A weak upward elevational shift in the distributions of breeding birds in the Italian Alps

Aim To test whether bird assemblages are shifting upwards in their elevational distribution in synchrony with current climate warming and/or habitat changes. Location A gradient of elevation in the Italian Alps (Alta Valsessera, Piedmont). Methods We used data from two recent atlas surveys performed on a 1 x 1 km grid at an 11-year interval (1992-94 and 2003-05). We modelled the elevational gradient of avifaunal composition, using a sample-based approach, in an effort to detect evidence for an upward elevational shift of bird zonation. Changes in species richness were controlled for. The results from this analysis were compared with those obtained using a species-based approach. Changes in climate and landscape between the two surveys were assessed using local meteorological data and Corine Land Cover maps, respectively. Results We detected small avifaunal changes between the two surveys: (1) mean elevations increased for the majority of species, but the average change was not significantly different from zero; (2) the species richness increased, but this was mainly due to an increase in sampling effort; and (3) a change in species composition was detected, which was at the limit of significance and corresponded on average to a 29-m upward elevational shift in the distribution of the avifauna. The shift was the same for open land and forest bird communities. During the same period, the mean temperature increased by c. 1 degrees C in the area, and a slight trend towards vegetation closure by woody plants was detected. Main conclusions The use of fine-scale breeding bird atlases in mountainous regions, together with ordination methods, provides a sensitive tool to test and measure elevational shifts in species ranges, but the results have to be interpreted carefully. In our case, the observed elevational shift in the distributions of the avifauna cannot unambiguously be attributed to climate warming. This shift is smaller than expected from the regional increase in temperature, which raises the question of how closely bird distributions match climate change.