The Early Development of Thermoregulation in Birds

Abstract

There is evidence for incipient thermogenesis in the embryos of precocial species of birds during the later stages of incubation. In ovo, nascent thermoregulation is limited by (1) the high thermal conductance of the egg and embryonic tissues, (2) the rate at which oxygen can diffuse into the egg through the shell and the chorioallantoic membrane ("oxygen-conductance limited"), and (3) the degree of development of the thyroid gland and the metabolic capacity of the embryo ("power limited"). In birds with a semiprecocial mode of development, hatching appears to be a climactic event in the development of the thermoregulation. In contrast, altricial species are at the mercy of the ambient temperature ("Arrhenius limited") even after hatching, and they never experience an oxygen-conductance limitation. Thus, there is a great disparity in the thermoregulatory competence of hatchlings varying from the altricial poikilotherm to the precocial homeotherm. Consequently, the development of thermoregulation in altricial species is an entirely posthatching event, whereas in precocial species there is an embryonic component in the development of thermoregulation and the process is subject to several extraneous limitations, notably the functional pore structure of the eggshell, and the chorioallantoic membrane.