Effect of Clutch Size and Time of Day on the Energy Expenditure of Incubating Starlings (Sturnus vulgaris)

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Abstract
The steady-state energy expenditure of incubating starlings (Sturnus vulgaris) was studied at different air temperatures, at day and at night, and on different clutch sizes. During the night, energy expenditure (oxygen consumption) increases linearly from 20 down to - 10 C air temperature. During the day, the values at 25 and 10 C air temperature are 50%-60% higher than during the night but only 10% higher at -5 C. This increase is due to the digestive calorigenic effect of food at 25 and 10 C, but at -5 C the heat of the calorigenic effect substitutes for the thermoregulatory requirements. Increasing clutch size results in greater energy expenditure of the incubating starling below 20 C air temperature but has no effect in the thermoneutral zone. Each increase of clutch size by one egg increased the energy expenditure by 3%-5%. The lower critical temperature increased from 14.1 C for a clutch of two eggs to 23.6 C for a clutch of eight eggs. The consequences of different clutch sizes for the time and energy budget of free-living birds are discussed.