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274 Kurze Mineilungen

P < 0.0 l; faterai wind: X'= l 04.6, d. f. = l, P< 0.0 l; Fig. 3). Moreover, regarding winds with tail and
head components (frequency 36 vs 53 hrs, x'= 2.87, df= l, p > 0.05), the difference between the lo-
tal numbers ofbirds observed under these conditions did not differ significantly ( 114 vs 96; x' = 1.38,
df = l, P > 0.05). A total of 44 Marsh Harriers was recorded during absent wind (frequency 16 h).

Discussion
Our results partially agree with those of a recent study made simultaneously by radar and visual obser-
vation during the auturnn migration of raptors at the southern coast ofSpain, about 100 km NE ofthe
Straits of Gibraltar (MEYER et al. 2000). Also at that si te, the opposi te coast was out of sight (distance
about 150 km). There, during weak winds (< 15 kmlh), the number ofharriers, falcons and ospreys un-
dertaking the crossing ofthe Mediterranean Sea was similar forali wind directions. In contrast, during
strong winds (> 15 kmlh), the number ofbirds crossing was higher during tail and latera! winds ifthe
laner provided at least partial tailwind support. Not one bird undertook the crossing during strong head-
wind. To explain these results, MEYER et al. (2000) suggested that this strategy leads to shorter crossing
times and lowers the risk of unpredictable weather changes. During strong lateral and headwinds the
crossing would require more energy while tailwind should allow a faster and energetically less expen-
sive flight over water (KERLINGER 1989). Over the sea there is no possibility to land and the probability
of changing weather (wind) conditions increases with the length ofthe crossing. Previous visual obser-
vations made at tbe Cap Bon promontory during spring migration showed that Marsh Harriers tend to
cross the Channel of Sicily both during weak and strong latera! winds such as Black Kites (MilvriS mi-
grans) and Montagu's Harriers (Circus pygargus; AGOSTINI & DuCHI 1994). However, once reached
western Sicily after the long sea crossing (approx. 150 km), tired birds have to decide whether to mini-
mize energy (for instance using mostly soaring-gliding flight over land or going hunting along the way),
orto fly aga in NE about 300 km across the Tyrrhenian Sea migrating also during the night minimizing
the time of migration. In fact, because over Ustica Marsh Harriers are observed mostly during midday
an d afternoon, probably a t least some of them cross the Tyrrhenian Sea immediately after the Channel
of Sicily flying about 500 km over water in one day also during the night. Our results suggest that,
avoiding the crossing of the larger body of water during strong wind independently from its direction,
Marsh Harriers seem to have a conservative.strategy flying slower but safely.
ln the Marsh Harrier the occurrence ofpolygyny would suggest an imbalance towards females
among adults (KJELLÉN 1992, CLARKE 1995, AGOSTINI 200 l), and in severa! populations of this
species females outnumber mal es (SIMMONS 2000). A similar unbalancing was a Iso recorded in a re-
ceni research on tbe spring migration of Marsh Harrier over the island of Marettimo, located about
130 km NE of the Ca p Bon promontory and 35 km W of western Sicily, where a total of 577 indi-
viduals were counted between the end ofMarch and the first half of Aprii 2000 (AGOSTINI 2001). In
contrast, in our study males outnumbered females. ln this species, mal es reach breeding areas some
days earlier than females (BROWN & AMADON 1968). Perhaps ma1es tend to choose, more than fe-
males, direct routes both between Tunisia and western Sicily (avoiding the is1and of Marettimo and
saving about IO% ofthe crossing) and between western Sicily and centralltaly, minimizing the t ime
ofmigration. Thus, as a rule, the differential arrivai time at breeding sites among sex classes in this
species, could occur not only because males begin northward migration earlier but also because
they tend to fly along shorter routes between wintering and breeding areas.
Zusammenfassung
Ùberquerung des Ty rrheni scben Meeres: Fruhjahrszug von Rohrweihen
(Cìrcus aentginosus) , Geschlechtervcrhaltnis und Einflus s der Windvcrhaltnisse .
Aufder lnsel Ustica (ca. 60 krn nèirdlich von Westsizilien) wurde zwischen 22. Marz und 16. April2002jeweils
9 Std./Tag der Fruhjahrszug von Rohrweihen uber das Tyrrhenische Meer bcobachtet. In dieser Zeit konnten