| Summary: | We studied the occurrence of the Siberian Tit Parus cinctus in relation to variation in the structure of forests and the size of homogeneous forest areas in northernmost Finland. Habitat structure was measured at sites where Siberian Tits were observed (mainly foraging, n = 100) in the summers of 1982-84 and at randomly selected sites (n = 91) in both virgin and managed forests. At each site 25 habitat variables were measured these were combined in eight variables used to analyze the data . We used two-group discriminant analysis (observation vs . random sites) of the whole data set, separately for virgin and managed forests, and for comparing the nestling and fledgling periods. The tits preferred habitats with dead trees, large coniferous trees and birches; they avoided very bushy areas. Canopy cover was the most important discriminating variable in virgin forests. In managed forests tits favoured a closed canopy with large coniferous trees and birches. During the fledgling period tits occurred in habitats with more birches than those used in the nestling period . The high proportion of correct classification of prior groups (89%) in the discriminant analysis of managed forests indicated that Siberian Tits selected these habitats non-randomly, contrary to the case in virgin forests: the proportion of correct classifications was relatively low (61%). The forest areas in which tits were observed were significantly larger than the randomly selected areas. The preference of sedentary Siberian Tits for large trees and large forest areas is probably important for the foraging of this foliage-gleaning species. Dead and large coniferous trees, reduced in number in thinned forests, also provide both nesting and roosting sites for Siberian Tits . The effects of fragmentation of old virgin forests and loss of habitat heterogeneity in terms of heavy thinning affect the Siberian Tit adversely.
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