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Migration ecology of raptors in the Mediterranean
and Libya rather than across the Channel of Sicily This concurs with a recent study made by satellite
(paper VII). tracking revealing that migrating raptors tend to
compensate the effect of lateral winds close to a
Migration and weather conditions natural barrier, in all probability to avoid the risk of
Weather conditions strongly affect bird migration; being blown off over the sea or desert (Klaassen et
this is easily understandable. For instance a strong al. 2010). A previous study investigating the effect
head wind makes a flight more difficult since of crosswinds on raptors of different ages during
birds move in a medium that is itself in motion, migration showed that juveniles are more likely to
namely air. The direction and speed travel of a be drifted by winds while adults compensate the
flying bird is the sum of the wind vector and the drift effect (Thorup et al. 2003). However, recent
bird’s own flight vector relative to the surrounding visual observations at some raptor migration
air (Alerstam 1990). This means that, to cover watchsites seem to suggest that juveniles are able
the same distance, a bird requires more energy to compensate the drift effect when migrating
in a strong headwind than when wind is absent close to landmarks or leading lines. For instance,
(Pennycuick 2008). Another easily understandable counts made on the island of Antikythira did
example is provided by heavy rain. Most birds are not show significant differences in numbers of
compelled to land during such weather conditions. juvenile honey buzzards recorded during lateral
However every case has its exceptions and I was winds and other wind directions (headwinds and
surprised to observe hundreds of steppe buzzards tailwinds; paper IV). Moreover, when focusing on
(Buteo buteo vulpinus) migrating under heavy the effect of prevailing winds during peak passages
rainfall in Georgia in October 2011. In this at the site (headwinds and tailwinds), juveniles
dissertation I present and discuss my observations were observed mostly with headwinds, while on
on the migratory behaviour of raptors relative to days with strong tailwind components very few
weather conditions in some Mediterranean areas. raptors were recorded (Fig. 7). This result is likely
Among the findings of these studies I would like to have been caused by the fact that juvenile honey
to underline that both adult honey buzzards and buzzards let themselves be carried away during
short-toed snake eagles are able to compensate tailwind, travelling toward Crete and bypassing the
the effects of crosswinds to avoid a drift effect island altogether. Tailwinds ofPfEeRrAPtIh.Ae.mminin fact a less
(papers IXP,ERXAIP)I.Aa.mnadPxEiRnAPbI.Ao.mthaxcases I recorded this energetically costly fPlEigRAhPtI.oA.vmeinr the sea.
behaviour when raptors migrate close to the coast. Another paper investigates the influence of wind
0 0
0 0
2
2
1 1 1 90
1 1 1
1 1
1 90
90 1
90
2
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Figure 7 - Prevailing wind directions during peak days (left) and days with no passage (right) of
juvenile honey buzzards over the island of Antikythira, autumn 2009.
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