You are here:
BBC >
Science & Nature >
Animals >
Birds >
Articles
 |
 |
The autumn migration gets under way. |
|
|
|
|
|
 |
|
|
 |
|
Bird migration is
the only mass animal movement that we can observe at first hand in the
British Isles. Thousands of individuals and dozens of species are the
players in a seasonal rhythm that was first noticed 5,000 years ago. Gone
are the days when people believed that swallows spent the winter hibernating
in mud at the bottom of reed-beds or that young barnacle geese hatched
from barnacles growing on the seashore: a myth that was not disproved
until 1891 when the first nest was found in Greenland.
|
|
|
|
|
The point of migration
is simple. Entire populations need to follow the seasonal ebb and flow
in the food supply. The onset of winter across the Northern Hemisphere
displaces millions of birds southwards. However, there is no panic. Routes
evolved since the world's climate changed in the wake of the last Ice
Age 10,000 years ago provide them with both a timetable and an itinerary
for twice-yearly journeys that, on average, span a third of the Earth's
circumference. Like a good detective story, we are still a little way
short of discovering quite how they manage to navigate accurately over
huge distances.
|
|
|
|
|
The
ubiquitous starling is hardly the first species that springs to mind when
epic flights are considered. Common as muck they may be but results from
an intensive ringing programme involving 11,000 individuals in the 1950s
produced some of the most important information ever gathered on bird
migration. A Dutch scientist named Perdeck captured and ringed many adults
and immature starlings during autumn in Holland and transported some to
Switzerland, where they were released. After several months, information
on recovery locations began to trickle in. Most of the immatures flew
south-west from Switzerland and finished up in southern France and northern
Spain. They had continued migrating in the same direction as other immatures
released in the Netherlands. In so doing, they overshot their intended
destination, which was southern Britain and northern France. Adults, on
the other hand, corrected their course by 90 degrees and arrived in winter
quarters remembered from the year before.
|
|
|
|
|
 This
proved that, by successfully completing its first migration, a bird acquires
a rudimentary map. Youngsters lacking the benefit of experience are programmed
to set off in a particular direction and to follow it for a genetically
determined length of time. The principle applies to many species, for
example ruffs that breed in wet meadows across northern Europe and Asia.
The bulk of the European population winters in Africa, south of the Sahara.
During autumn migration flocks of juveniles are commonplace in the British
Isles whereas adults are decidedly scarce. Like adult starlings, the adult
ruffs have learned the destination at each end of their migratory journey.
Having wandered west during their 'apprenticeship year' they stick thereafter
to a direct route through the Mediterranean to Africa.
|
|
|
|
|
Before juveniles migrate
in earnest, they disperse in random directions. Even for species that
are long-distance migrants, a period during which they wander about may
be beneficial in helping to familiarise them with the general area around
their home. However, some exploratory flights stretch youthful exuberance
to extremes. An arctic tern ringed as a chick on the Northumberland coast
in July was recovered a month later in Sweden. It had initially dispersed
north-east instead of heading for the southern oceans. If mistakes are
to be made, autumn is the time to make them. Migration at this season
is a more relaxed affair and birds tend to wait for optimum weather conditions
before starting a long flight. After all, there is no need to find a breeding
territory and attract a mate.
|
|
|
|
|
Radar has shown that
the height of migration tends to be greater in spring than in autumn.
This fits in well with the general finding that birds heading for breeding
grounds are more purposeful and hurried. It may be that there is a connection
between the greater urgency and the greater height of spring migration,
in much the same way that aircraft climb to higher altitude when making
long hops.
|
|
|
|
|
Once
underway, birds travel by both day and night and along broad and narrow
fronts. In this general scheme different strategies are employed. Some
species can feed as they go and travel in daylight. Sand martins catch
insects over water and are mainly river valley migrants. In Britain they
concentrate along large river systems like the Trent and Severn and, not
surprisingly, once in Africa they follow the course of the Nile and avoid
inhospitable deserts. The route of other day-time migrants is influenced
by the nature of the land over which they fly. Coasting - the habit of
following a coastline - is a common practice with linnets and meadow pipits.
Escarpments also serve to direct and cluster birds along a narrow front.
Cols in the Alps and passes around the ends of the Pyrenees funnel thousands
of birds along flight paths only a few hundred metres wide.
|
|
|
|
|
After dark many more
birds fly quietly through the black skies above our heads. Most are small
insectivorous species like warblers and flycatchers. Before embarking
on a non-stop nocturnal flight they need fuel. To reach peak migratory
fitness they feed hungrily on insects and late summer crops of caterpillars
and berries, which they convert to subcutaneous body fat. Sedge warblers,
which are reputed to overfly the Sahara in one major hop to reach wintering
grounds near the equator, lay down so much fat that they effectively double
their weight. Are there advantages to migrating at night? Smaller birds
reduce the chances of being preyed upon but not significantly. Another
factor that does not stand up to close scrutiny is the argument that the
night sky offers better cues for navigating. Although stars are used for
orientation, the Sun, polarised light, the Earth's magnetic field and
other global positioning criteria now thought to be used by birds are
available during daytime.
|
|
|
|
|
Perhaps the best explanation
for nocturnal migration may be the birds' need to conserve water during
long-haul migratory flights. At night the air is cool and less body fluid
is lost through evaporation. Continuous flight generates a significant
amount of heat. Warblers with resting body temperatures of 38 degrees
centigrade clock temperatures of up to 44 degrees centigrade during powered
flight. Heading southwards through warmer climates militates against flying
in the heat of the day. Furthermore, not only is the night sky cooler
but also the air itself is normally more calm and stable. Most migrants
fly at between 15 and 30 miles-per-hour and desire a straight and level
course free of turbulence. We can be sure of one thing: they have analysed
how this planet works and made themselves masters of survival.
|
|
|
|
|
 |
|
|
|
 |
|
