Visible Migration of Short-Toed Snake-Eagles: Interplay of Weather and Topographical Features

Migratory behavior of raptors is affected by several factors, including weather, geography, and topographical features. Here we provide information on how these factors may affect the behavior and detectability of the Short-toed Snake-Eagle (Circaetus gallicus). We observed passage of Short-toed Snake-Eagles at two watchsites along mountain chains in northeastern Greece (Mount Olympus) and northwestern Italy (Arenzano) during the peak periods of migration in autumn 2009 and spring 2010. More Short-toed Snake-Eagles were observed in spring than in autumn; this difference was more evident at Arenzano. Temperatures influenced the number of migrants observed. In particular, the number of individuals observed decreased drastically when temperatures were higher than 24°C during post-reproductive movements. At both sites, daily patterns showed a lower proportion of raptors observed during midday and early afternoon in autumn than in spring. These results suggest that, during autumn, individuals may pass undetected by flying at higher altitudes during midday and in early afternoon at both sites. The lack of difference in number of eagles observed during westerly (lateral) winds compared to other wind directions at Mount Olympus suggests that these birds may be able to compensate for drift effect toward the Aegean Sea. Wind strength and lateral northerly wind negatively affected the number of migrants observed at Arenzano during spring movements