Patterns and variations in the structure of forest bird communities in southern Finland

Abstrakti

Species richness and density of forest birds in relation to habitat area and characteristics were studied on the basis of census data from various localities in southern Finland . The number of species increased with increasing size of the study area, firstly steeply up to a plot size of about 10 ha, and then progressively more gently sloping. The number of species increased significantly with increasing bird density as well . The expected number of species per ha averaged 7.0±0 .4 (SE) and ranged from 3 to 12 . In the standardized sample size of 50 pairs, the number of species averaged 20 .7±0 .3 (SE) (range 16-26) . Species richness had a significant positive relationship with the amount of edges. There was a significant dependence between the number of species and forest productivity in larger (5 .0-55.0 ha) study plots, but not in a set of smaller (0 .2-10.0 ha) ones . The number of species had a significant positive relationship both with the age and amount of trees, and it also coincided with the intensity of thinning of forests. The bird density decreased with increasing study area . The relative but not absolute amount of edges showed a highly significant positive relationship with bird density . Densities of birds were, in general, higher in the more productive study sites. The average bird densities of the main forest types ranged from about 440 to 1060 pairs/km2. The general average bird density in forests was 668.8±57.2 (SE) pairs/km2. Bird densities varied significantly between forest types, coniferous forests differingnsignificantly from mixed and deciduous forests. Forests that were relatively light (open) due to a moderate amount of trees seemed to be most attractive for birds. The evenness of communities decreased significantly with increasing productiv- ity of habitat. Species abundance distributions, in general, approximately fitted to the lognormal distribution . Two dominant species of the total data set together comprised a third of the total number of pairs. Some general features of habitat (such as patch size, amount of edges and productivity) explained part of the variation in the structure of forest bird communities. This proportion was not very high, however, particularly at larger geographical scales.