MIGRATION AND THE WIND
Griffon vulture (Gyps fulvus). Photo: Francisco J. Cazalla
The strong winds from the East (Levanter) and the West (Poniente) that
batter the coasts of the Strait are, without a doubt, very influential
on the behavior of migrants (Bernis 1980). Studies with radar have
demonstrated that the birds tend to fly high with a tail wind, whereas
they descend and fly low,close to the ground into a headwind to minimise resistance. In the case of the non soaring birds, their
movements are affected considerably by the winds,causing them to use
different coastlines depending on wind direction. This gives rise to
very visible movements along the Atlantic coast when the Levante blows
but along the Mediterranean coast with a Poiniente.
Something similar
happens with the soaring birds, whose migration are subject to identical
lateral displacements by the direction of the winds. But other factors
also come into play here. The birds have to calculate accurately the
place and the moment at which they cross the Strait, after their flight
down in pursuit of favourable thermals. Given the narrow profile of the
Iberian and African coasts of the Strait, a strong cross wind can blow
them away from the coast making for a much more ardous flight over the
water. It even can fatally push them out over the open sea, as can
happen with those birds in the Autumn migration that get displaced
beyond Cabo Espartel by the Levante, where the African coast stands out
abruptly towards the South.It's hardly surprising,therefore,that there
can be complex movements of the soaring birds in order that they can
find the place and moment most appropriate to initiate this dangerous crossing. For that reason, sometimes it is difficult to state whether a
group of birds that passes overhead at one of the observatories has
opted or not to cross the Straits at that time.
