Abstract
The energetic cost of incubation of a free-living Blue Tit (Parus caeruleus) female was studied during two breeding seasons by measuring the rate of oxygen consumption in a nest box converted into a metabolic chamber. Like its congeners, only the female Blue Tit incubates and during that time is fed by the male. Just before and during the egg-laying period the female spends the night in the nest. Because of the progressive development of incubation behavior during this period, it is possible to measure the oxygen-consumption rate of a nonincubating female (resting metabolism) and to compare it with values obtained later when the bird is incubating a full clutch under otherwise similar conditions. The air temperature in the metabolic chamber was regulated experimentally. The results show that the energy cost of incubation is relatively important below the lower critical temperature (about 15°C). With a fall in the air temperature, energy expenditure increased in relation to that of the resting metabolism. The energy cost of incubation also increased with clutch size, by about 6-7% for each additional egg. At air temperatures around 0°C, which are frequent under natural conditions in Fennoscandia, the female must increase her metabolic rate by 50-90% to keep the eggs in a normal-size clutch (10-13 eggs) warm. During the last days of incubation we accounted for the metabolism of the embryos, which on the day before hatching contributed about 15% of the total oxygen consumption when the female was incubating a clutch of 13 eggs.