Daily energy expenditure (DEE) of free-living birds during the incubation and subsequent brood-rearing stages was measured using the doubly-labelled water technique for seven species of bird. These data are combined with published information to provide a data set of 17 species of bird covering the mass range 12–85 g.
Allometric relationships between DEE and body mass for the two stages of breeding were constructed using three models (log-transformation regression, geometric mean regression, and iterative non-linear regression). The geometric mean regression model was rejected as inappropriate. The iterative non-linear regression model provides the best predictions of DEE given the average body mass of a species, but is of less value for comparing data sets. The most commonly used model (log transformation regression) was thus favoured for comparative purposes as the predictions it generates do not significantly alter the conclusions that would be obtained with use of the iterative non-linear regression models (Marquardt model) for the data sets in this work. Comparison between the average DEE for the two stages of breeding suggest higher DEE during the brood-rearing period, although this was significant in only five species.
The difference in body mass among species can account for more of the variation in average DEE during incubation (r2= 82·6%) than during brood-rearing (r2= 67·3%1), indicating that other effects such as activity are probably more important determinants of energy expenditure during brood-rearing.
The residual variation in DEE during incubation, after the effect of body mass had been removed, was considered for groups of species occupying different ecological niches. It is shown that the level of activity and ambient temperature can explain much of the residual variation in DEE during incubation.
The blue tit was used as an example to demonstrate the role of energy balance as a constraint on reproductive success during the incubation period.