Partitioning heat loss from mallard ducklings swimming on the air-water interface.

Water birds whose young begin swimming while downy are interesting because hypothermia and mortality are associated with wetting. While wetting is known to increase heat loss, little is known about basic issues, such as the amount of heat lost to air vs water during surface swimming. To partition heat loss to air and water, we measured the body temperature, metabolism and thermal conductance of 2-3-day-old mallard ducklings (Anas platyrhynchos) swimming under different combinations of air and water temperature. Ventral down remained dry or was wetted only on the surface, and most ducklings could maintain Tb>39 degrees C for 1 h while swimming on water as cold as 5 degrees C. Ducklings were at or below thermal neutrality when swimming in water at Tw=30 degrees C even when air temperature Ta=45 degrees C. Heat loss from ducklings with dry down to air and water was partitioned by fitting data to a heat transfer model of the form M=G(Tb-Tw)+Ke(Tb-Ta). For an average 48 g duckling, thermal conductance to water increased with water temperature, G=0.0470(1+1.059 x 10(-6)Tw4)W/ degrees C-animal. Conductance to air was Ke=0.0196 W/ degrees C-animal for all air temperatures. Thus, a minimum of 70% of metabolic heat production is lost to water, and this fraction increases with increasing temperature.