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 present greater risk to diurnal birds, since their vision is not well adapted to dimness and both nocturnal and diurnal predators may be active at these times. If the birds are not time-limited, selection should favour activity patterns where crepuscular activity is avoided. Individuals in dominance-structured groups may differ in their time-limitation. The roost-entering and emerging times of willow tits (Parus montanus) spending the winter in flocks with a social hierarchy were examined. In early winter, dominants initiated their daily activities later and roosted earlier than subordinate individuals. As a result, the duration of roosting was longer for dominants than for subordinates. By the next breeding season, significantly more subordinates than dominants had disappeared (presumably died), and the individuals which disappeared had the longest days. There were no sex differences in daily activity in early winter, but in late winter, males emerged earlier in the morning than females. A feeding experiment revealed that the birds were restricted by food availability: the fed birds reduced their roosting period less than simultaneous controls. The greater change in the controls is understandable, because the days grew longer (and the roosting period thus shorter) during the experiment. We suggest that the longest active birds were forced to extend their day in order to obtain sufficient food, which perhaps resulted in greater mortality through predation.