Flock formation, flock size and flock persistence in the Willow Tit Parus montanus

Abstract

The study was carried out in the alpine birch region in central Norway. According to flock attachment and behaviourjuvenile Willow Tits were classified into five categories : regulars (permanent flock members), vanishers (flock members disappearing before New Year), irregulars (unsteady flock members), floaters and transients. Although the vanishers on the average arrived earlier in the area than the regulars and thereby, according to the current view, should benefit priority of being permanent flock members they nevertheless disappeared from the flocks after some time; the majority evidently emigrated from the area in October. It is suggested that the different behaviour of regulars and vanishers is hormonally based. The winter territories of flocks overlapped considerably . Flock size averaged initially 4.2 individuals (range 2- 8 birds) and was composed of 2.3 adults and 2.0 juveniles. The sex ratio irrespective of age was 1 .3, i .e . males outnumbered females. Thirty-two percent of the flocks contained more than 2 adults, some of which bred in the winter territory besides the dominant adult pair . Occasionally, even juvenile flock members bred successfully together with the adult pair within their common winter territory. Owing to strong fidelity, the birds usually kept to the same winter territory for life . Occasional territory (and flock) shifts were usually combined with pairing of individuals from different flocks, or divorces . Prior to establishment in winter flocks, the juveniles probably evaluate the resident pair and its habitat. It is furthermore suggested that juvenile flock members continuously try to improve their fitness, for example by frequently exploring areas outside their main territory, and that they are ready to change strategy if this seems beneficial .