Spacing and Survival in Winter Groups of Willow Tit Parus montanus and Crested Tit P. cristatus--A Removal Study

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Abstract
Spatial distribution, local movements and survival of willow tit and crested tit populations outside the breeding season are examined by removal experiments, and analyzed in relation to social structure. In autumn, willow tits gather in small, stable and highly stationary groups using largely non-overlapping ranges, or they adopt a floater strategy. Crested tits form similar groups, but floaters were not unambiguously identified. Removed tit groups were quickly replaced by new groups largely within the same group ranges. Floating willow tits switched to a stationary habit in response to removals in autumn. By mid-winter, replacement birds were solely recruited from border groups, and willow tit floaters could not be detected; they probably disappeared due to a local mortality in autumn. Experimental reduction of the willow tit density improved winter survival, which compensated for the removals. Juveniles in mixed-age groups survived less well than adults, but in pure juvenile groups they survived equally well as did average members of mixed-age groups. These results suggest that: the number of groups is limited; group size is limited; group members compete with each other; juveniles are most severely affected by competition, probably through interference from adults; and group size is best understood as the result of individual birds seeking a compromise between advantages of a group life and reduced resource availability imposed by dominant group members.