Daily Energy Expenditure and Water-Turnover Rate of Adult European Starlings (Sturnus vulgaris) during the Nesting Cycle

-We measured rates of carbon-dioxide production and water turnover in adult starlings by the doubly-labeled water technique. CO2 production of females was 4.23 cm3g-l h-1 during the incubation period, 4.86 cm3 g-' h-I during the early part of the nestling period, and 6.86 cm3 g-1 h-I during the middle of the nestling period, the time of greatest food requirement by the brood. During the last period, the rate of CO2 production by males was 5.50 cm3 gh-1. CO2 production was independent of brood size (3-7, approximately a two-fold range of brood mass) during both the early and middle parts of the nestling period. Water-turnover rates paralleled CO2 production over the nesting cycle, the average values for each sample varying between 0.048 and 0.070 per h. Within each of the samples, waterturnover rates were independent of CO2 production but appeared to be influenced by weather conditions. During the middle of the nestling period, water-turnover rates were higher on 2 cold, rainy days than on 2 milder days. The maximum daily energy expenditure for females was about four times predicted BMR, similar to values reported in other studies. Received 19 August 1983, accepted 4 March 1984. DAILY energy expenditure (DEE) of adult birds has been the subject of many recent studies (Walsberg 1977, 1978, 1983; Mugaas and King 1981). A few investigators have attempted to relate DEE to variations in the time and energy demands of activities during the nesting cycle and to conditions of the environment. For example, Drent and Daan (1980) concluded that the maximum daily work capacity of birds during the nesting cycle is about four times the basal metabolic rate. Some investigators, using time and energy budget methods, have suggested that there is an increase in DEE between the incubation and nestling periods (Walsberg 1977, Mugaas and King 1981); others have found none (Custer 1974, Withers 1977, Holmes et al. 1979, Ettinger and King 1980). Part of this discrepancy may arise from uncertainty over factors converting activity to energy expendi-