Development of Endothermy in Nestling Bank Swallows (Riparia riparia)

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Abstract
Body temperature measurements after exposure to air temperatures of 20 C or 27 C for 2 h in conjunction with metabolism measurements were used to describe the timing of and basis for developing temperature regulation in nestling swallows. As growth proceeds from hatching to 10 g there is a gradual increase in the after exposure to low . As growth continues beyond this point, the ability of nestlings to maintain above improves rapidly, such that nestlings weighing over 14 g are completely homeothermic when exposed to 20 C. Conductance (C, in cal [g∙h∙ °C]⁻±) decreases throughout growth following the equation: log C = -0.501 log m + - log 6.902, where m = mass in grams. Conductance values and the relation of to metabolism were used to predict the Tb after exposure to 20 C for 2 h, assuming the nestlings cooled passively. These calculations demonstrate that large changes in - can occur with growth even in the absence of active metabolic regulation. The major conclusions of this study are: (1) in bank swallows, mass is a better predictor of thermoregulatory ability than age; (2) conductance values are influenced mostly by increasing mass, whereas the developing plumage during the nestling period does not significantly retard heat loss; (3) apparent improvement in thermoregulation with growth in nestlings weighing less than 10 g is due to changes in passive resistance to cooling, not improvement in active metabolic regulation; and (4) active metabolic regulation develops rapidly as nestlings increase in mass above 10 g.