Biometrics Variation and Directional Preferences of Immature Robins (Erithacus rubecula) Caught in Northern Italy during Autumn Migration in 2005

Inter-seasonal changes of biometry and preferred migration directions of Robins were studied according to data collected during autumn migration in northern Italy at the Arosio Bird Observatory (45°43'N, 9°12'E). Altogether 598 immature Robins were caught and 187 orientation tests were performed. Wing, tail and tarsus length, wing shape and weight were analysed in subsequent five migration waves distinguished according to migration dynamics. General pattern of migration as well as graphs with distribution of preferred directions in subsequent waves were prepared. In the case of tail length and weight their average values in subsequent waves were significantly different. Decrease of wing length was noted along the season. On the contrary, increasing trend was observed in the case of tail length and wing shape. Results of orientation tests showed that SSE direction was predominant (34%). SW direction was not clearly marked and its percentage was 23%. Distribution of directions slightly changed in subsequent migration waves. Noted results suggest passage of Robins heading to the Mediterranean basin and Apennine winter quarters. Obtained inter-seasonal changes of biometry and preferred directions can be an effect of differences in migration time between this groups or gradual inflow of more northern populations what the authors discuss here.