Energetic strategies are of adaptative significance to living organisms that allow them to cope with high energetic expenditure. Energetic strategies for reproduction allow birds to optimize both their reproductive success and survival. New information about energetic strategies improves our understanding of the 'life-history strategies' of birds. In recent years, measurement of energy expenditure of free-ranging animals is made possible by using a non-invasive isotopic method. The ecological characteristics of reproduction in the Tengmalm's owl are well known, and this species therefore presents an ideal model for studies of energy expenditure. The major objective of this project is to determine the factors (clutch size, territory quality, food availability) that lead to a high energetic expenditure during the period of reproduction and the energetic strategy used. For this, the daily energetic expenditure and the energetic time-budget during the reproduction period will be evaluated.