Development of endothermy in birds: patterns and mechanisms

Endothermy is a conspicuous and important adaptation in birds. Even though juvenile and adult birds are endothermic and maintain a constant, high body temperature by means of internal heat production, they begin life expressing an ectothermic phenotype. Depending on where a species falls along a continuum of maturity at hatching, from precocial to altricial, they begin to express endothermic traits either close to the time of hatching or as nestlings over a period of 1-3 weeks. Developing endothermy requires attaining a high basal metabolic rate and associated aerobic scope to produce sufficient internal heat, insulation to retain the internally produced heat, and a thermostat that "turns on" heat production in response to cooling ambient temperatures. To support the high metabolic costs of endothermy, the animal must have the capacity to deliver sufficient oxygen and nutrients to the heat-generating tissues. In this review, we examine the development of physiological and morphological traits that are required for endothermy and discuss their potential to limit the development of endothermy. These include ventilatory and cardiovascular function, contribution of visceral organ masses, membrane lipid composition, substrate supply pathways, and skeletal muscle physiology. The developmental trajectories of each of these systems in precocial and altricial species can have significant effects on the development of an endothermic phenotype.