Ecological barriers promote risk minimisation and social learning in migrating short-toed snake eagles

The short-toed snake eagle (Circaetus gallicus) uses mostly soaring flight over land during migration to avoid long sea crossings. In particular, birds breeding in central Italy cross the Mediterranean Sea at the Strait of Gibraltar, using a route through northwestern Italy during both autumn and spring migration. Birds breeding in Greece, such as those breeding in Italy, are expected to use the same strategy passing through northeastern Greece and avoiding the longer sea crossing between southern Greece and Libya. In order to verify this hypothesis, contemporaneous observations were made at two watchsites, in northwestern Italy (Apuane Alps) and northeastern Greece (Mount Olympus), during autumn 2009 and spring 2010. During autumn migration 376 birds were seen migrating at Mount Olympus, nearly all heading NNE. Most birds were seen migrating in flocks, and at least 23 flocks contained both adults and juveniles. Over the Apuane Alps a total of 1042 short-toed snake eagles, all migrating NNW, was counted. At this watchsite the proportion of juveniles was lower than that reported at Mount Olympus. During spring migration, 606 birds were seen at Mount Olympus, 602 heading south. At the Apuane Alps 1307 birds were counted, all heading SSE. The orientation behaviour of short-toed snake eagles confirms that those breeding in Greece, like those breeding in central Italy, use a circuitous route during both spring and autumn. In particular those breeding in Greece are expected to cross the sea at the Dardanelles and/or at the Bosphorus. In addition, the higher proportion of juveniles reported at Mount Olympus during autumn migration would suggest that social learning could have been much favoured by natural selection in the case of birds breeding in Greece rather than in Italy, highlighting a relationship between the length of the barrier and the tendency of juveniles to follow the adults.