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.

3 .4 . Mechanisms of formation and functioning of passerine communities According to the current views, resource sharing and segregation o f avian e co ­ logical niches take place due to the difference in the m ethods o f resource exploita­ tion. Foraging behaviour is viewed as an integral param eter that characterises the species’ niche as a functional unit. Peculiarities o f avian foraging behaviour depend on the food type and the habitat structure. Birds o f different orders usually show fundam ental difference in diet, whereas diets o f members o f one order are general­ ly similar. The food objects o f most passerines are small invertebrates and seeds o f plants found in tree and bush crowns, on the grass and on the ground. Therefore, ecological segregation is secured not by food, but rather by the features o f foraging behaviour and spatial distribution (Khlebosolov 1999, 2004, Khlebosolov et al. 2006b). We marked out three ecological groups o f passerines that inhabit fund am en tal­ ly different landscape types o f Pasvik: forest, swamps and scrub, and the m ountain tundra. The habitat structure o f these landscapes significantly differ, therefore they are inhabited by birds that forage by fundamentally different methods. The forest habitats are occupied by birds that forage mainly in tree and bush crowns. Swamps and the scrub are inhabited by the birds that feed on the ground, in the grass, in the crowns or und er the crows o f bushes. In the tund ra birds o f open habitats breed. As m icrohabitats w ith sim ilar structure occur in different landscapes, some birds are shared by all the three habitat types. For instance, the ground-feeding B luethroat occurs in the forest, in marshland and in the tundra. The C omm on Redpoll that forages in the crowns o f trees and bushes occurs in all habitats where at least low willow scrub is available. Birds belonging to the same ecological group may be spatially separated or c o ­ occur, relatively to their foraging tactics and strategy. Ecologically and taxonom i- cally close species differ in their foraging tactics. D ifference in foraging behaviour o f these birds is due to using foraging substrates w ith varying structure, or m ic ro ­ habitats, so that they are usually spatially separated. The degree o f spatial isolation depends on the position o f th e ir preferred m icrohabitats. In the forest, birds in some cases forage in different parts o f crowns, sometimes on different species o f trees, and not infrequently in different habitats. For example, Willow Warblers and Comm on Redpolls often co -occu r in the crowns o f the same trees, but use differ­ ent parts o f the crown. Willow Warblers forage in the inner, and Redpolls in the outer parts o f the crown. Willow Warblers and Bramblings prefer to feed in differ­ ent trees: the former species mainly occurs in birch stands, whereas the latter one, 162