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2 M. Panuccio et al. 35

KEY WORDS: migration, flocking behaviour, orientation, information transmission,
geography, morphology Circaetus gallicus.

INTRODUCTION

During migration, birds can organise their travels according to alternative 40
criteria, namely time, energy expenditure or safety (ALERSTAM & LINDSTRÖM 1990). In 45
particular, strategies that minimise energy consumption and mortality risk evolved in 50
broad-winged species, such as eagles, vultures and storks (KERLINGER 1989; NEWTON 55
2008; BILDSTEIN et al. 2009). In these species, long powered flapping flights over water 60
require the expenditure of huge amounts of extra cost (PENNYCUICK 1972, 1975) and 65
increase mortality risk (e.g. more than 1300 raptors were found dead along a beach 70
of the Mediterranean coast of Israel during April 1980 as reported by ZU-ARETZ & 75
LESHEM 1983). For these reasons, the routes of broad-winged raptors are constrained
by the geographical distribution of land-masses, leading to detours that often involve
complex changes of course in order to concentrate at straits where sea-crossings are
narrower (KERLINGER 1989; ALERSTAM 2001; BILDSTEIN 2006). In some species these
migration pathways can even involve movements that are opposite to the main migra-
tion direction, thus leading to the so-called “circuitous migration” (AGOSTINI et al.
2002a; YAMAGUCHI et al. 2008). Flocking behaviour, and in particular mixed-age flocks,
are of paramount importance in these species, since inexperienced juveniles cannot
know the safest route during their first migration (MARANSKY & BILDSTEIN 2001;
AGOSTINI 2004; CHERNETSOV et al. 2004,) and, hence, the selective pressure to migrate
together with the adults should be higher than in species more adapted to flapping
flight.

The Short-toed Snake Eagle (Circaetus gallicus) is a summer resident in Europe,
wintering in tropical Africa (FERGUSON-LEES & CHRISTIE 2001). In Greece a breeding
population of 350–500 pairs has been estimated with a wide distribution over the whole
mainland (HANDRINOS & AKRIOTIS 1997; BIRDLIFE 2004). There is lack of information
concerning migration pathways used by birds belonging to populations breeding in the
Balkans. This species mostly uses soaring flight over land during migration, avoiding
the crossing of water surfaces and concentrating at the Strait of Gibraltar and at the
Bosphorus (MEYBURG et al. 1998; KIRWAN et al. 2008; PAVON et al. 2010). As sug-
gested by AGOSTINI & MELLONE (2008) “the benefits associated with the low cost of
thermal soaring flight, compared to flapping flight over water, are probably great for
these birds during both spring and autumn migrations, as this species has a low aspect
ratio and is thus less well adapted to flapping flight than some other raptors”. As a
result, Short-toed Snake Eagles breeding in the Italian Peninsula follow a circuitous
route rather than directly crossing the central Mediterranean, entering and depart-
ing Europe via the Strait of Gibraltar, and travelling through northwest Italy, France
and Spain (AGOSTINI et al. 2002a, 2002b; PREMUDA 2004). In particular, hundreds of
Short-toed Snake Eagles, mostly adults, are observed annually migrating northwards
during autumn and southwards during spring along the Tyrrhenian coast, exploiting
thermal currents and updrafts on the western slopes of the Apuane Alps (PREMUDA
et al. 2010). Since the water surface separating the Greek mainland from African coasts
is approximately three times longer than the Channel of Sicily (Fig. 1), it is expected
that Short-toed Snake Eagles breeding in central and southern continental Greece, like
those breeding in the Italian Peninsula, avoid that long sea crossing using a circuitous

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