DOMINANCE, DAILY ACTIVITY AND WINTER SURVIVAL IN WILLOW TITS - DETRIMENTAL COST OF LONG WORKING HOURS

Predation risk can vary both in space and in time. Dawn and dusk may p resent greater risk to diurnal birds, since their vision is not well a dapted to dimness and both nocturnal and diurnal predators may be acti ve at these times. If the birds are not time-limited, selection should favour activity patterns where crepuscular activity is avoided. Indiv iduals in dominance-structured groups may differ in their time-limitat ion, The roost-entering and emerging times of willow tits (Parus monta nus) spending the winter in flocks with a social hierarchy were examin ed. In early winter, dominants initiated their daily activities later and roosted earlier than subordinate individuals. As a result, the dur ation of roosting was longer for dominants than for subordinates. By t he next breeding season, significantly more subordinates than dominant s had disappeared (presumably died), and the individuals which disappe ared had the longest days. There were no sex differences in daily acti vity in early winter, but in late winter, males emerged earlier in the morning than females. A feeding experiment revealed that the birds we re restricted by food availability: the fed birds reduced their roosti ng period less than simultaneous controls. The greater change in the c ontrols is understandable, because the days grew longer (and the roost ing period thus shorter) during the experiment. We suggest that the lo ngest active birds were forced to extend their day in order to obtain sufficient food, which perhaps resulted in greater mortality through p redation.