Daily Energy Expenditure (DEE) in the Common Kestrel has been extensively studied in the netherlands, making use of modern stable isotope techniques (the Doubly Labeled Water method; DLW). We anticipate that at latitudes closer to the equator, where brood sizes are smaller, and possibly adult mortality is reduced, that birds are adpted to work at reduced overal levels of DEE. this will be staudied by applying the same DLW technique to Common Krestels during the breeding season in Souther Spain. Comparison of data obtained at both latitudes will be applied to ansewr the question of latitudinal adaptation. By exchanging nestlings between broods, the question ehether the parental the parental DEE levels represent physiological maxima will be adressed.