Annual variation of northern Finnish forest and fen bird assemblages in relation to spatial scale

Abstract

Censuses made yearly in 1983-89 were used to study the year-to-year variation in bird species assemblages in four localities in virgin coniferous forest and four localities in open flark fen. The censuses in forests were made in an area of 100 km2 and those in fens in 1000 km2 . The variation in the density of the assemblages did not differ between the pooled samples from forests and fens or between the separate localities . The temporal density variation (V2 ) of the assemblages in separate fens was significantly larger than that ofthe pooled data of the other fens in three out of four cases, whereas no difference was observed between separate and pooled samples in coniferous forests. In the passerine species the density variation in the different sites was similar in magnitude in both forests and fens, whereas the variation in waders in fens differed in magnitude between the sites. Compensatory fluctuations in the density of species prevailed in the fens, but neither compensatory nor parallel fluctuation patterns were observed in the bird assemblages in coniferous forests. The results indicate that the variation patterns of waders in fens are different from those of passerines in fens or forests. Waders are probably more susceptible than passerines to environmental variation between sites, such as differences in the water level . The fact that the variation of bird assemblages in fens was observed to be smaller on a larger spatial scale than the variation in separate localities, while this was not the case in forests, may be due to differences in species characteristics between fens and forests or to the larger spatial scale adopted in the study of birds of fens .