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JUNE 2010 WESTERN MARSH HARRIER MIGRATION
Panuccio et al. 2005b). This latitudinal segregation of
sexes in some populations of Western Marsh Harriers
may be explained by Bergmann’s rule, which predicts
that larger individuals are better adapted to survive in that this strategy resulted in shorter crossing times
colder climates. Other hypotheses, based on social
dominance theory, are less supported by available da-
ta, including similar proportions of juveniles and
adults in Italy (Panuccio et al. 2005b).
Water Crossings During Migration. The limited
data available from satellite telemetry suggest that
Western Marsh Harriers can make nonstop cross-
ings over water of 300–550 km or more. For exam-
ple, one adult male summering in the Ukraine and
studied with satellite telemetry crossed the central
Mediterranean Sea either via Corsica and Sardinia
or directly over water (R. Strandberg et al. 2008;
Fig. 1). Although locating birds using satellite te-
lemetry produces large gaps in information in the
area around Italy because of an unknown interfer-
ence (R. Strandberg pers. comm.), it was known
that the male passed through central Italy and
crossed the Tyrrhenian Sea from a starting point
north of Rome, continuing either to the east of or
across the island of Sardinia, reaching northern Tu-
nisia, and heading southwest through north Africa
toward the Sahara Desert. Thus, this male either
made a nonstop powered flight over water of ap-
proximately 550 km, avoiding the passage through
Corsica and Sardinia, or traveled via the islands,
likely with at least one night of rest. When crossing
the central Mediterranean during both spring and
autumn, Western Marsh Harriers use several small
islands to enable soaring flight or as stopover sites
for roosting (Sammut 2005, Panuccio and Agostini
2006) and/or hunting (Thake 1983, Frost 1994, Re-
bassa 1995, Agostini and Logozzo 1998, Jonze´n and
Pettersson 1999, Agostini et al. 2003, Panuccio et al.
2004, Sammut and Bonavia 2004, Panuccio and
Agostini 2006). Western Marsh Harriers are able
to cross water bodies during the night and tend to
cross the large body of water between western Sicily
and central Italy (.300 km; Fig. 1) during weak
rather than strong winds, without regard to wind
direction, or during windless periods (Panuccio et
al. 2002, Panuccio and Agostini 2010). However, a
study made simultaneously with radar and direct
observations during autumn migration on the
southern coast of Spain (Fig. 1) showed that the
number of harriers undertaking a shorter sea cross-
ing (approx. 150 km) was similar for all wind direc-
tions during periods of weak winds. Conversely, dur-
ing strong winds (.15 km/h) the number of birds
139
crossing was higher during tail and lateral winds if the
latter provided at least partial tail-wind support
(Meyer et al. 2000). Meyer and colleagues suggested
and thus lowered risk of unpredictable weather
changes. However, the probability of changing weath-
er (wind) conditions increases with the length of the
crossing (see also Meyer et al. 2003). In the case of the
central Mediterranean, the flight strategy of Western
Marsh Harriers, moving hundreds of kilometers over
water in weak or no wind and using islands as stop-
over sites (Panuccio et al. 2002, Panuccio and Agos-
tini 2010), might be seen as a conservative strategy.
During autumn migration, large numbers of
Western Marsh Harriers are seen each season leav-
ing the coasts of Tuscany (central Italy) heading
southwest apparently en route to Corsica via the
islands of Elba and Pianosa (Fig. 1; Paesani and Po-
liti 2002, Vanni and Paesani 2007). In contrast,
Western Marsh Harriers leaving central Italy from
the Circeo Promontory (Fig. 1; Latium), south of
Tuscany, are expected to cross the sea en route to
Africa, flying about 500 km nonstop over water be-
cause they face the open sea when heading south-
west (Agostini et al. 2001, 2003, Panuccio and Agos-
tini 2010). If the simplified plotted flyway reported
by Strandberg et al. (2008) is correct, the male from
the Ukraine expended a large amount of energy
and maximized its risks relative to the long sea cross-
ing. However, based on the timing of the sea pas-
sage, it appears that this bird might have used the
Corsica-Sardinia route to Tunisia.
In the western Mediterranean, birds leaving
northeastern Spain apparently passed via the Balea-
ric Islands. These birds likely make ,300 km of
nonstop, powered flight over water during the sec-
ond stage of that crossing, between the Balearic Is-
lands and northern Africa.
SPRING MIGRATION
Migration Route. There are fewer data on spring
migration than on autumn migration. However, avail-
able visual observation data indicate that Western
Marsh Harriers cross the Mediterranean on a broad
front during spring migration (Gensbol 1992, Handri-
nos and Akriotis 1997, Gustin and Pizzari 1998, Zalles
and Bildstein 2000, Agostini 2001, Ferguson-Lees and
Christie 2001, Panuccio et al. 2002, Pandolfi and So-
net 2003), as they do during autumn migration.
Migration of Different Sex and Age Classes. Stud-
ies in the central Mediterranean (Gustin and Pizzari
1998, Pandolfi and Sonet 2003, Panuccio et al.