The birds of Pasvik / E. I. Khlebosolov, O. A. Makarova, O. A. Khlebosolova [et al. ; English transl. Nikita Chernetsov]. - Ryazan : Golosgubernii, 2007. - 175 с. : ил., портр.

Willow Warblers are confined to forests with a high proportion of birch, which is probably due to the fact that in rather sparse forests of Pasvik uniform illumination is conductive to the for­ mation o f spherical crowns o f birch trees. In such crowns branches grow in different directions and form a rather uniform space filled with twigs without pronounced horizontal or vertical lay­ ers (Fig. 3.3). Such crown structure facilitates foraging of this bird species (Khlebosolov 1993). The most typical feature o f forag­ ing behaviour o f Willow Warblers is that, unlike many other Phylloscopus warbler species, they do not maintain a certain direction o f movements when searching for prey. The birds mainly use hops and short flights above the branch­ es, and broadly move in both horizontal and vertical planes (Fig. 3.4, Khlebosolov et al. 2003). Brambling. Unlike the Willow Warbler, the Brambling occurs in differ­ ent types o f forest. The decisive factor is the presence o f tall trees with well developed crowns that provide oppo r­ tunities for foraging by its characteristic method. Brambling numbers may be rather high in pure middle-aged or mature birch forests (Fig. 3.1, type 2). Unlike Willow Warblers, Bramblings do not occur in young birch stands. They also avoid young pine forests and carrs (Fig. 3.1, type 4). In pine and mixed pine and birch forests the Brambling numbers vary in relation to the avail- Figure 3.3. Structure o f tree crowns pre­ ferred by Willow Warblers. The dashed line shows the foraging zone o f Willow Warblers. N(S) = 73 (2393) Figure 3.4. Succession and frequency o f for­ aging manoeuvres o f Willow Warblers: P - pick, H - hop, F - flight, N(S) - number of successions, in parenthesis the overall obser­ vation time (s). Numbers show the frequency o f foraging manoeuvres after the preceding ones, arrows show the direction o f hops and flights. 121