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Differential migration in Western Marsh Harrier
Table 1 – Results of the GLM investigating the relationship between the
distance covered to reach wintering grounds (dependent variable) and
coordinates of breeding sites and age class (adults vs. juveniles).
Explanatory Term F P (>F )
Latitude of the breeding site 11.45 <0.01
Longitude the breeding sites 13.23 <0.01
Age class 0.05 >0.05
Figure 1 – Graph showing relationship between consequence northerly populations start later their
the length of the migratory journey in relation to migration in autumn finding the closer wintering
the latitude of the breeding site. areas already occupied. For this reason they are
Discussion forced to move further. 2) “Competition and the
Our results indicate that northerly and easterly predictability in spring”: the suitability of an area
populations of Western Marsh Harriers move diminishes with the increase of bird density, as a
further away than southerly and westerly ones. result of intra-specific competition. Individuals
Starting from 46° of breeding latitude (Fig.1), move showing different densities within different
some individuals start to show very short distance areas, but birds breeding at the southerly
migration or even residency. This migratory pattern territories, differently from the northern ones,
could be largely explained by leap-frog migration have the advantage to winter as close as possible to
as shown also by other examples in literature their breeding area reacting quickly as the spring
(Alerstam 1990, Boland 1990, Marques et al. weather becomes favorable. This occurs because
2009, Kylin et al. 2011). Alerstam (1990) gave earliest breeding pairs of birds show highest fitness
two possible explanations of leap-frog migration: (Alerstam 1990). On the other hand individuals
1) “Competition and the timing of migration”: belonging to northern populations cannot arrive
birds breeding in the southernmost part of the too early at the breeding sites, at least not before
species range start and finish reproduction earlier that their feeding source is available. These birds
than birds breeding in the northerly area. As a wintering far away from the breeding sites, cannot
react immediately to detect the upcoming spring
as southerly breeders do. Other hypotheses were
based on optimal time allocation rather than
effects of competition. Greenberg (1980) showed
that northerly breeders, which have to spend the
longest annual period away from the breeding
range, may minimize their total mortality during
the long non-breeding period by migrating to the
farthest south wintering grounds. On the other
hand Bell (1996, 1997) predicted that northerly
breeders with a late spring migration could use the
surge of spring food at southerly latitudes before
starting the migration. All these assumptions
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