Page 1 - Eagles_Circaetus_gallicus_2016

Italian Journal of Zoology, 2016, 543–548
Vol. 83, No. 4, http://dx.doi.org/10.1080/11250003.2016.1240833

Short-toed snake eagles Circaetus gallicus (Gmelin, 1788)
(Aves: Accipitridae) approaching a water barrier show reverse
direction of migration

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1
N. AGOSTINI *, M. GUSTIN , & M. PANUCCIO 1,3
1 2
MEDRAPTORS (Mediterranean Raptor Migration Network), Rome, Italy, LIPU (Lega Italiana Protezione Uccelli),
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Parma, Italy, and DSTA, Department of Earth and Environmental Sciences, University of Pavia, Italy
(Received 14 January 2016; accepted 19 September 2016)

Abstract
We investigated the directions of migration (reversed vs. expected) of raptors approaching a geographical strait in relation to
local wind conditions, time of day, flock size and location of the observation post (coastal zone vs. inland zone). Fieldwork
was conducted during autumn migration in 2011, 2012 and 2013 at a migratory bottleneck located in the southernmost part
of the Italian Peninsula (Calabrian Apennines), using four watch points. In this area, migrating birds face the narrowest
water surface between continental Italy and Sicily, the Strait of Messina. The only species showing substantial reverse
migration was the short-toed snake eagle (Circaetus gallicus). In particular, eagles, mostly first calendar year (cy) birds,
showed this behavior when passing closer to the coast (5 km inland of the Strait of Messina). Our results could reflect the
reluctance of these birds to head south when approaching this relatively short stretch of sea, even before reaching the
coastline. This behavior could be evidence of the strong selective pressure, which would have led to the evolution of the
extremely detoured flight path of birds breeding in Italy.

Keywords: Reverse migration, short-toed snake eagle, sea-crossing, detour, Mediterranean

Introduction
showing a reversed direction of migration increases
Reverse migration is a behavior shown by birds mov- at night (Bruderer & Liechti 1998). In the case of a
ing in the opposite to expected direction of migration sea barrier, soaring raptors show reverse migration
(Åkesson et al. 1996). Three factors appear to cause on coastal areas, flying inland and sometimes flying
this behavior in birds. During spring movements, back from the sea (Agostini et al. 1994, 2000;
some birds overshoot breeding sites because of Agostini & Panuccio 2004; Panuccio et al. 2004,
their “crude” navigation system (Mueller & Berger 2011; Mellone et al. 2013). In this case, among
1969; Rabøl 1993). These birds may use reverse factors affecting reverse migration, wind condition,
migration to reach their goals, sometimes showing a flock size and time of day are of paramount impor-
high degree of directional scatter (Karlsson et al. tance in explaining the flight direction of raptors
2010). Birds perform reverse migration because of (Agostini & Duchi 1994; Panuccio & Agostini
adverse weather conditions, with birds turning back 2010). The reluctance of raptor species to fly across
when conditions ahead are bad (Nilsson & Sjöberg the sea proportionally increases with the body size of
2015). Alternatively, birds may travel along the coast the bird species. Their morphology and weight affect
in order to find suitable stop-over sites to feed inland the energy consumption rates and therefore their
and increase their fat reserves before setting out over chance to safely cross the sea barrier (Panuccio
water (Alerstam 1978, 1990; Åkesson et al. 1996; et al. 2013; Agostini et al. 2015a). For this reason,
Bruderer & Liechti 1998; Åkesson 1999; some heavy species with broad and rounded wings
Komenda-Zehnder et al. 2002; Smolinsky et al. evolved an extremely detoured (circuitous) route
2013; Deppe et al. 2015). The proportion of birds involving reverse migration at the end (spring) or at

*Correspondence: N. Agostini, MEDRAPTORS (Mediterranean Raptor Migration Network), Via Mario Fioretti 18, 00152 Rome, Italy. Tel: +39 328
6549590. Email: nicolantonioagostini@gmail.com
© 2016 Unione Zoologica Italiana