Development of temperature regulation in the grey partridge Perdix perdix

Abstract

The development of temperature regulation in grey partridge Perdix perdix, Galliformes chicks at the age of 0–40 days was studied using three different approaches: 1) heat production was measured by indirect calorimetry of chicks subjected to various ambient temperatures; 2) the ability to resist cooling was studied by subjecting chicks to an ambient temperature of 0°C with either dry or wetted plumage; 3) behavioural responses were studied in a thermal gradient using a video camera and an image processing system. The development of homeothermy was not linear. Absolute metabolic rates increased with age at all temperatures, while the maximal mass‐specific heat production (W×kg ‐0.75) doubled between days 1‐2 and day 5, but remained unchanged after that. Assuming that the allometric exponent accounts for mass‐induced changes in the metabolic rate, this suggests that the increase in maximal heat production before the age of five days can be attributed to both maturation and growth, while the increase after five days can be explained by growth only. At the age of 1–2 days, the chicks' capacity to increase heat production in cold was negligible. In five and 10‐11‐day‐old chicks the capacity was markedly higher, but the maximal heat production occurred already at 25°C. A quantum leap occurred between 10–11 and 15–16 days, when the point of maximal heat production shifted from 25°C to 5°C, which indicates a significant improvement of insulation. Wetted chicks cooled faster than dry chicks until 21 days of age. The relative effect of wetting was maximal at 10–11 days. Behavioural observations suggest a drop in the time spent in the warmest section of the thermal gradient after five days and that the coldest part of the gradient (5°C) was hardly visited before 25 days of age. The sequence for attaining homeothermia in the partridge is thus comprised of an early development of significant thermogenesis from day 5 onwards, followed by a rapid increase in insulation around two weeks of age. Despite this fact, especially wet chicks strongly depend on parental heat in cold spells during the first three weeks of their life. The increase in body mass and the development of a water‐impermeable plumage enhance the cold resistance in older chicks.