Corticosterone metabolites in blue tit and pied flycatcher droppings: effects of brood size, ectoparasites and temperature.

The hypothalamic-pituitary-adrenal (HPA) axis of birds induces the secretion of corticosterone (CORT) as a response to different ecological variables. In this study we tested experimentally if manipulations of brood size or ectoparasitism led to subsequent differences in the concentration of excreted CORT metabolites of adult and nestling blue tits (Cyanistes caeruleus). No significant effect of the manipulation of brood size was detected in adults or nestlings. No significant effect of ectoparasitism was detected in males or nestlings, although females from uninfested nests showed lower concentrations of excreted CORT metabolites. In addition, we analysed if weather conditions had an influence on the concentration of excreted CORT metabolites of blue tits and pied flycatchers (Ficedula hypoleuca) breeding in the same forest. We detected no effect of weather conditions on adults, but nestlings of both species showed a negative correlation between their excreted CORT metabolites and the average mean temperatures they were subjected to during their growth. This effect was not found in blue tits in a colder year, suggesting that the sensitivity of the HPA axis to ambient temperature may be subjected to interannual variation. Moreover, we found a positive effect of the maximum temperature on the day of sampling on the concentration of CORT metabolites of blue tit nestlings in one of the years. These results suggest that weather conditions may act as environmental stressors to which the HPA axis of blue tit and pied flycatcher nestlings may be sensitive.