PARENTAL ENERGY-EXPENDITURE IN RELATION TO MANIPULATED BROOD SIZE IN THE EUROPEAN KESTREL FALCO-TINNUNCULUS

Parental daily energy expenditure (DEE(par)) of European kestrels Falc o tinnunculus with manipulated brood sizes was measured with the doubl y labelled water (DLW) method. The reproductive output of the experime ntal broods increased with brood size. DEE(par) was positively associa ted with the number of young that were added to the brood, while no as sociation with original brood size could be established. The average l evel 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 affect ed 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 (layin g date, mortality), and characteristics of experimental individuals (b ody 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 obse rved direct link between parental energy expenditure and fitness costs . We discuss potential causes of fitness costs as a result of hard wor k.