5.1. Ontogeny of endothermy

Cold tolerance improved with age and the increasing capacity to produce heat. Precocial birds obtained an ability to increase heat production in response to decreasing ambient temperature before or at the age of 1–2 days (I, II). The mass-specific heat production was age-dependent: a major increase in heat production occurred a few days after hatching, and thereafter the mass-specific heat production (W·kg^-1 or ml·min^-1·kg^-1) declined showing a biphasic pattern. In the grey partridge, the maximal mass-specific heat production was achieved before or at 5 days of age. In the Japanese quail, the corresponding age was 7 d. In the domestic fowl, the maximum occurred between days 1–2 and 7. The altricial species studied, the domestic pigeon, was able to increase heat production slightly in cold at 4 days of age but an intense thermogenesis did not occur until 6 days of age. During the 8 posthatching days observed, the mass-specific heat production increased smoothly. In all the species studied, both ambient temperature and age had a major effect on body temperature.

The wetting experiment in newly hatched grey partridge chicks (I) revealed that the ability to withstand cold ambient temperatures decreases dramatically if the insulation of plumage declines. The insulation of downy plumage was vulnerable to wetting, and at the age of three weeks its water impermeability was still improving although the insulation of dry plumage did not change appreciably. Due to the limited insulation of newly hatched chicks, the role of behavioural thermoregulation in resisting cooling was noticeable during the first two post-hatching weeks. In the experiment to study behavioural responses in the thermal gradient, chicks were found to be dependent on the external heat source. Moreover, the utilization of insulation in Japanese quail chicks was dependent on the nutritional state (IV). On the one hand, in chicks exposed to short-term fasting, the most crucial decline in conductance occurred before heat production started to increase during cold exposure. On the other hand, control quails increased insulation and metabolism simultaneously to defend their body temperature in cold. Furthermore, behavioural thermoregulation in the form of huddling was used within 10 h from the beginning of the fasting.

In all the species studied, body temperature measured from the cloaca increased gradually during the post-hatching development. Fig. 1 shows the development of body temperature in a domestic fowl chick between days 0–43. The major increase occurred during the first 10 days and thereafter body temperature increased only slightly but steadily. The diurnal rhythm in body temperature was only weakly visible soon after the hatching but the rhythm became more evident with age as the amplitude increased.