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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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