Blood metabolite levels in normal and handicapped pied flycatchers rearing broods of different sizes.

We measured levels of select metabolites (glucose, triglycerides, free fatty acids, glycerol, uric acid) and corticosterone in the blood plasma of adult pied flycatchers Ficedula hypoleuca while they were rearing broods whose sizes were modified experimentally. We also made it more difficult than normal for some pairs of birds to forage by removing certain wing and tail feathers (handicapping them). Both procedures have been shown previously to change parental workload. We did this in order to determine if the birds alter their use of nutrients in response to differences in their workload. Metabolite levels were not influenced by handicapping or brood size. However, the concentration of free fatty acids in the plasma of females and of triglycerides in the plasma of males was directly related to the frequency with which the adults fed their nestlings. These findings suggest that the two sexes have different ways of coping with the work associated with rearing the brood: females apparently undergo brief daily fasts while feeding their chicks, whereas males take more time to feed themselves while providing food for their young, and spend more time doing so as their workload increases. The flycatchers exhibited high concentrations of uric acid and corticosterone in the blood plasma; corticosterone and glycerol were positively correlated in females; and corticosterone and triglyceride levels were negatively correlated in males; all of which suggest that gluconeogenesis provides some of the energy required for their parental activities.