The Catalan Breeding Bird Atlas (CBBA): methodological aspects and ecological implications

2in north-east Spain and stands as a pioneer landmark in bird mapping in the Mediterranean region. Here, we present the context, novelties and main results of the new CBBA conducted 20 years later, between 1999 and 2002. This new atlas has included a series of methodological innovations amongst which we should emphasize the establishment of small scale timed censuses on a sample of 1x1 km UTM squares within each of the original squares of the 10x10 km UTM grid. These smallscale censuses (about 3,200) allowed a consistent sampling of the territory by covering uniformly about 10% of the total study area, and make possible the production of species distribution maps at local scales (e.g. 500 m resolution). These maps were generated using spatial logistic regressions in the framework of niche based modelling approaches. Such models included information from 45 environmental variables ranging from land use and relief to direct human influence and climate, as well as information on the spatial structure of the data collected for each sampled species data to account for spatial autocorrelation. Currently, the CBBA stands amongst the best current quantitative mapping of bird distribution in Europe.We also used data from the 1x1 km UTM squares to develop species-time curves and model the number of species found in each 10x10 km squares per unit of time. The application of these models to the first and the new CBBAs allowed an accurate estimation of changes in bird distribution between the two atlases by taking into account local differences in sampling effort (e.g. time spent on a given square) between the two periods. Overall, the new CBBA provides one of the best large-scale pictures of the changes that occurred during the last 20 years in southern Europe. The results obtained strongly support the hypothesis that strong changes in bird community patterns have occurred in large areas of the country, often associated with changes in land use patterns.