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

(Aquila pennata; Agostini et al., 2015). During these the Sicilian mainland, most of them head towards the
movements, European Honey Buzzards sometimes Straits of Messina (the shorter water crossing between
interrupt their continuous, powered flight over water and Sicily and Italian Peninsula) while others bypass the Straits
concentrate over small islands to exploit thermals and crossing the Tyrrhenian Sea via the islands of Ustica and
undertake further crossing at higher altitude (Agostini, Panarea (Figure 1). Here, we analyse the influence of
2002; Panuccio et al., 2004, 2013; Meyburg et al., 2010, wind patterns on the spring pathways of European Honey
2013; Agostini and Panuccio, 2015). This strategy could Buzzards flying through this portion of the Mediterranean
lower the risk of facing unpredictable weather changes area, through field observations performed during eight
during the crossing, and may perhaps result effective years at these five watch sites.
from the point of view of a conservative strategy (Agostini
et al., 2005a, 2005b, 2007). When crossing the central
Mediterranean, these raptors fly through the Channel 2. STUDY AREAS AND METHODS
of Sicily, between the Cap Bon Peninsula (Tunisia) and
western Sicily (approx. 150 km wide), concentrating over 2.1 Fieldwork
the islands of Marettimo and Pantelleria. Once reached
Observations, using binoculars and telescopes, were
made over the islands of Marettimo, Pantelleria, Ustica,
Panarea, and at the Straits of Messina, between 09:00 and
19:00 (local time) from 20 April to 20 May 2006–2013,
during the peak of the spring migration of the European
Honey Buzzard in the Mediterranean basin (Agostini and
Panuccio, 2005). This 8 year multisite study allowed the
collection of a large data set. Marettimo is a small (12 km²)
mountainous island, approximately 30 km distant from
western Sicily. This island is located about 130 km NE
of the Cap Bon Promontory (Tunisia, Figure 1). Monte
Falcone is its highest point, at 686 m above sea level.
Pantelleria is a volcanic island (84 km²), approximately
110 km SW of western Sicily and 70 km SE of the Cap Bon
Peninsula, in the southern part of the Channel of Sicily
(Figure 1). In this island the highest elevation (Montagna
Grande) reaches 836 m. Ustica is a small volcanic island
(8.5 km²) about 60 km N of western Sicily, 270 km NE
of the Cap Bon Promontory (Tunisia), 140 km NE of
(a)
the island of Marettimo and 230 km W of the Italian
Peninsula (Figure 1). Like Pantelleria and Ustica, Panarea
2
is a volcanic island (3.5 km ), located approximately 65
km NW from the Straits of Messina (Figure 1). At each
site the monitoring was made using the observation post
of previous surveys (see Agostini et al., 2005b, 2007).
To investigate the circadian pattern of migration, each
observation day was divided into three time periods:
morning (09:00–11:59), midday (12:00–14:59) and
afternoon (15:00–19:00, solar time). In the analysis, we
considered for each watch site only peak days, i.e. those
in which at least 9% of the total number of birds counted
in the season was reported.

2.2 Wind data
(b)
Wind fields were obtained from ERA-Interim (Dee et al.,
Figure 1 The study area (Pnt=Pantelleria; M=Marettimo; 2011), a global atmospheric reanalysis product realised
U=Ustica; P=Panarea; SoM=Straits of Messina; V=Ventotene) by assimilating available meteorological measurements
(a) and approximate paths over water used by European within a state-of-the-art forecast model in order to estimate
Honey Buzzards during spring migration across the central the evolving state of the global atmosphere. Gridded
Mediterranean (b). fields of relevant atmospheric variables are available at