Begging Response in Nestling Red-winged Blackbirds (Agelaius phoeniceus): Effect of Body Temperature

Altricial birds are essentially ectothermic at hatching. Parental brooding is the primary thermoregulatory mechanism, and body temperature (Tb) is about 5° C lower and more variable than adult Tb. Thermoregulation improves with growth, and a stable Tb near adult levels can soon be maintained. We hypothesize that the thermal performance curves for whole-animal behavior and for neuromuscular contractility should shift during development to match the normal Tb range. We tested this hypothesis with 1- and 8-d-old nestling red-winged blackbirds (Agelaius phoeniceus). We measured several parameters of the begging display used to release parental feeding behavior at Tb's from 15° to 42° C. We also measured some of the isometric contractility properties of the sciatic-gastrocnemius system as a function of tissue temperature from 15° to 42° C. The optimal temperature for begging height, begging speed, gape width, and isometric tetanic force increased 5°-15° C between 1-d-old and 8-d-old birds. The temperature sensitivity (Q10 or R10) of all variables except gape width also increased with age, resulting in a narrower range of near-optimal performance. The behavior parameters were more temperature-sensitive than the neuromuscular tetanic responses. The absolute performance of 8-d nestlings was better than that of 1-d nestlings except at low temperatures.