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Differential migration in Western Marsh Harrier
Figure 2 – The map shows ring (summer) and recovery (winter) sites of adult and
juvenile Western Marsh Harriers breeding in Europe.
are not mutually exclusive and could integrate arriving is associated to higher fitness (Myers
with each other explaining leapfrog migration 1981, Alerstam 1990). Concerning this topic,
of Western Marsh Harrier. Its life-history traits it has been observed among Western Marsh
clearly indicates that this species is strictly linked Harriers breeding in Sweden that delayed arrivals
to wetlands that are mostly frozen during winter lead to a lower breeding success and this is also
in north, central and eastern Europe and therefore linked to the difficulties in crossing the ecological
feeding opportunities drastically decrease. barriers such as the Sahara desert (Strandberg et
Individuals belonging to western and southern al. 2009). Therefore there are several strategies
Europe could remain close to their breeding sites that a bird could use to reduce the time needed
since wetlands do not freeze, especially on coastal to perform the migratory journey (i.e. the use of
areas (like western France), they probably take more direct routes, limiting the use of stop-over
advantage of milder temperatures in respect to sites, extending the travelling time also within the
other areas located further East (inland) at the night). Among them reducing the length of the
same latitude. As a consequence Western Marsh migratory journey: wintering as closer as possible
Harriers from northern and eastern Europe to the breeding sites or, vice versa, breeding as
should be forced to migrate farther to avoid closer as possible to their wintering grounds.
competition with resident (or shorter-distance Finally, it is important to consider the different
migrants) birds in western and southern Europe. starting dates of reproduction; in fact individuals
Moreover in many species of migrating birds early belonging to northerly breeding range start the
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