Energy expenditure and body mass changes as measures of reproductive costs in birds

1 The relationship between short-term reproductive costs (indicated by changes in body mass, body condition and daily energy expenditure) and brood provisioning rates is explored using simple models and field observations and experiments on 10 bird species. 2 Where sample sizes were large, energy expenditure could usually be shown to increase with higher rates of nest visiting. No significant trends were found in relation to the number of young in the brood. 3 Energy expenditure was a linear function of brood provisioning rates in house martins, an accelerating function in blue-throated bee-eaters and perhaps a decelerating function in a third species, the pied kingfisher. 4 The relationship between energy expenditure, mass changes and provisioning rates differed between the three main study species. 5 Estimates of energy expenditure alone will usually be inadequate to indicate prospects for parental survival.