Page 6 - 1Agostini_wind_patterns_honey_buzzard_ABR2016

164 N. Agostini, M. Gustin, J. von Hardenberg and M. Panuccio

Figure 5 Strength and direction of the winds averaged only for the days before (–1 day) a peak passage at Panarea
and at the Straits of Messina.

at the Channel of Sicily the day before the peak at these to compensate their drift effect. This wind condition was
two watch sites we noted a difference due to very weak the worst recorded during the peak passages in our study.
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(≤2 m s ) westerly and lightly stronger (3–4 m s ) north- European Honey Buzzards observed at the Cap Promontory
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westerly winds, respectively (Figure 5). Finally, when tended to cross the open sea heading NE during weak lateral
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comparing the medians of flock size reported at each site winds (≤4 m s ), such as Black Kites (Milvus migrans), when
(Table 1), the difference was significant with larger flocks migrating in larger flocks. In particular, they showed a strong
observed at Pantelleria and smaller ones over Marettimo tendency to remain together in front of the water barrier since
(Kruskal–Wallis test, H=98.3, df=4, P<0.001). the first individuals taking a decision (crossing or not) were
followed by the others (Agostini and Duchi, 1994; Agostini
et al., 1994). Since migrants of both species often hesitate
in front of the open sea waiting for better wind conditions
4. DISCUSSION
for crossing (e.g. weaker lateral winds), it has been suggested
that flock size affects the water crossing tendency in these
Our study shows that wind patterns affect migration flyways species because an increase in flock size increases the
and flock size of European Honey Buzzards crossing water probability that the flock will contain one or more individuals
surfaces. Raptors select weak tailwind conditions when facing with a pronounced tendency to continue migrating. The
long flights over water, and show a curvilinear path when physiological state of the bird such as a previous experience
crossing during moderate crosswinds. In agreement with the could play a role in this decision (Kerlinger, 1989). This
preliminary study realised in a single season in spring 2004, behaviour would explain why the flock size recorded over
European Honey Buzzards reach the island of Pantelleria Pantelleria was the larger one. As a matter of fact, during such
when they can exploit moderate NW winds (Agostini et al., wind conditions (lateral wind >4 m s ), we can expect a
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2007). During this first stage of the crossing between the Cap greater hesitation and the formation of larger flocks at the Cap
Bon Peninsula and western Sicily, they probably save energy Bon Peninsula among birds facing the Channel of Sicily and
thanks to the tail support of such winds, with Pantelleria trying to cross the water barrier heading NE, before some
located SE of the Tunisian peninsula. Observations made at birds decide to fly towards SE dragging the flock. Conversely,
the Cap Bon Promontory reported flocks of European Honey birds passed via Marettimo in smaller flocks during weak S–
Buzzards leaving the coast and heading SE, apparently SE winds. Perhaps, during such favourable wind conditions,
towards this mountainous island that they probably see when few birds passed via Marettimo, while most flew directly
soaring at several hundred metres above sea level before towards western Sicily exploiting its tail component (see also
crossing (Thiollay, 1975; Agostini et al., 1994). However, Agostini et al., 2007). This would explain why the passages
during the following flight between Pantelleria and south- over Pantelleria and Marettimo did not show any significant
western Sicily (ca 110 km long), they have to fly through correlation. Weak tail winds were apparently the better
lateral winds blowing on average 5 m s , and are forced wind conditions to fly both through the Channel of Sicily
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