Parental energy expenditure and offspring size in the pied flycatcher Ficedula hypoleuca

To determine if parental daily energy expenditure (DEE) is an appropriate measure of parental care with a positive effect on offspring fitness, the DEE of pied flycatchers Ficedula hypoleuca at 24 nests was measured with the doubly-labeled water method in a population breeding in nest boxes in a pine plantation in central Spain. DEE was measured when parents were feeding chicks of 12 d of age. In 17 of the nests, DEE of both parents was measured simultaneously. To expand the range of parental efforts observed, clutch size was manipulated by adding or removing one egg after laying and keeping some nests as controls. The significant brood size differences between treatments at hatching disappeared before chicks were 13 d old. All nestlings were weighed and measured (tarsus and wing length) at the age of 13 d. Chicks in enlarged nests had significantly shorter tarsi than chicks in control and reduced nests. The mean DEE was 64.4 kJ/d. There was no relationship between the DEE of mates measured simultaneously. Feeding rates of the labeled birds were not correlated with DEE. Treatment or brood size did not affect parental DEE. Female DEE was positively correlated with mean nestling mass and tarsus length. The effect on tarsus length was also significant when controlling for treatment and brood size. Male DEE was correlated with mean nestling tarsus length only when controlling for treatment. The relationship of parental DEE with tarsus length was linear. Female DEE explained more of the variation in mean tarsus length than did the midparent tarsus length. Parental and especially maternal DEE is a good measure of parental care as it positively affects offspring fitness, and should be considered as a parental performance effect on offspring skeletal size.