Page 35 - Panuccio2012

Circannual variation movements of Black Kite
vary across the Mediterranean basin (Strait of
Gibraltar vs. other flyways). This may be due to
their highly plastic behaviour, which is affected by
different factors. Among them, different density
at the breeding sites could play an important
role. Black Kites migrating through the Strait of
Gibraltar belong to populations of western Europe,
where they have been estimated to be roughly 30-
40000 pairs (Birdlife International 2004, Bensusan
et al. 2007). If higher competition for resources
occurs in western Europe rather than in Italy or
eastern Europe, where the population density is
lower (Ferguson-Lees and Christie 2001), birds
belonging to the populations breeding in Spain,
Portugal and France could be forced to reach their
nesting sites as soon as possible in spring. This
could explain the evident peak passage of this
species at the Strait of Gibraltar at the beginning of
March and why the migration occurs later in the
central Mediterranean and around the Black Sea
(Kirwan et al. 2008, Panuccio and Agostini 2010).
Several studies, focused on population dynamics,
provide further support to this conclusion; in
particular some authors showed that younger
Black Kites delay territoriality as a voluntary
restraint in order to enhance longevity and the
quality of the territory eventually acquired (Sergio
et al. 2009). This hypothesis has been developed in
a context of high density of breeding pairs. Sergio
et al (2009) showed that age alone could not be the
sole predictor of territorial status because a wide
overlap in the age structure of floaters and holders
occurs. The other important predictor is the spring
arrival date, with early arriving birds having higher
likelihood to acquire a territory (Sergio et al. 2007,
2009). Moreover, high individual philopatry in a
population would result in a surplus of individuals
in relation to vacant places, and would retard the
first breeding attempt until birds are between 3
and 5 yr old (Forero et al. 2002). In this picture
the delayed spring migration in the central
Mediterranean and in Turkey could be a result of
low density and therefore less competition in the
breeding areas of those populations. Black Kites
breeding in areas with a low density of pairs could be
less motivated to arrive as early as possible from the
wintering ground. This hypothesis could explain
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the late migration peak observed in early May
at the Strait of Messina and in Turkey (Agostini
and Duchi 1994, Panuccio and Agostini 2010,
Kirwan et al. 2008). Moreover it is reasonable that
in areas with low density of territorial individuals,
less competition could allow a higher percentage
of younger individuals to hold and defend a
territory and to attempt reproduction, since it
has been demonstrated that young individuals are
physiologically ready for reproduction (Blas and
Hiraldo 2010).
It is not possible to exclude, however, a relationship
between timing of migration and the distance
between breeding and wintering grounds. In the
case of the tracked kite breeding in Germany and
wintering in western Africa, the migratory distance
was about 4600 km (Meyburg and Meyburg
2009), while it is likely that Black Kites wintering
in Israel are closer to their breeding grounds.
Further investigations should provide information
on the wintering grounds of Black kites belonging
to the population of eastern Europe and Russia.
Loop-migration strategy?
Another evident difference concerning spring
and autumn movements occurs in the central
Mediterranean area. In this area Black Kites
show a narrower front, associated with a higher
concentration in time, during autumn rather
than spring migration. This behaviour is unusual:
a stronger concentration in time should be
associated with a time-minimization strategy, and
thus with a stronger tendency to cross the sea in
order to reach the breeding grounds as soon as
possible (see Agostini & Mellone 2007). Birds are
expected to migrate more quickly during spring,
in a scenario of intra-specific competition during
the breeding season (Kokko 1999). In agreement
with research made by visual observations that
we reviewed, also the information provided by
satellite tracking showed that a Black Kite breeding
in Germany covered the distance during autumn
migration more rapidly (2007: 234 km/day, 2008:
256 km/day) than during spring movements
(215 km/day in spring 2008 and 191 km/day
in spring 2009; Meyburg and Meyburg 2009).
Agostini (2002, 2003) speculated on the different