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Differential migration in Western Marsh Harrier

reproduction in average some weeks later than in north of Sahara, probably reflects a weaker drift
western and southern Europe (Ferguson-Lee and effect in Europe, as previously reported by data
Christie 2001, Génsbøl 2004). collected with satellite telemetry (Klaassen et al.
Considering the length of the migratory journey 2010a, 2010b). Many different research made in
in relation to the age class, our results show that central and eastern Mediterranean area through
there aren’t differences among adults and juveniles. visual observations showed a stronger tendency
In this regard, a previous survey made in Italy of adult male Western Marsh Harriers to migrate
showed that the proportions of wintering adults over longer distances than adult females (Agostini
and juveniles were similar while in other areas and Logozzo 2000, Agostini et al. 2003, Panuccio
of Europe the proportion of wintering juveniles et al. 2005, Agostini and Panuccio 2010).
appears to be increasing in the last years (Panuccio Unfortunately from the EURING dataset it is not
et al. 2005, Castelijns and Castelinjns 2008, possible to evaluate if and how sex classes interact
Strandberg et al 2008). Moreover, no juvenile with the leap-frog migration pattern of Western
satellite tracked from Sweden has wintered south Marsh Harrier, and therefore further studies are
of the Sahara, although sample size (3) was too needed to make clear other aspects of different
small to draw any conclusion (Strandberg et al. migration strategies of this species.
2008). Acknowledgments: We are grateful to Chris du
Analyzing the distribution of individuals recovered Feu, EURING and to the Bird Ringing Centre of
in winter south of Sahara desert in relation to the Russia for the permission to use their data set.
age class, juveniles wintered significantly westerly
than adults, even if there are no differences in
this respect concerning the breeding site. Across
the Sahara desert in autumn easterly winds are
dominant and this largely affects the migration of
raptors, including the Western Marsh Harriers in
particular (Klaassen et al. 2010a, 2010b). In other
species of raptors it has been observed that adults
(experienced individuals) compensate the drift
of lateral winds during migration while juveniles
(inexperienced individuals) do not do it, being
drifted by crosswinds (Thorup et al. 2003). This
happens because adults have higher orientation
abilities allowing accurate compensation for wind
drift. On the opposite, juveniles are more inclined
to be drifted by lateral winds being at their first
migration and without any specific prior winter
site as final destination. We suggest that, differently
from adults, juvenile Western Marsh Harriers are
more likely to be drifted during their first flight
across the Sahara desert and, as a result, their
wintering sites are located western than in the case
of adults and their paths are differently oriented.
It remains unclear if and how survived juveniles
would shift their wintering grounds towards east
during the subsequent winters. The fact that we
haven’t recorded any difference in the direction
of migration of juveniles and adults wintering

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