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Across and around a barrier:
migration ecology of raptors in the Mediterranean basin
Introduction Spaar 1997). Finally Kerlinger (1989) suggests that
Migration is an event that involves a huge number raptors with higher aspect ratios (aspect ratio =
of birds moving from one habitat to another. (Wing Span)2 / Wing Area)) are better adapted to
Among birds breeding in the Paleartic area many powered flight than species showing a lower aspect
long-distance migrants are found. Some species ratio (relatively short vs. relatively long wings). In
travel thousands of kilometres twice a year. The this context it is readily understandable that large
repeated journeys between breeding and wintering raptors fly mostly over land and avoid flying over
areas involve a considerable effort associated with large bodies of water where they cannot use soaring-
high mortality risks. Since the selective forces acting gliding flight. Seas and large lakes act as ecological
on migrating birds are considerable, birds have barriers for most land birds since they do not offer
developed a range of adaptations and strategies to birds the possibility to land or feed, and because
cope with their journeys. “These include not only weather conditions are likely to change suddenly.
morphological features for efficient locomotion and For soaring-gliding birds the disproportionate
storage of energy but also behavioural adjustments” increase in energy consumption adds to the threats
(Åkesson and Hedenström 2007) in order to inherent to flying over water surfaces (Zu-Aretz
overcome barriers and adverse weather conditions. and Leshem 1983).
Since migration is only a fraction of the life of a The aim of this thesis is to investigate which factors
bird, morphological adaptations for migration affect the migration strategies of European raptors
should be integrated with other features allowing in relation to the ecological barrier represented by
the bird to exploit resources all year long, in order the Mediterranean Sea. Morphological, ecological,
to reproduce successfully. geographical and meteorological elements shape
Most birds move using powered flapping flight the paths of migrating raptors moving through
which also allows long non-stop flights and wide or around this water barrier. The study of bird
water crossings (Alerstam 1990, Klaassen et al. migration can help us to achieve a more solid basis
2011). By contrast, Accipitriformes raptors are that will allow us to protect and conserve these
species that move using largely passive flight. They species and their habitats as well as increase our
use soaring gliding flight in particular, exploiting knowledge of the dynamics of evolution.
thermal updraughts and ridge lifts; these kinds Study area
of currents occur on land and obviously not over The Central Mediterranean
water. This particular flight strategy is mostly due The shortest sea crossing available to raptors to reach
to the morphology of these species; the energy Europe from Africa and vice versa in this area is the
expenditure in powered flapping flight increases Channel of Sicily, between the Tunisian Peninsula
rapidly with the increase of body mass, while of Cap Bon and western Sicily (Fig. 1). Since even
soaring gliding flight is convenient since it only at its narrowest point the sea crossing still exceeds
requires the application of a small percentage of one hundred kilometres, broad-winged raptors
the basal metabolic rate (Pennycuick 1989, 2001, are rarely observed migrating in this area; the
2008). Moreover, heavier species glide faster than most common species are honey buzzards (Pernis
smaller ones, allowing large raptors to reach a high apivorus), marsh harriers (Circus aeruginosus), black
cross-country speed, comparable with the highest kites (Milvus migrans) and Montagu’s harriers
speed of flapping migrants (Hedenström 1993,
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