ENERGETIC EXPENDITURE OF MALE OSPREYS PROVISIONING NATURAL AND MANIPULATED BROODS

This study examined the relationship between brood size and energetic expenditure of male Ospreys. The daily energetic expenditure (DEE) of male Ospreys, provisioning nestlings and providing post-fledging paren tal care, was estimated using detailed time-energy budgets. Factors af fecting DEE were investigated, and DEE and the maximum rate of energy assimilation were compared to determine whether physiological constrai nts limit energetic expenditure. The DEE of male Ospreys was independe nt of brood age, but increased linearly with brood size. Males provisi oning three-chick broods had a mean DEE of 1336 kJ/day, significantly more than the mean of 1084 kJ/day expended by males provisioning singl e-chick broods. Male Ospreys, however, did not respond to brood size m anipulations by altering their energetic expenditure, indicating that male DEE is not directly determined by brood size. Male Ospreys also d id not alter their intake rates when provisioning enlarged broods and the mean brood growth rate following brood enlargement was consequentl y reduced. Weather conditions had a small affect on DEE; male Ospreys had a lower energetic expenditure on days when the water surface was c alm for a longer time. The DEE of male Ospreys in this study was estim ated to be well below the maximum they are able to sustain. Physiologi cal constraints therefore appear unlikely to limit brood size in Ospre ys. It is suggested that individual differences in age or current cond ition may influence both brood size and the level of energetic expendi ture of male Ospreys as these factors could affect an individual's res idual reproductive value and the magnitude of the costs associated wit h various levels of reproductive effort.