Daily Energy Expenditure of Male and Female Marsh Harrier Nestlings

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Abstract
We used the doubly labeled water (DLW) method to measure daily energy expenditure (DEE) in eight brother-sister pairs of free-living Marsh Harrier (Circus aeruginosus) nestlings. We calculated metabolizable energy intake (ME) from DEE and body-mass change. In each pair, males had lower body mass, DEE, and ME than their female siblings. On average, male body mass was 20%, DEE 19%, and ME 20% lower than that of female siblings. Thus, energy turnover was proportional to body mass. Because the average sex ratio at fledging in Marsh Harriers is 55% male, and the energy requirement of sons is 45% of the son-daughter pair, this sex ratio matches exactly that predicted by Fisher's theory (1930). A literature review revealed that DEE (kJ/day) measured by DLW when nestlings were approximately 95% of asymptotic or fledging mass scaled as 4.58 M (0.76) (where M = body mass in g) in 11 species of altricial nestlings, including the Marsh Harrier