Mertens J.A.L. (1977) The energy requirements for incubation in Great Tits Parus major L. ARDEA 65 (3-4): 184-196

It is a controversial point of discussion whether the normal metabolic energy expenditure is sufficient to cover the; energy requirements for incubation (King 1973). This paper is an attempt to contribute to the solution of this problem by presenting a model of the thermal properties of nest and nestboxes, by which the energy requirements for incubation in relation to the clutch size and the dimensions of the nest and nestbox can be calculated. The calculations performed with this model show that the energy requirements for incubation increase with clutch size. For a Great Tit incubating ten eggs, this implicates that she has to increase her heat production when the ambient temperature drops below 27ŚC. In a nestbox with one egg she is forced to do this only at temperatures below 19ŚC. The metabolic rate of an incubating Great Tit averaged over the whole incubation period is about 1.5 times the basal metabolic rate. The free-existence metabolic rate of non-incubating birds of the size of a Great Tit is about 2.5 times the basal metabolic rate. It must be therefore concluded, that the energy expenditure for free-existence exceeds the energy requirements for incubation. It must be realized, however, that the time available for foraging by an incubating bird is severely limited, so that an energy expenditure of 1.5 times the basal level may be the maximum output a bird is capable of during incubation.