Physiological and biochemical aspects of the development of endothermy in an altricial bird, the red-winged blackbird.

Abstract

Young of the altricial red-winged blackbird (Agelaius phoeniceus) undergo dramatic changes during postnatal development. Neonates of this species are small (approximately 3.5 g), largely naked, and incapable of metabolic thermoregulation. However, body mass subsequently increases 8- (females) to 11-fold (males) during their relatively short 9-11 day nestling period. Most of the growth occurs in the first 5 days of the postnatal period, and growth rates during this period are among the highest reported for any bird (K in logistic equation = 0.59 and 0.67 for females and males, respectively). In addition, the thermoregulatory capabilities of nestlings improve markedly during the nestling period, based on measurements of two indices of thermogenesis (total metabolic rate and shivering intensity in the pectoralis and gastrocnemius muscles) during a cold challenge. Most of the improvements occur after the period of most rapid growth of the body as a whole; 7-day old nestlings achieve peak $\rm VO\sb2s$ at 15 C that average 2-fold higher than their corresponding SMR, and 2.3-fold higher than the peak values in 6-day olds. This profound difference exists despite inconsequential changes in insulation and significant overlap in the masses of the nestlings between nestlings of these two ages. Dramatic ontogenetic changes in the pectoralis muscle correlate well with the time course of the development of endothermy. This muscle is the principal site of shivering thermogenesis in both nestling and adult blackbirds, and the threshold for shivering coincides with the lower critical temperature in $\rm VO\sb2$ at all ages. However, in contrast to the transient and ineffective shivering detected in 3- and 5-day old nestlings, the pectoralis of 8-day old nestlings achieves larger factorial increases in shivering intensity (20- to 22-fold increase over levels at thermoneutral temperatures). Consequently, these nestlings are better able to defend normothermic body temperatures during exposure to cold. The gastrocnemius muscles also contribute to shivering heat production in older nestlings, but this muscle is always secondary in importance to the pectoralis. The ontogenetic increases in the absolute value and range of dominant frequencies in the EMG signal during shivering, the concomitant hypertrophy (and possibly hyperplasia) of fast twitch muscle fibers, and increases in the catabolic capacities and maturation of the contractile apparatus in the pectoralis probably account in part for the dramatic improvements in thermoregulatory abilities during the nestling period in this species.