Wintering avifauna in the agricultural landscapes of northern Spain. I. Biogeographical patterns
Published: Volume 32(2), December 1985. Pages 203-225.
Original Title: Avifauna invernante en los medios agrícolas del norte de España. I. Caracterización biogeogrÁfica
This paper analyzes the features of the distribution of passerines (crows excluded) wintering in the agricultural fields of Northern Spain (Basque Country and sourrounding regions; figure 1). This area is located on the boundary of the Eurosiberian (forested landscape with grass fields as dominant cultures) and Mediterranean (deforested landscape with cereal fields -Triticum, Hordeum- in the coldest areas and more diversified cultures cereals, vineyards and orchards in the more thermic sectors) bioclimatic regions. We have delimited 23 sectors where the bird communities have been censused by means of lie-transects and some agricultural-climatical gradients have been defined by means of a Principal Component Analysis (table I and figure 2). Factor I defines a thermic gradient opposing the Eurosiberian grassfields and the supramediterranean cereal fields on the hot and cold extremes respectively. Factor II defines a structural gradient ranging from the forested grassfields (associated with high precipitations) to the Mesomediterranean diversified cultures, through the cereal fields of the colder Supramediterranean sectors at higher elevations. In this factorial space, the 52 species of wintering birds have been situated in order to illustrate their mean winter distributions (figure 3). On the other hand, and to test the relationships among the species and the gradients defined by factors I and II, we have calculated the correlations between specific abundances of every sector and the factor scores of the sectors. Correlations with factors I and II are significant for 48 and 31%, respectively, of the bird species. A 55,8 % of the species (accounting for 85,9 % of the 35000 birds censused) have a significant multiple correlation with factors I and II, so that both gradients offer an adequate representation of the wintering bird patterns in the study area. The isopleths obtained from the distribution of abundance values (IKA) for insectivorous and granivorous species on the factorial space (figures 5,6 and 7), show a concentration of the former in the warmest sectors (according to the thermic exigences of their invertebrate preys). The greatest abundances for granivores are recorded in the coldest Mediterranean sectors, where cereal fields are the dominant landscape. In these sectors the high densities of some granivorous species (Alauda, Passer, Fringilla) are the main factor accounting for the entire regional patterns of variation in density (D), biomass (BM) and dominance (ID). Insectivores and granivores have distinct biological constraints that determine their different geographical patterns and strategies of local distribution (figure 8). The spatial aggregation of insectivorous species (measured as the coefficient of variation of their sectorial densities) shows a strong increase in the most unfavourable cold and deforested areas, while granivores have a spatial pattern much more homogeneous.