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Differential migration in Western Marsh Harrier (Circus aeruginosus):

a ring recoveries analysis

Panuccio M.1,2, Mellone U.1,3, Muner L.1

1 MEDRAPTORS (Mediterranean Raptor Migration Network) via Mario Fioretti 18, 00152
Rome, Italy - www.raptormigration.org
2 University of Pavia,“DISTA – Dipartimento di Scienze della Terra e dell’Ambiente”,Via Ferrata 9,
27100 Pavia, Italy.
3 Estación Biológica Terra Natura, Instituto Universitario de Investigación
CIBIO, University of Alicante;Apdo. 99. E– 03080;Alicante (Spain).

Abstract: Among migrating birds different strategies evolved in response to biomechanical constraints and
motivational asymmetries between ages, sex and species. In this paper we analyze differential migration of
Western Marsh Harriers using ring recovery data of 321 individuals from the European population. The
results show that with the increasing of latitude and longitude of the breeding sites, also the covered distance
to reach the wintering areas increases, i.e. Western Marsh Harriers breeding in Northern and Eastern Europe
migrate farther than individuals belonging to the populations of Western and Southern Europe, with no effect
of age classes in respect to the latitude of wintering. The results are consistent with the hypothesis that this species
shows a leap-frog migration pattern. On the other hand, when taking into account only birds wintering south
of Sahara desert, juveniles winter further West than adults. We suggest that this longitudinal difference could
be related to the different ability of experienced individuals to compensate the wind drift of dominant easterly
winds during migration over the desert in autumn. Our study confirms that ringing recoveries, despite not
providing detailed data on migratory routes such as satellite tracking does, still allow to perform large scale
analyses on a larger sample of individuals.

Introduction population winter farthest south (Salomonsen
Among migrating birds, differential migration, a 1955, Smith et al. 2003).
phenomenon in which migratory patterns differ Despite many different cases of leap-frog migrants
between age and/or sex classes of a population are reported among American raptors (Bildstein
(Cristol et al. 1999), largely occurs. Variations 2006), only few cases are known in Europe:
could be related either to different timing or the Kestrel (Falco tinnunculus), the Northern
to average length of migration (Marques et al. Sparrowhawk (Accipiter nisus) and the Eurasian
2009). Among factors that give rise to differential Buzzard (Buteo buteo, Wallin et al. 1987, Ferguson-
migration social dominance and competition Lees and Christie 2001). The third case is the most
are of paramount importance (Alerstam and documented one; in particular individuals breeding
Hedenström 1998). Two mechanisms can in Finland and Siberia migrate leapfrogging over
trigger latitudinal segregation among migratory partially migratory and sedentary populations in
populations: the “chain migration”, where winter central and southern Europe as well as in Japan
quarters of different populations are situated in the (Alerstam 1990).
same latitudinal sequence as the breeding areas, The Western Marsh Harrier (Circus aeruginosus)
and “leap-frog migration” where winter ranges are is a good model species to investigate differential
in a reverse latitudinal arrangement compared with migration since the European breeding population
breeding areas, i.e. the most northerly breeding is widely distributed, also at different latitudes. As

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