Ibis (2016), doi: 10.1111/ibi.12401

       Individual variation in orientation promotes a
 3000-km latitudinal change in wintering grounds in a

                   long-distance migratory raptor

                          UGO MELLONE,1* GIUSEPPE LUCIA,1 EGIDIO MALLIA2 & VICENTE URIOS1
          1Vertebrates Zoology Research Group, Departamento de Ciencias Ambientales y Recursos Naturales,

                                         University of Alicante, Apdo. 99, Alicante E-03080, Spain
                 2Parco Gallipoli Cognato Piccole Dolomiti Lucane, Localita Palazzo, Accettura IT75011, Italy

            Migrating juvenile birds rely on endogenous information in choosing the direction in
            which to fly, but such input may be overridden by social interactions with experienced
            individuals. We tagged seven juvenile Short-toed Eagles Circaetus gallicus with GPS
            transmitters in southern Italy. This trans-Saharan migrant flies mainly by soaring and is
            therefore not well adapted to performing long water crossings. Five of the seven tagged
            juveniles used the longer but apparently safer route towards the Strait of Gibraltar, and
            two migrated along a southerly trajectory and subsequently spent the winter in Sicily,
            apparently forced to do so by the 150-km-wide Sicily Channel. One of these individuals
            took the longer route the following autumn. These results, combined with long-term
            (15 years) visual field observations involving thousands of individuals, suggest that inex-
            perienced Short-toed Eagles may learn their migratory routes from experienced adults,
            whereas some of them migrate south in response to an innate orientation instinct. Trans-
            port costs, inherited information and geography apparently interact, forcing some Short-
            toed Eagles to winter 3000 km to the north of the majority of their conspecifics.

Keywords: flocking, migration, navigation, raptors, Short-toed Eagle, social interactions.

The behaviour of migrating birds can be subject to   age-related difference has also been confirmed by
great intraspecific variation, depending upon popu-   continent-wide displacements, with adults being
lation, age or experience, and sex (Newton 2008,     able to adjust their heading and juveniles continu-
Thorup et al. 2010, Panuccio et al. 2013). Differ-   ing to migrate in their original direction (Thorup
ences in orientation are determined by the geo-      et al. 2007).
graphical origin of the individuals, leading to the
development of migratory divides (Bobek et al.          In soaring species, such as broad-winged raptors
2008), and by their experience, which is mainly      and storks, there are many social diurnal migrants
shaped by age. Generally adult individuals have      (Newton 2008). This behaviour probably evolved
navigational capabilities that allow them to reach   to facilitate the location of thermal currents and
their goal precisely, even when using complex        the learning of optimal routes by inexperienced
routes that involve changes of course and correct-   individuals through conspecific guidance, in
ing for displacements (Holland 2014). However,       response to the mortality risk and high energy con-
juveniles rely mainly on a clock-and-compass strat-  sumption associated with orientation mistakes
egy, as they have inherited information that tells   (Kerlinger 1989, Maransky and Bildstein 2001,
them in which direction to fly and for how long,      Agostini 2004, Chernetsov et al. 2004). Any incor-
thanks to an endogenous circannual clock that is     rect decision could lead birds to fly over water sur-
under direct genetic control (Berthold 2001). This   faces such as seas and oceans, where thermal
                                                     currents are absent (thus forcing them to perform
*Corresponding author.                               flapping flight; Kerlinger 1989). Water bodies also
Email: u.mellone@gmail.com                           provide no landing possibilities for raptors in
                                                     adverse weather conditions, eventually causing

© 2016 British Ornithologists’ Union