Adrenocortical responses to stress in breeding Pied Flycatchers Ficedula hypoleuca: relation to latitude, sex and mating status

The adrenocortical response to acute stress, as measured by increases in plasma levels of corticosterone following capture and handling, has been shown to vary between individuals within a population as well as with season in a variety of species. However, the ecological bases for such endocrine phenomena remain unclear. One hypothesis predicts that populations of birds breeding under extreme conditions such as in sub-arctic habitats where the nesting season is short, decrease the amplitude of the adrenocortical response to acute stress so as to allow non-interrupted breeding under potentially severe conditions such as bad weather conditions and/or food shortage. To test this hypothesis we measured increases in circulating levels of corticosterone following capture and handling in populations of Pied Flycatchers Ficedula hypoleuca breeding in southern (Goteborg) and northern (Ammarnas) Sweden. We also sampled breeding females that did not have a male helping them to feed young (only in south Sweden). As predicted by the hypothesis we found male and female Flycatchers breeding in northern Sweden to be adapted to the short breeding season and harsh unpredictable sub-arctic climate by having a reduced adrenocortical response to stress, as indicated by the pattern of corticosterone levels in blood. Maximum corticosterone levels generated by the capture stress protocol were significantly higher in single females than all other groups. Unlike other passerines studied at high latitudes Pied Flycatchers showed no significant relationships between corticosterone levels and body mass or fat score.