INDIVIDUAL VARIATION IN MATE CARE BY ALPHA MALES IN CRESTED TIT WINTER FLOCKS

Winter flocks of crested tits Parus cristatus, typically consisting of two adults and one or two non-kin 1st-year birds, were observed to sp lit up into subflocks in a way related to ambient temperature. On warm days, when they were foraging in subflocks without 1st-year birds, al pha males always occupied the most preferred upper foraging sites, as expected from their top dominance rank. On cold days, when foraging in flocks with 1st-year birds, 8 out of 13 alpha males shifted to lower (less preferred) positions below their alpha mates while allowing the latter to forage at the best sites. As enhanced access to preferred mi crosites on days with high energy stress is believed to increase overw inter survival probability, this shifting behaviour of alpha males can be considered as a form of mate care. Out of 13 alpha males, however, 5 did not shift and always occupied the best foraging sites irrespect ive of flock composition. As (i) these non-shifting males were in poor er physical condition than shifting males, (ii) they scanned significa ntly less for predators than either females or shifting males when for aging in the uppermost tree parts, and (iii) four out of five non-shif ting males were replaced by immigrants in early spring, absence of mat e care during winter may be caused by constraints due to condition. Hi gh-quality-territory owners in poor condition at the end of autumn wer e most vulnerable to replacement by immigrants. Therefore, as four out of five replacements affected high-quality territories, selective int rusion by immigrants is suggested.