Parental energy expenditure in relation to manipulated brood size in the European kestrel Falco tinnunculus

Parental daily energy expenditure (DEE(par)) of European kestrels Falco tinnunculus with manipulated brood sizes was measured with the doubly labelled water (DLW) method. The reproductive output of the experimental broods increased with brood size. DEE(par) was positively associated with the number of young that were added to the brood, while no association with original brood size could be established.

The average level of DEE(par) did not differ between the sexes, but female DEE(par) increased with age of the brood, whereas male DEE(par) was not affected by age of the brood.

Other potentially important effects on DEE(par) were examined, such as environmental factors (wind speed, rainfall, and ambient temperature), characteristics of experimental nests (laying date, mortality), and characteristics of experimental individuals (body mass). None of the tested factors contributed significantly to the explained variance in DEE(par).

The negative association of DEE(par) and local survival rate of experimental parent birds is the first observed direct link between parental energy expenditure and fitness costs. We discuss potential causes of fitness costs as a result of hard work.