TEMPERATURE REGULATION IN NEONATES OF SHOREBIRDS

In the laboratory we determined cooling rates, resting and peak metabo lic rates, and minimal thermal conductances in neonates of nine North American and five European shorebird species, with neonatal body masse s between 4 and 55 g. We measured the initial (T(i)) and final body te mperature (T(f)) of chicks during 30-min exposures to an ambient tempe rature (T(a)) of 18-degrees-C. For each trial, the change in body temp erature was converted to an index of homeothermy (H) by dividing the f inal temperature difference between the chick and surrounding air by t he initial difference, such that H = (T(f) - T(a))/(T(i) - T(a)). The interspecific relation between H and body mass (M; grams) could be des cribed by H = 0.073 + 0.464 log10(M). Among neonates, the interspecifi c relationships of effective net peak metabolic rate (peak metabolic r ate minus the evaporative heat loss) to body mass could be described b y PMR(n) (W-ind 1) = 0.0161M0.922, of minimal dry thermal conductance (minimal thermal conductance minus the evaporative heat loss) by h(na) (W.degrees-C-1.ind-1) = 0.0114M0.359, and of the ratio between heat p roduction and heat loss by PMR(n)/h(na) = 1.412M0.563. Thus, the highe r degree of homeothermy in larger neonates could be explained by their more favorable ratio of heat production to heat loss. Peak metabolic rates of shorebird neonates were 1.9 times the resting metabolic rates in the zone of thermoneutrality. Rate of decrease in body temperature could be adequately simulated with a Newtonian cooling model that inc orporated metabolic measurements corrected for evaporative heat loss. The degree of homeothermy of shorebird neonates appears to be related primarily to body mass. The occupation of cold, arctic regions during the breeding season by some species does not depend on adjustment of e ither minimal thermal conductance or thermogenic heat production by ne onates, compared to other species that breed in more temperate climate s.