The primary goal of the analyses demonstrated in this case study is to describe the variability of bird communities, related to the differences in their habitat.

The data set originates from a field study by Mirek E. Šálek et al. (unpublished data) in Velka Fatra Mts. (Slovak Republic) where the bird assemblages were studied using a grid of equidistant points placed over the selected area of montane forest, representing a mix of spruce-dominated stands and beech-dominated stands. There was a varying cover of deforested area (primarily pastures) and the individual quadrats differed in their altitude, slope, forest density, cover and nature of shrub layer, etc. (see the next section for description of environmental variables). The primary data (species data) consist of the number of nesting pairs of individual bird species, estimated by listening to singing males at the centre points of each quadrat. The data values represent an average of four observations (performed twice in each of two consecutive seasons).

Data manipulation

The data are contained in the Excel spreadsheet file named birds.xls. This file has two sheets. We will use the data contained in the first sheet (named birds), where the bird species observed in only one of the quadrats were omitted (the full data are available in the other sheet, labelled birds-full). Note that the birds sheet contains both the species data (average abundance of individual bird species) and the environmental data (description of habitat characteristics for the particular quadrat).