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Migration behaviour of Short-toed Snake Eagles 5

DISCUSSION

Our results confirm the hypothesis that birds breeding in central and southern 150
Greece use a circuitous route during both spring and autumn migration and proba- 155
bly cross the Mediterranean Sea at the Dardanelles’ Strait and/or at the Bosphorus. 160
At both sites and seasons Short-toed Snake Eagles showed a conservative migration 165
strategy, avoiding the crossing of large stretches of water. This was already known 170
just for Italy (AGOSTINI et al. 2002a) but it is a new finding for Greece. Thus, these 175
migration patterns are the result of a migration strategy performed by individuals 180
breeding in peninsulas facing wide bodies of water. Our field observations agree with 185
the theoretical model: in fact the huge increase of energy consumption during pow- 190
ered flight means that a 400 km crossing across the Mediterranean using flapping flight 195
is equivalent to a soaring-gliding flight of approximately 3500 km over land around
the barrier. In this picture, social learning plays an important role in the evolution of
this migration strategy since, during their first migration, inexperienced birds migrat-
ing alone would tend to move southwards along an innate direction of migration
(KERLINGER 1989). Probably the higher survival rate of juveniles migrating together
with adults rather than alone favoured high synchronicity of departure times from
the breeding grounds in populations of Italy and Greece. The flocking behavior is the
“tool” allowing for information transmission between birds belonging to different age
classes (COUZIN et al. 2005). In addition, the higher proportion of juveniles recorded
in Greece rather than in Italy during autumn migration, if compared with the breed-
ing success of the Greek and Italian populations of this species (FERGUSON-LEES &
CHRISTIE 2001; BAKALOUDIS et al. 2005), suggests that nearly all juveniles belonging
to the Greek population use the circuitous route, learning this flyway by following the
adults. Conversely, as expected, five juveniles that did not migrate in mixed-age flocks
at Mount Olympus were seen heading southwards along the innate direction of migra-
tion. Perhaps the length of the sea crossing, longer between southern Greece and Libya
rather than between western Sicily and Tunisia, would negatively affect the survival
of juveniles in Greece more than in Italy. As a result, the overlap of migration periods
of birds belonging to different age classes could have been much favoured by natural
selection in the case of birds breeding in Greece rather than in Italy. This assumption
would agree with recent autumn surveys made at the islands of Antikythira (located
33 km NW of Crete) and Marettimo (approximately 30 km off western Sicily; Fig. 1).
While, at the first site, LUCIA and colleagues (in press) reported the passage of a few
dozen juvenile Short-toed Snake Eagles moving southwards late in the season, dur-
ing the first half of October, in the same period higher numbers of juveniles (at least
150–200 birds) were reported undertaking the crossing of the Channel of Sicily (approx-
imately 150 km wide) via Marettimo, between western Sicily and Tunisia (AGOSTINI
et al. 2004b, 2009). Social learning concerning migration routes has been suggested
to occur in other species of birds such as the Black Kite (Milvus migrans; AGOSTINI
et al. 2004a), the White Stork (Ciconia ciconia; CHERNETSOV et al. 2004) and, occasion-
ally, the European Honey Buzzard (Pernis apivorus; AGOSTINI et al. 1999; AGOSTINI
2004).

In conclusion, like Short-toed Snake Eagles breeding in central Italy, birds breed-
ing in central-southern Greece use the route that probably reflects the colonisation
process during both migrations: a further example of how colonisation history, ecologi-
cal barriers, and morphological characteristics of migrants probably interact in shaping
migratory routes and migration strategies of birds belonging to different age classes.

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