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Wind patterns affect migration flyways and flock size       165


          and the western side of the Tyrrhenian Sea via Ustica. We   north-western Sicily exploiting their tail component, but did
          cannot exclude that during such wind conditions a significant   not support their flight towards Ustica. Conversely, the day
          number of migrants flew directly from Tunisia to central Italy   before the peak at the Straits, weak north-westerly winds
          undertaking a water crossing of about 500 km. In particular,   (≤4 m s ) occurred over the Channel of Sicily, a favourable
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          on 6 May 2012 at least 300 European Honey Buzzards were   condition for the water crossing bypassing both Pantelleria
          seen reaching central Italy  via Ventotene, a small island   and Marettimo, but with a probable drift effect towards SE. In
          located about 500 km NE of Tunisia and 230 km NNE of   conclusion, this study shows that wind conditions promote
          Ustica (Figure 1), passing during late afternoon (16:00–20:00;   route flexibility and inter-annual variability in a soaring
          Ferri pers. obs.). At the same time that day, 398 birds were   raptor, with longer flights over sea performed with tailwind
          counted at Ustica, while few birds were recorded over   assistance (see also Panuccio  et al., 2011). In agreement
          Marettimo. Although European Honey Buzzards travel   with the ‘optimal use of wind’ hypothesis (Alerstam, 1979),
          significantly faster and farther in tail winds (Vansteelant et   with less favourable winds for sea crossing (moderate lateral
          al., 2014), and a stronger tail wind would allow birds to   winds) European Honey Buzzards use a curvilinear path over
          save both time and energy during the crossing, radar studies   the Channel of Sicily. In doing so, they fly in larger flocks
          showed that unlike smaller birds (e.g. passerines), larger birds   exploiting the tail component to reach a mountainous island
          like raptors do not fly as fast as they can by selecting altitude   (Pantelleria) minimising the risk of long non-stop flapping
          with the best tail wind support; in addition, they are limited   flight over water, and are expected to overcompensate during
          to speeds close to their optimal speed of minimum power   the final stage of the crossing to reach their goal.
          consumption (Bruderer and Boldt, 2001; Mateos-Rodriguez
          and Liechti, 2012). In a preliminary study made along the
          continental  slope  of  the  Straits  of  Messina  during spring
          migration, European Honey Buzzards were seen flying close   5.  ACKNOWLEDGEMENTS
          to the ground, making abrupt rises followed by dives toward
          the ground, with tail winds blowing at 9–10 m s  (Agostini,   We thank Andrea Corso, Michele Cento, Renzo Ientile,
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          1992). Since raptors fly at lower altitudes over open water,   Giovanni  Cumbo  and Lucio Maniscalco  for the field
          the crossing during weak tail winds would allow them to   assistance. The work was supported by grants of LIPU-UK.
          maintain a steady forward movement. This flight strategy
          could be the result of a conservative strategy (i.e. fly slower but   Published online: 6 July 2016
          safer) to avoid grounding that results in death (Agostini, 1992;
          Agostini et al., 2005b). This conclusion is in agreement with
          recent research made using radar, concerning the behaviour   6.  REFERENCES
          of soaring birds, storks and raptors (and, among them, the
          European Honey Buzzard) while exploiting thermals over
          land. In particular, it confirmed that large birds did not fly as   Agostini, N. (1992) Spring migration of honey buzzards (Pernis
          fast as they could, but they glided in a risk-sensitive manner   apivorus)  at  the  Straits  of  Messina  in  relation  to  atmospheric
          to avoid “the grounding/flapping risk associated with a failure   conditions. J. Rapt. Res., 26, 93–96.
          to detect adequate thermals farther ahead” (Horvitz et al.,   Agostini, N. (2002) La migrazione dei rapaci in Italia. In: Brichetti,
          2014). While it is a conservative strategy over land, it is even   P. and Gariboldi, A. (eds), Manuale di ornitologia. Vol. 3., pp.
          more over water where, as mentioned above, thermals are   157–182. Edagricole-Il Sole 24 Ore, Bologna.
          weak and scarce and grounding would result in death since   Agostini, N., Baghino, L., Coleiro, C., Corbi, F. and Premuda, G.
          raptors cannot rest at sea. At the Straits of Messina the greatest   (2002) Circuitous autumn migration in the Short-toed Eagle
          concentration of European Honey Buzzards occurred during   (Circaetus gallicus). J. Rapt. Res., 36, 111–114.
          both spring and autumn (Agostini and Panuccio, 2005).   Agostini, N. and Duchi, A. (1994) Water–crossing behaviour of
          This is expected for a soaring bird, since this is the shorter   Black Kites (Milvus migrans) during migration. Bird Behav., 10,
          water crossing between Sicily and the Italian Peninsula and   45–48.
          thus the safer pathway. Moreover, the benefits associated   Agostini, N.,  Gustin, M. and Cardelli, C. (2007) Factors shaping
          to the pathway  via Panarea are not great considering the   pathways of European Honey-buzzards (Pernis apivorus) during
          distance of the crossing relative to the distance around the   spring  migration  in the  central  Mediterranean  basin.  J. Rapt.
          water barrier. However, during very weak westerly winds,   Res., 41, 57–61.
          a substantial number of migrants concentrate over Panarea   Agostini, N., Lucia, G., Mellone, U., Panuccio, M., von Hardenberg,

          bypassing the Straits. In this view, we cannot exclude that   J., Evangelidis, A. and Kominos, T. (2012) Loop migration of
          the passage through this island could be the result of wind   adult European Honey Buzzards Pernis apivorus through the
          conditions on the Channel of Sicily the day before the peak.   central-eastern Mediterranean. Ital. J. Zool., 79, 280–286.
          In particular, migrants could have concentrated their passage   Agostini, N., Malara, G., Neri, F. Mollicone, D. and Melotto,
          along the Tyrrhenian coast of Sicily and over Panarea, such as   S. (1994) Flight strategies of Honey Buzzards during spring
          other Aeolian islands, when wind conditions (weak westerly   migration across the central Mediterranean. Avocetta, 18, 73–
          winds) on the Channel the day before allowed them to reach   76.
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