The stress response and autumn dispersal behaviour in willow tits

The present study tests the hypotheses that an increase in circulating levels of corticosterone, during the period when flocks were being established, redirects the behaviour of juvenile willow tits, Parus montanus, away from hock establishment towards dispersing from their natal areas. As dispersing is a poorer strategy than joining a flock, juveniles should be less sensitive to environmental stressors during this period. An experimental increase in circulating corticosterone in free-living willow tits affected dispersal behaviour differently in juveniles and adults during the period of flock establishment (July-September). Adults did not leave their territories, whereas the majority of corticosterone-treated juveniles disappeared from the forest. Corticosterone is thus likely to be a factor, involved in the dispersal behaviour of juvenile willow tits during early autumn. When the experiment was repeated at a time when permanent winter flocks had been established (October-November), the corticosterone treatment did not make the juveniles leave their territories. Adults showed the same behavioural response as in late summer. To test the hypothesis that juvenile willow tits are less sensitive to environmental stressors during the period of flock establishment, willow tits were captured at three different times of the year and exposed to handling stress. In late July, neither juveniles nor adults showed any marked response to capture stress. In mid-September juveniles still showed little response to handling stress, whereas adults had become very sensitive. In January both age groups responded rapidly to capture stress. (C) 1997 The Association for the Study of Animal Behaviour.