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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.
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raptors cannot rest at sea. At the Straits of Messina the greatest (2002) Circuitous autumn migration in the Short-toed Eagle
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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,
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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.
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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
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