CONCENTRATIONS OF MIGRANTS IN THE STRAIT
Flock of white storks (Ciconia ciconia) on migration
The birds that migrate across the Strait of Gibraltar come, in the most, from areas found in western Europe. Throughout that latitudinal
gradient, which reaches as far north as Scandinavia, different
populations exhibit different migratory behaviour. Some end the trip
along the European Atlantic coastline , a more hospitable environmental
than the interior of the continent and the more northerly areas. The
majority however ,continue south. Some stay in the warmer zones of the
Iberian Peninsula, and the Maghreb. Others, fly to the trans-Saharan
parts of Africa. The Strait of Gibraltar occupies a strategic position
in all these movements. Terrestrial birds, that tend to avoid long sea
crossings, make towards the Iberian Peninsula by routes along the coasts
of Europe, a continent looking like a terrestrial wedge orientated
towards Africa. The Iberian peninsula becomes a great springboard for
the journey towards the Maghreb. In fact, many nocturnal migrants , that
fly at high altitude are able to cross on a broader front over the sea,
on their way to Africa, from the Western half of Iberia (Bernis 1962,
Hilgerloh 1988). For them, the Strait seems to be of little relevance .
But the diurnal migrants, not so keen to navigate over the the sea, are
obliged to use the Iberian coasts before deciding to cross towards North
Africa. From this perspective, the northern side of the Strait of
Gibraltar is the end of the great Iberian funnel in which many of the
migrants congregrate, where the Atlantic and Mediterranean coasts meet.
The endless stream of hirundines, larks, wagtails and finches that pass
along the beaches of La Atunara and Los Lances on autumn mornings is the
result of the amazing phenomenon (something similar, but in the reverse,
happens on the southern side of the Strait during the Spring). The Strait
of Gibraltar plays a much more important role in the migration of
soaring birds during both autumn and spring. The Storks, Kites ,
Vultures, and Honey Buzzards, take advantage of the thermals, the hot
air that rises from the ground heated by the sun. They climb to high
altitudes from which they glide towards their destination. By this
method, they are able to cross great distances with minimum expenditure
of energy. In fact, many of these birds usually are not able to hold
long periods of flapping flight. As the thermal currents are formed on
the land (the water of the sea absorbs the radiation and it does not
warm up the air), they cannot leave the coastline. This concentrates
them in the Strait from which, having ascended with the hot air thermals
, they can pass over to the other side (Bernis 1980).The Strait of
Gibraltar is, along with the Strait of Messina and the Bosphorus, the
route of hundreds of thousands of soaring birds moving into Africa
(Bernis 1980, Finlayson 1992). This area is of enormous strategic
importance in the field of conservation for this group of species and
gives a great opportunity to people to see the impressive spectacle of
migration. From any observatory in the Strait it is possible to observe
thousands of individuals of these birds - which make up the majority
-together with a smaller numbers of other birds , like the hawks, falcons
and harriers, which are not limited so much in their movements as are
the more specialized species of soaring birds. Something similar happens
with marine birds (gulls, shearwaters, gannets, scoters…) and also with
other migratory animals (turtles, cetaceans, tuna…), that have to use
the Strait when moving between the Atlantic and the Mediterranean.
