Notas Breves SPRING MIGRATION IN RELATION TO SEX AND AGE OF MARSH HARRIERS CIRCUS AERUGINOSUS OVER A CENTRAL MEDITERRANEAN ISLAND

grates in a broad front, undertaking crossings of large bodies of water using powered flight (Cramp & Simmons, 1980; Kerlinger, 1989). During spring migration in the Mediterranean basin, the largest concentration of birds occurs on the Straits of Messina, between the Italian Peninsula and Sicily, where the peak has been observed between the end of March and the beginning of April (Giordano, 1991; Agostini et al., 1995; Agostini & Malara, 1997; Agostini & Logozzo, 2000). In particular, during the springs of 1994 and 1997, about 1000 and 1300 birds, respectively, crossed the Straits at its narrowest point (Agostini et al., 1995; Agostini & Logozzo, 2000). If compared with counts made at the Straits of Messina, very few birds are observed each spring at the Strait of Gibraltar (usually under 200; Bernis, 1980; Finlayson, 1992), while over the Bosphorus the passage of this species is virtually non-existent (Collman & Croxall, 1967). At this important bottleneck area of southern Italy, birds crossing the Mediterranean Sea between Tunisia and Sicily (150 km) converge with birds crossing between Libya and Sicily via the island of Malta (>400 km) (Beaman & Galea, 1974; Sultana & Gauci, 1982; Agostini & Duchi, 1994; Agostini & Logozzo, 1998). A substantial number of birds using the first route concentrate over the island of Marettimo (Agostini & Logozzo, 1998), where the greatest passage of Accipitriformes during post-reproductive movements on the central Mediterranean occurs (Agostini et al., 2000). The aim of this study was to investigate the spring migration of this species on this site and during the peak movement, recording the passage of birds belonging to different age and sex classes. Marettimo is a small mountainous island (12 km2) about 30 km off western Sicily and 130 km NE from the Cap Bon peninsula (Tunisia; Agostini & Logozzo, 1998). Observations were made from 26th March to 14th April 2000. I used a single post at an altitude of about 500 m. From this post it was possible to dominate both the western and eastern coasts of the island. The observation period has been divided into five four-day periods. It was possible to determine the age and sex of some birds by observation of their plumage (Porter et al., 1981; Forsman, 1999) generally when they were very close (<150 m). I estimated the total number of individuals belonging to age and sex groups according to their proportions among identified individuals for each four-day period, following the method used by Kjellén (1992) in his study on the autumn migration of raptors at the Falsterbo peninsula. In particular, the proportion of females and juveniles was estimated dividing unidentified individuals of the group female/juvenile between the two age groups according to their proportions among the identified individuals (Kjellén, 1992; Agostini & Logozzo, 2000). 160 hr of observations were carried out using a 10 × 40 binocular. I counted 577 Marsh Harriers, with a peak between 7th and 10th of April (Fig. 1). I observed the plumage of 322 (55.8%) birds; 235 (73%) of them were adults and 87 (27%) juveArdeola 48(1), 2001, 71-73