At Idyllic we try to bring to the attention of browsers interesting Wild Life happenings in Southern Africa

Frequent flyers of the animal world
 

The Witness, 29 Apr 2008
By Roddy Smith

March-April is a particularly interesting period for bird-watching because it is a time of transition as the seasonal migrants are gearing up to head north. There are periods of days or weeks when huge flocks of birds congregate — thousands of Barn Swallows swooping over the river or perching on the banks, shifting clouds of storks wheeling high up in the sky or standing in and around pans catching frogs and fish, and many others. Some of these bird species have been here all summer, while others have spent the season further south and are passing through or making a “refuelling stop”. All these birds are building up their body fat and energy reserves in preparation for their epic journeys, and then one day they are gone.

The term migration is generally applied to a regular seasonal movement using set routes and going from and to the same areas, as opposed to irregular movements of populations following food availability, such as the irruptions of thousands of queleas, or escaping unusually severe climatic conditions.

The phenomenon of migration is one of the great marvels of ornithology and, although thanks to bird ringing and other scientific experiments much more is known about it now than formerly, there are many aspects which are still not fully understood. Why do birds migrate at all? Why some and not others? In general it is about food supply. Many migrants are birds which live by catching flying insects, which tend to be in short supply in winter-time, so the birds head for the opposite hemisphere where they are plentiful; for example, the insect-eating kingfishers are all migrants while the fish-eating kingfishers are year-round residents. Most waders which live on insects in mud also migrate. It is a mystery, however, why some species of Bee-eaters migrate but others don’t, and why even some members of the same species (such as European Bee-eaters) migrate while others are resident in South Africa all year round.

Migrants are classified in two groups. Intra-African migrants travel back and forth within the continent taking advantage of the abundant food available during rainy seasons, and many of these breed in Southern Africa.

Palaearctic migrants are intercontinental travellers. Most of these breed in the far north of Europe and Asia, where the long summer days offer increased feeding opportunities, and then move south to avoid the extreme winter conditions. Some of these birds cover huge distances; the Arctic Tern’s return trip can be up to 50 000 kilometres each year.

This long-distance flight uses huge amounts of energy, so different types of bird travel in different ways, tending to use the method which is most energy-efficient for them. Big birds such as storks and raptors soar on thermal currents and travel at great heights. There are no thermals at night or over water, so they only travel by day and their routes from Europe or Asia to Africa pass over Gibraltar or Israel to eliminate long sea-crossings.

At the height of the migration season there are millions of birds in the sky over Israel, an animal movement that dwarfs the Serengeti and which presents certain problems in an airspace which also contains the world’s busiest fighter aircraft traffic.

Other large to medium-sized birds fly in V-shaped or similar formations which allow them to slipstream each other, saving as much as 20% in energy. Small birds which cannot soar use flapping flight at lower levels and frequently fly at night when it is cooler and the air denser.
Many species have rest areas where they stop to feed and build up energy, especially the soarers who can only travel in the heat of the day. Others, however, stop only briefly or not at all: Bar-tailed Godwits fly non-stop for 11 600 kilometres across the Pacific Ocean from Alaska to New Zealand in six days. These “endurance flyers” have to feed intensively before starting their journey in order to store energy in the form of body fat; many increase their body weight by 50% or more in the month before departure.

Migrants don’t necessarily return along the same route by which they came. The Red-backed Shrike for instance makes an anti-clockwise loop from Europe to southern Africa and back, possibly to take advantage of prevailing winds. Carmine Bee-eaters complicate things with a three-stage migration: they arrive at their breeding areas in Zambia, Zimbabwe and Botswana in August-September and after breeding head further south for the summer before returning to equatorial Africa in April. Some of them then simply say the hell with it and stay on for the whole year. Many frequent fliers will find this attitude understandable, especially as there are no upgrades awarded for bird migration miles.

• Roddy Smith works as a guide and conservationist in the lower Zambezi valley.

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