Energy expenditure, nestling age, and brood size: an experimental study of parental behavior in the great tit Parus major

A brood manipulation experiment on great tits Parus major was performed to study the effects of nestling age and brood size on parental care and offspring survival. Daily energy expenditure (DEE) of females feeding nestlings of 6 and 12 days of age was measured using the doubly-labeled water technique. Females adjusted their brooding behavior to the age of the young. The data are consistent with the idea that brooding behavior was determined primarily by the thermoregulatory requirements of the brood. Female DEE did not differ with nestling age; when differences in body mass were controlled for, it was lower during the brooding period than later. In enlarged broods, both parents showed significantly higher rates of food provisioning to the brood. Female DEE was affected by brood size manipulation, and it did not level off with brood size. There was no significant effect of nestling age on the relation between DEE and manipulation. Birds were able to raise a larger brood than the natural brood size, although larger broods suffered from increased nestling mortality rates during the peak demand period of the nestlings. Offspring condition at fledging was negatively affected by brood size manipulation, but recruitment rate per brood was positively related to brood size, suggesting that the optimal brood size exceeds the natural brood size in this population.