Tuesday, October 5, 2010

Travellers under the cover of darkness

Sunset over the Atlantic in south-western Iberia at this time of the year signals the start of a wave of small migratory birds that rise from the vegetation into the darkness above. They take a south-westerly heading which takes them out over the open ocean towards Africa. They reach land once again over Morocco. Some may land but others press on south towards the western Sahara Desert. Flying along its western margin minimizes an otherwise dangerous and tedious journey over a sea of sand dunes. This means that, even though many migrants overfly the Sahara, there is a real concentration of birds along the west, coming down the coast of Portugal and  making the most of the wetter Atlantic coast of Iberia. Predators have learnt that the coasts of south-western Iberia and Atlantic Morocco are a source of migrants at this time of year and they make the most of the bonanza. Eleonora's Falcons breed late in the year, on rocky islands, and feed almost exclusively on migrants. Less known but far more dramatic is the predation on migrants by noctule bats. These mammals fly high into the night sky and track down the migrants that are flying at night precisely to avoid falcons and other diurnal predators!

Willow Warblers are among the commonest of these migrants, many yellowish juveniles among them. These tiny birds were only born a few weeks ago and instinctively find their way to their winter quarters. When they come down during the daylight hours, they make the most of the time to feed and regain lost energy.

Some of these migrants, among them Pied Flycatchers, may remain in an area for a few days before moving on. They are fiercely territorial and may stick to a small clump of trees where they will spend the day pouncing on insects on the ground or flycatching others in mid-air.
Pied Flycatchers track a south-westerly route across Europe to reach northern and central Portugal where they spend time refattening. From here, many make a direct flight south to cross the Sahara Desert. They rely on fat reserves deposited in their bodies. These get broken down during the journey to produce energy and water.

Greenland Wheatears are among the greatest of these travellers. These birds make a long sea crossing from Greenland to the British isles, via Iceland. From there they travel south towards the Atlantic coast of Iberia and then move south into Africa from here. They move south later than their smaller European cousins and October is their month in Iberia.

Diurnal trans-Saharan migrants are also on the move down the western coast of Iberia and many of these make the most of the swarms of grasshoppers, locusts and other large insects on the way. The last of the White Storks continue to make their way towards the Strait of Gibraltar. It is the Lesser Kestrels that are now flocking in the open fields, as at la Janda, where they are actively hunting ahead of their journey to winter quarters in West Africa.

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