The adrenocortical responses to stress in breeding Willow Warblers Phylloscopus trochilus in Sweden: Effects of latitude and gender

1. In the present study we tested the hypothesis that birds suppress their adrenocortical response to stress when breeding in a severe environment, 2. We measured increases in circulating levels of corticosterone following capture and handling in populations of Willow Warblers, Phylloscopus trochilus, breeding in a rich deciduous forest in southern Sweden and in Willow Warblers breeding in a subarctic environment in Swedish Lapland. Data from an 8-year study on survival rates and nest predation in southern and northern Sweden are also included. 3. By comparing the changes in corticosterone levels immediately following capture, handling and restraint, and then up to l-h post-capture across gender and latitude, we were able to assess possible ecological influences. 4. Willow Warblers had about the same survival rates in northern and southern Sweden (0.27 and 0.30, respectively). Depredated clutches were normally replaced in southern Sweden (72%). but not in northern Sweden (1%). 5. A reduced adrenocortical response to stress was found in Willow Warblers of both sexes breeding in northern Sweden where conditions were more severe and the nesting season shorter than in southern Sweden. At the southern site, males had a significantly more robust adrenocortical response to stress than females, whereas at the northern site changes in corticosterone following capture were identical in males and females, Maximum corticosterone levels generated by the capture stress protocol were significantly higher in males at the southern site than in all other groups, 6. There were marked individual variations in the patterns of corticosterone changes following capture in all groups, but there were no significant relationships of dynamics of corticosterone changes to body mass or fat score in males, However, in females at the southern site, the maximum corticosterone level and the rate of corticosterone increase during the capture stress protocol were positively correlated with body mass. In contrast, at the northern site females showed negative correlations of body mass with maximum corticosterone and rate of corticosterone increase. These differences may reflect different ecological bases for modulation of the adrenocortical response to stress with latitude, and with gender.