Temporal trends in abundance and phenology of migratory birds across the Italian Alps during a 20-year period

Introduction: Bird migration across mountainous regions has been studied usually at single sites due to the difficulty in employing and support a multi-site sampling effort. This may affect inference on migration whose scale is larger that a single site. The Progetto Alpi has been monitoring post-breeding bird migration through the Italian Alps with a network of several ringing stations since 1997. Until 2017, 666,471 ringing data of 191 different species were collected.
Methods: Phenology of bird migration in terms of date of capture and the related elevational distribution during the sampling season (August-November) were analysed for 69 species. For a subset of 45 species the inter-annual variation in phenology along with trends in the number of ringed birds and in the ratio between the number of juveniles and adults ringed, were also analysed.
Results: Migration through the Italian Alps occurred differently between species, with heterogeneity across species in the median date of capture and their elevational distribution. No linear trends in phenology were detected. For four species an annual linear increase of the ringed individuals was detected, while for other four species an annual linear decrease of the captures was detected. For two species, an increase of the ratio between the number of the juveniles and the number of adults ringed was detected.
Conclusions: The long-term standardized monitoring of post-breeding migration allowed us to consolidate and increase knowledge of bird migration through Italian southern Alps. The information gathered allowed us to define the temporal and elevation distribution of passerine birds crossing the mountainous area, and to test interannual trends in the number of captures. The main purpose of this work was to report on the information collected in twenty years of field activity, providing a contribution to the understanding this complex phenomenon. Further investigations and more in-depth analyses are necessary to understand how environmental factors and species-specific eco-physiological traits interact and affect migratory strategies of passerine birds in the Italian Alpine region.