Verhulst S. & Tinbergen J.M. (1997) Clutch size and parental effort in the Great Tit Parus major. ARDEA 85 (1): 111-126

We experimentally reduced clutch size of Great Tits Parus major to investigate the effects on parental care (including Daily Energy Expenditure, DEE, measured with doubly labelled water). and the relationship between DEE and the residual reproductive value. The length of a working day was not affected. but the nestling phase was 0.7 days (4%) shorter in experimental clutches. Males fed reduced broods at a lower rate, but DEE and mass (loss) were not significantly affected by the experiment. However, male DEE was positively correlated with feeding frequency, and this relationship did not differ between control and experimental males. This suggests that the experiment resulted in a slight reduction in male DEE, and lack of statistical significance can be attributed to the power of the test applied. Females fed reduced broods at a lower rate, but female DEE and mass (loss) were not affected by the clutch size manipulation. Female DEE was not correlated with feeding frequency, the original number of eggs in the nest or the actual number of young. The division of labour (feeding rate, DEE) was not affected by the clutch size manipulation. Female DEE was significantly higher than male DEE, and this could be caused by the fact that only the q roosts with the young. There was no consistent relationship between DEE and parental survival or future reproductive success. Since the experimental reduction in clutch size led to an increase in residual reproductive value (more second clutches), but not to a decrease in DEE, this costs of reproduction is apparently not contingent on increased rate of energy turnover. It is suggested that birds with equal DEE may differ in their residual reproductive value as a consequence of variation in energy allocation between reproductive effort and e.g. aspects of physiological maintenance and repair.