Inertial Thermostability and Thermoregulation in Broods of Redwing Blackbirds

Metabolic rate and body temperature (Tb) were measured during 2 h of exposure to ambient temperatures (Ta) of 5-32 C in (a) broods of three or four redwing blackbirds (Agelaius phoeniceus) in a nest and (b) single redwings of comparable ages, isolated from the nest. Between 2 and 6 days of age, broods maintained markedly higher Tb's than individuals; for instance, 6-day-old broods usually kept Tb above 35 C even at 5 CTa, whereas the Tb of 6-day-old single animals usually fell to approach Ta at 15 CTa and fell below 35 C even at 25 C Ta. Single 8-day-olds thermoregulated at 15 C and sometimes at 5 C but did so at a higher mass-specific metabolic cost than broods. Thermogenesis is not brought under thermoregulatory control until 4-5 days of age in redwings, although broods can modulate their insulation homeostatically at least as early as 3 days. Two phases are recognized in the life of broods of altricial nestlings. Phase 1 (inertial phase): although thermogenesis is not under thermoregulatory control, Tb's of broods in the nest can remain well above Ta during long parental absences by virtue of thermal inertia; metabolic rates, and probably growth rates, are correspondingly kept from falling. Phase 2 (regulatory phase): broods thermoregulate physiologically, resulting in more effective stabilization of Tb and higher metabolic costs than in phase 1. In redwings and three other passerine species, at least 53%-72% of the increase in body energy content resulting from growth between hatching and fledging occurs during phase 2. Thermoregulatory altriciality has been claimed to promote efficiency of growth by precluding use of energy for thermoregulation. Study of the thermal and metabolic status of broods reveals that although the claim may be valid early in nestling life (phase 1), it can apply only during completion of less than half of nestling growth.