Oxidative status and reproductive effort of great tits in a handicapping experiment

Life-history theory predicts a trade-off for allocation of limited resources to reproduction and self-maintenance; however, many of the underlying physiological mechanisms remain elusive. There is growing evidence for oxidative stress to play an essential role in this trade-off because some by-products from the immune system and from normal metabolism generate reactive oxygen species that can cause oxidative damage. We manipulated reproductive effort of male and female great tits shortly before reproduction by clipping feathers of either the male or female parent of pairs of known age, given that parental effort may differ between the sexes and change over the lifetime of an individual. We quantified the effect of the treatment on morphological, physiological, behavioral, and reproductive traits. We found that feather clipping led to a decrease in parental body mass and to a reduced clutch size. Nestlings raised by clipped fathers showed reduced body mass although feeding rate was equally high between clipped and control individuals. In contrast to our predictions, we found that the feather clipping did not affect oxidative status. However, independently of the treatment, adult males had higher antioxidant capacity than females and older males showed higher oxidative damage compared with yearlings. Thus, our results suggest that the self-maintenance was prioritized over reproduction. It suggests that males are more susceptible to increased workload than females and thus more likely to reduce allocation of resources to reproduction.