INTERSPECIFIC VARIATION IN DAILY ENERGY-EXPENDITURE DURING AVIAN INCUBATION

Daily energy expenditure (DEE) of free-living birds during the incubat ion and subsequent brood-rearing stages was measured using the doubly- labelled water technique for seven species of bird. These data are com bined with published information to provide a data set of 17 species o f bird covering the mass range 12-85 g. Allometric relationships betwe en DEE and body mass for the two stages of breeding were constructed u sing three models (log-transformation regression, geometric mean regre ssion, and iterative non-linear regression). The geometric mean regres sion model was rejected as inappropriate. The iterative non-linear reg ression model provides the best predictions of DEE given the average b ody mass of a species, but is of less value for comparing data sets. T he most commonly used model (log transformation regression) was thus f avoured for comparative purposes as the predictions it generates do no t significantly alter the conclusions that would be obtained with use of the iterative non-linear regression models (Marquardt model) for th e data sets in this work. Comparison between the average DEE for the t wo stages of breeding suggest higher DEE during the brood-rearing peri od, although this was significant in only five species. The difference in body mass among species can account for more of the variation in a verage DEE during incubation (r2=82.6%) than during brood-rearing (r2= 67.3%), indicating that other effects such as activity are probably mo re important determinants of energy expenditure during brood-rearing. The residual variation in DEE during incubation, after the effect of b ody mass had been removed, was considered for groups of species occupy ing different ecological niches. It is shown that the level of activit y and ambient temperature can explain much of the residual variation i n DEE during incubation. The blue tit was used as an example to demons trate the role of energy balance as a constraint on reproductive succe ss during the incubation period.