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140 AGOSTINI AND PANUCCIO VOL. 44, NO. 2

Figure 2. The occurrence of migrating Western Marsh Harriers (Circus aeruginosus) over the Strait of Messina, southern
Italy, between 27 March and 25 May 2004, during six ten-day periods (M. Panuccio unpubl. data). In this survey, the totals
for males, females, and juveniles were derived by multiplying their proportions in the sample of identified individuals in
each period, following the method used by Kjelle´n (1992) in his study on the autumn migration of raptors at the
Falsterbo peninsula (Sweden). In particular, to eliminate a bias resulting from the easier identification of the adult
males, we estimated the proportion of females and juveniles by dividing unidentified individuals of the group female/
juvenile between the two age groups according to their proportions among the identified birds.

2004) and at the Strait of Gibraltar (Finlayson 1992) Among long-distance migrants, adults tend to mi-
showed that adult males tended to migrate earlier grate earlier than juveniles during spring (Kerlinger
than adult females in spring (M. Panuccio unpubl. 1989). In the case of the Western Marsh Harrier,
data; Fig. 2). Strandberg et al. (2008) did not report however, observations made at several sites of the
distinct differences in timing (departure and arrival central Mediterranean indicate a partial overlap in
dates) of spring migration between males and fe- the migration periods of birds of different ages
males; however, the duration of spring migration (Fig. 2; Agostini and Logozzo 2000, Agostini 2001,
tended to be shorter for females. In spring, Western Panuccio et al. 2004). Perhaps, as suggested in a pre-
Marsh Harriers tracked by satellite telemetry vious study (Agostini 2001), the tendency of juveniles
showed a narrower front of migration over water, to migrate shorter distances than adults may explain
concentrating the passage in the area of the Strait this partial overlap in their migration periods during
of Gibraltar. One bird crossed the sea apparently via spring, a result of the contemporaneous passage of
Balearic Islands. In contrast with Strandberg et al. juveniles wintering in the Mediterranean basin and
(2008) and in agreement with several migration adults wintering in tropical Africa but beginning their
counts, previous investigations have shown that northward migration earlier in the season.
males tend to arrive on the breeding grounds earli-
er than females (Brown and Amadon 1968 quoted POPULATION TRENDS AND CONSERVATION
by Kerlinger 1989). We recommend that future re-
search focus on elucidating any differential migra- At the beginning of the second half of the last
tion strategies, as well as any effects of those strate- century, the European population of the Western
gies on reproductive fitness. Marsh Harrier decreased dramatically as a result of
shooting, poisoning by pesticides, and habitat loss