The doubly labeled water method was used to measure daily energy expenditure of adult Ovenbirds (Seiurus aurocapillus) feeding nestlings in large and small forests in northern New England. Carbon dioxide production for all birds averaged 7.67 ± 1.29 mL g−1 hr−1 (n = 27), equivalent to 90.1 ± 15.0 kJ day−1. Carbon dioxide production did not differ between males and females within the same pair measured over the same period. Ratio of daily energy expenditure to basal metabolic rate (BMR) for Ovenbirds (3.4) was closer to the mean value derived for aerial foragers (3.8) than that for ground foragers (2.3) in a sample of passerines feeding nestlings. High daily energy expenditure for Ovenbirds may be related to their relatively brief breeding season in northern climates and their use of cool, closed-canopy forests. Although daily energy expenditure for Ovenbirds was high when compared to other ground-foraging passerines, most individuals were not working near the hypothetical maximum of 4× BMR (Drent and Daan 1980). Daily energy expenditure for Ovenbirds feeding young was greater in large forests than small forests, although the difference was not statistically significant. We speculate that time used for foraging (and hence energy expenditure) may be lower in small forests as a result of increased prey density, or alternatively, greater risk of nest predation in forest fragments leads to greater vigilance at the nest site and less time available for foraging. Further studies of avian energetics in large and small forests, including detailed time–activity budgets, may reveal hidden costs of forest fragmentation.
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Vol. 119 • No. 4