Geographical and land-use influences on bird species richness in small woods in agricultural landscapes

In this paper we examine the influence of geographical and land-use factors on breeding bird species richness in small woods within farmland. In many European agricultural landscapes, small woods represent the major component of semi-natural habitat available to wildlife. We used census data (for woods of up to 15 ha) from six study areas in four countries (The Netherlands, UK, Denmark and Norway) to construct species-area relationships for woodland birds. An additional relationship for Sweden was obtained from the literature. Species richness, and the parameters of the species-area relationships, were then related to the latitude, longitude and percentage woodland cover of each study area. Overall, species richness across all woods in each study area declined with increasing latitude; the proportion of resident species also declined, but that of migrants increased. Numbers of migrant species also increased with increasing woodland cover. Both the slopes and intercepts of the species-area relationships declined with increasing latitude. Thus not only were fewer species available to colonise individual woods at higher latitudes, but the return in terms of species richness for a given increase in area was also less than at lower latitudes. From the species-area relationships, the numbers of species expected to breed in woods of 1 ha and of 15 ha decreased from 13 and 21, respectively, in The Netherlands (latitude 51.7°) to 7 and 13, respectively, in Denmark (latitude 56.3°). Other factors influencing species richness on both local and geographical scales are discussed