERMOREGULATION COSTS IN KNOT CALIDRIS-CANUTUS

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