Page 4 - Agostini_2005

, ,

Letters

J Raptor Res. 39(2) :184—186
© 2005 The Raptor Research Foundation, Inc.

Are Earlier Estimates of Accipitriformes Crossing the Channel of Sicily
(Central Mediterranean) During Spring Migration Accurate?

Each spring thousands of raptors cross the Channel of Sicily, between the Cap Bon Peninsula and western Sicily,
during northward migration. The Cap Bon Peninsula in northeastern Tunisia is a 25-km-wide plain dominated in
the north by a promontory that reaches a height of 392 m (Fig. 1). At this promontory, Thiollay (1975, Nos Oiseaux
33' 109-121; 1977, Alauda 43:115-121) made counts of migrants between 26 March-14 April 1974 and between 2-
18 May 1975 to estimate the migratory flow of raptors using this route (Table 1). Thiollay divided the period 22
March-20 May into six 10-d periods and extrapolated numbers of migrants in the periods for which observations
were lacking. Three decades later these counts and estimates are still quoted in publications (Ferguson-Lees and
Christie 2001, Raptors of the world, Helm Edition, London, U.K.; Brichetti and Fracasso 2003, Ornitologia Italiana,
Perdisa Editore, Bologna, Italy; Sara 2003,/. Raptor Res. 37:167-172). However, subsequent studies carried out at the
Cap Bon Promontory, at the Strait of Messina and over the islands of Marettimo and Ustica (Eig. 1) were not
consistent with these earlier data (Giordano 1991, Birds Prey Bull. 4:239-249; Agostini and Duchi 1994, Bird Behav.
10’45-48; Agostini et al. 1994a, Avocetta 18:73-76; Agostini et al. 1994b, Atti VI Conv. Ital. Ornitol. 451-452; Agostini
and Malara 1997, Riv. Ital. Ornitol. 66:174-176; Agostini and Logozzo 1998, Riv. Ital. Ornitol. 68:153-157; Agostini
2001, Buteo 12:99-102; Corso 2001, Br. Birds 94:196-202; Panuccio et al. 2004, Br. Birds 91:400-403).
At the Cap Bon Promontory, several collaborators and I collected data focusing on the flight behavior of raptors
on the coastline in the spring of 1990 and 1992 (Agostini and Duchi 1994, Agostini et al. 1994a, 1994b). During
these observations, upon reaching the coast near the promontory, most raptors hesitated at the water barrier; they
often stopped migrating and exhibited an unexpected series of movements, first disappearing across the sea, then
returning to the coast, and then flying back inland. During our observations, my colleagues and I recognized indi-
viduals or entire flocks circling over the promontory, sometimes repeatedly over several days. Because of this behavior,
it was impossible to avoid recounting migrating raptors and we concluded that the Tunisian promontory was unsuit-
able for assessing numbers of passing raptors. Factors probably influencing a hawk’s decision whether or not to cross
the Mediterranean included the morphology and age of the hawk, weather, and in the case of Honey-buzzards {Perms
apivorus) and Black Kites (Milvus migrans), flock size. Was the hesitation to cross this water barrier not conspicuous
when Thiollay made his observations? Was it possible to avoid recounting migrants during spring 1974 and 1975^
The Channel of Sicily is ca. 150 km wide, implying that long, powered flight, with a considerable expenditure of
energy, is necessary to make landfall. Because thermals are mostly absent over water, raptors cannot alternate soaring
with gliding during the crossing, as they do over land (Kerlinger 1989, Flight strategies of migrating hawks, Univ.
Chicago Press, Chicago, IL U.S.A.) and this could explain their hesitation at the Cap Bon Promontory when faced
and over the islands
with a substantial water crossing. In particular, counts made at the Strait of Messina (Table 1 )
of Marettimo and Ustica (Giordano 1991, Agostini 2001, Agostini and Logozzo 1998, Corso 2001, Panuccio et al.
2004, Br. Birds 97:400-403), showed that few broad-winged raptors, such as Common Buzzards {Buteo buteo). Booted
Eagles {Hieraaetus pennatus) Short-toed Eagles {Circaetus gallicus) and Egyptian Vultures {Neophron percnopterus) cross
the Channel of Sicily. However, Thiollay (1977) estimated the total passage of more than 2800, 450, 400, and 620
birds, respectively (Table 1). Moreover, at the Strait of Messina, the Long-legged Buzzard {Buteo rufinus) and the
Lesser Spotted Eagle {Aquila pomarina) were irregular migrants (Table 1; see also Zalles and Bildstein 2000, Raptor
watch: a global directory of raptor migration sites, BirdLife International, Cambridge, U.K. and Hawk Mountain
Sanctuary, Kempton, PA U.S.A). Whereas, Thiollay estimated a regular passage of more than 200 and 150 individuals
of these species. Undoubtedly, our counts missed some proportion of the birds that passed overhead; however,
difference between data recorded in southern Italy since 1980s and the estimate made by Thiollay in the 1970s was
substantial. Moreover, the few broad-winged raptors recorded in southern Italy was consistent with the notion that
species with a lower-aspect ratio (shorter) wings are not suited to undertake crossings of large bodies of water. Such
wings increase the induced drag, and thus, the energy needed for powered flight (Kerlinger 1985, Wilson Bull. 97:
109-113).
I suggest that the earlier estimates offered by Thiollay in the 1970s were too high probably because, during his
survey, the author recounted birds returning after they abandoned the water crossing attempt. Other explanations

184