Flight behaviour of Honey Buzzards (Pernis apivorus) during spring migration over the sea Migrační chování včelojeda lesního (Pernis apivorus) při jarním tahu přes moře AGOSTINI N. (1) , PANUCCIO M. (2) & MASSA B. (3) (1) Nicolantonio Agostini, Via Carlo Alberto n°4, 89046 Marina di Gioiosa Jonica (RC), Italy; e-mail: nicolantonioa@tiscalinet.it (2) Michele Panuccio,Via Mario Fioretti n°18, 00152 Roma, Italy (3) Bruno Massa, S.En.Fi.Mi.Zo, Università degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy ABSTRACT. The aim of this research was to study the light behaviour of mi- grating Honey Buzzards (Pernis apivorus) over water in relation to atmospheric conditions considering the lock as sampling unit. Observations were made be- tween 29 April and 16 May 2001 over the island of Ustica (southern Italy), lo- cated about 60 km N of western Sicily, 270 km NE of the Cap Bon Promontory (Tunisia), 140 km NE of the island of Marettimo and 230 km W of the Italian Peninsula. A total of 124 locks were counted, of them 101 undertook the cross- ing of the sea towards NE. The highest proportion of crossing locks was seen during midday, but those seen leaving the island during the afternoon, about two-three hours before sunset, are evidence of the capacity of these raptors to migrate across water surfaces during the night. Considering both crossing and not crossing locks, ideal weather conditions for crossing existed with following and weak lateral winds. During following wind, Honey Buzzards, although lew at low altitude, used soaring light over water successfully. INTRODUCTION During migration, many species of raptors mostly use soaring light exploiting ther- mal currents and limiting powered light to reduce energetic costs (KERLINGER 1989). As a rule, because thermals are almost absent over water, these birds are reluctant to un- dertake water crossings. Factors inluencing the decision whether to cross or not a body of water include weather, length of the crossing, distance around the water barrier, physiological state of the bird, lock size, time of day and experience (age dependent), while the risk of mortality probably increases with the absolute distance of the crossing (KERLINGER 1984, 1989, AGOSTINI & DUCHI 1994, AGOSTINI et al. 1994a, b, 2002, 2004, MEYER et al. 2000, THORUP et al. 2003). Moreover, also the light morphology plays a role; in particular, species with relatively long wings (high aspect ratio) are more suited to undertake crossings of large body of water as this feature decreases the in- duced drag, and thus the energy needed for powered light (KERLINGER 1985). Among Accipitriformes, Osprey (Pandion haliaetus), harriers (Circus sp.) and kites (Milvus sp.) show higher aspect ratio while the Honey Buzzard (Pernis apivorus) has intermediate light morphology between them and buzzards (Buteo spp.) (KERLINGER 1989). Eagles (Aquilla sp.) and vultures (i.e. Gyps sp.) show lower aspect ratio (KERLINGER 1989). The Honey Buzzard is a summer resident in Europe wintering in west-central equato- BUTEO 14 (2005): 3-9 3