Ecological requirements: graphs and tables

The main objective of any bird atlas is to discover the distribution of birds in the area under study. Nevertheless, as explained in the previous section, birds depend on the occurrence of a series of environmental factors that are indispensable for the completion of their life cycles. These ecological requirements, which vary in strictness from one species to another, are described in terms of ranges in temperature, altitude, slope gradients, forest cover, salinity and so forth. In the texts for each species various experts discuss some of the species' main ecological requirements; also included in this section (when available) is information regarding the selection of altitudinal ranges and habitat composition, as well as abundances in the principal landscape types. In a mountainous country such as Catalonia, height above sea level is an essential factor in a species' distribution, since it strongly influences climate, vegetation and human activity. The other significant group of environmental characteristics is related to habitats and how they combine to form landscape units. Therefore, both the species' main habitats and the selections they make, as well as abundances in the principal landscape units, are shown whenever possible.

		Graph of landscape types (Tawny Owl).Distribution bars per habitat (light blue, units along the lower X axis) show the mean percentage that each habitat occupies within the 1x1 UTM squares in which the species in question was detected. The index of selection bars (dark blue, units along the upper X axis) show the ratio between habitat occurrence in the squares where the species in question was found and habitat occurrence in the whole set of sampled squares in Catalonia. To make the interpretation of these graphs easier, 1 is subtracted from this ratio so that the values of the selection index are positive when the species positively selects the squares that contain that habitat, and negative when the species negatively selects them. It should be noted that this strictly means that the habitat is present in those squares chosen by the species, regardless of whether it uses it or not.
Altitude graph (Robin). This shows the altitudinal ranges in which the species has been detected and which ranges are selected. Distribution bars (light blue) show the percentage of all observations of a species found in each altitudinal range; the sum of all the values for each range is 100%. Preference bars (dark blue), on the other hand, show the percentage of squares within an altitudinal range in which the species was found and indicate the selection for each altitudinal range.

Table of SOCC densities (Firecrest). Densities in each of the main landscape units in Catalonia in pairs/km2 or, for some species, in individuals/km. These results are based on the data from the SOCC (Catalan Common Bird Survey) project.