Page 109 - Panuccio2012
P. 109
Southbound direction of colonization of Short-toed Snake Eagle
al. 2002a, b, Panuccio et al. 2011). Since birds Rappole 2005). Rappole & Jones (2002) provide
breeding in Italy cross the sea at the Strait of evidences that most of actual migrants descend
Gibraltar, in spring they reach the Country about from tropical residents species, in particular for
1000 km NW of southern regions of the Peninsula the Paleartic-African migration system: 42 of
via southern France. As suggested by a previous 185 species of migrants (23%) have conspecific
research (Agostini and Mellone 2008), the actual populations that breed in the Tropics while 139
distribution in Italy probably reflects the particular (75%) have Tropical-breeding congeners. Among
migration path used by this population and agree snake eagles (Circaetus spp.) only the Short-toed
with the hypothesis that Short-toed Snake Eagles Snake Eagle is a complete migrant species while
are still colonizing the country moving from north the other five species (Beaudouin’s Snake Eagle,
to south of the peninsula mostly along its western C. beaudouini, Black-chested Snake Eagle, C.
slope. pectoralis, Brown Snake Eagle, C. cinereus, East
These considerations lead us to further questions: African Snake Eagle, C. fasciolatus, Banded Snake
from where the Short-toed Eagle is colonizing Eagle, C. cinerascens) live in tropical Africa and
the Italian peninsula? Which is the original range are mainly resident (Ferguson-Lees and Christie
distribution of Short-toed Snake Eagle? These are 2001). However it is hard to generalize a model
topics linked to the origin of the Paleartic-African like the southern-home theory since birds show a
migration system and of migration itself. Among continuous adaptation of their migratory behavior
hypotheses concerning the origin of migrations, according to the change of resource availability
authors mostly agree that birds started to migrate (Bruderer & Salewski 2008). In fact the current
from tropical to temperate areas (Alerstam 1990, migratory system evolved starting about 15000
Safriel 1995, Rappole 1995, Rappole and Jones years ago at the end of the last glaciation and is still
2002, Berthold 2001, Böhning-Gaese & Oberrath evolving while the consequence of this process is
2003, Jahn et al. 2004). In this picture migration not predictable (Berthold 2001). Rappole & Jones
is an adaptation of tropical birds to use seasonal (2002) proposed a mechanism for the evolution of
abundant resources in temperate regions in order long distance migration, via gradual colonization
to optimize breeding success or to avoid seasonal at the edge of the breeding range furthest from
resource depression (Alerstam 1990, Alerstam et the wintering area, with populations on the edges
al 2003, Rappole 1995, Rappole & Jones 2002). more exposed to extinction risk. In this picture,
Alerstam and Enckell (1979) indicate competition we suggest that the Italian population of the
in Africa savannas during the breeding season as Short-toed Snake Eagles should be considered
the prerequisite for the evolution of the Paleartic- as part of a metapopulation comprising those
African migration system. In any case glaciations of Western Europe (France, Spain). Moreover,
were events of paramount importance in the within the Italian population, small and isolated
evolution of migrations. During glacial ages the populations of southern Italy could be considered
earth surface available for birds species drastically as small patches of this metapopulation system cut
decreased (Moreau 1972). In particular since the off from the bulk of the population of Western
northern polar front shifted south, temperate Europe. It is interesting to note that several studies
forests and bush vegetation reduced into small clearly show that isolation could lead small and
shelter, moreover the range extension of tundra, periphery patches to local extinction, although
deserts and steppes reduced the Mediterranean such evidence is not always found (Lomolino et
and the tropical vegetation to a minimum al. 1989, Peltonen & Hanski 1991, Hanski et al.
(Hooghiemstra et al. 2006, Bruderer & Salewski 1995, Whitcomb et al. 1996, Smith & Gilpin
2008). During interglacial periods (like the one 1997, Thomas & Hanski 1997, Hanski 1998,
we are living into) original species split in the so- Dunham & Rieman 1999, Clinchy et al. 2002,
called twins species, distinctly sedentary, partially Wahlberg et al. 2002). In agreement with this
migrants and completely migrants (Berthold 2001, conclusion, historical data confirm that the species
109
