By now, most
our visiting summer birds will have left us and headed for sunnier
pastures. Replacing them, as winter draws near are millions of birds
joining us from their breeding grounds in Northern Europe and the
Arctic. These tortuous journeys are made worthwhile by the advantages
of summertime habitat with an abundant food source plus similar
advantages on wintering grounds.
migration is a topic that has fascinated mankind for centuries and
it is only relatively recently that we have begun to understand
more about migration and in particular, migration routes and wintering
grounds. Although a lot remains to be discovered we've come along
way since thinking that Swallow's winter at the bottom of lakes
and Barnacle Geese (left) summer
in shellfish of the same name.
Birds put a
lot of effort into preparing for their journeys, as timing is critical.
Leave too early and you may not have the reserves to last the journey,
but leave it too late and you may encounter harsh weather and too
strong an opposing wind. Navigation is possibly the most important
factor of a successful journey. Birds use three main compasses to
ensure they fly in the right direction for the correct length of
time; the sun, the stars and the earth's magnetic field. Which they
use often depends on the time of day the bird is travelling, another
factor they have to consider.
Birds use a
huge variety of techniques when migrating, often dependent on the
terrain they are crossing or the distances they are travelling.
The chicks of many seabird species have not yet learnt to fly when
they fledge, so, accompanied by their moulting parents, swim from
their breeding grounds to their chosen wintering quarters. A lot
of songbirds double their body weight in autumn enabling them to
migrate long distances without having to keep stopping to refuel.
The Sedge Warbler, for example, averages an additional fat load
equal to 100 per cent of its normal weight, with which it can probably
cover more than 3,000 km in one non-stop flight of perhaps three
or four days. Large birds such as Storks, Cranes and Ibis's have
to rely on hot wind thermals to carry them, as they cannot rely
on flapping their wings as that would not be energy efficient. Nor
can they land on water so this collection of birds have to take
a migration route that avoids large bodies of water and seek out
the kinds of landforms that generate rising thermal currents. This
often results in large numbers of birds being channelled through
an area relatively small, which fulfils their requirements. On such
place is the straight of Gibraltar, which provides adequate thermals
and the shorting crossing distance across the Mediterranean Sea.
migrations undertaken by birds truly are awe-inspiring. Perhaps
the most renowned migrator in the bird kingdom is the Arctic Tern
(right). These birds breed
in the Northern Hemisphere, from temperate latitudes to the most
northerly land in the world, then winter on the edge of the Antarctic
pack ice. The complete round trip for a single Arctic Tern may be
as much as 40,000 km, an astonishing feat. Another feature of the
Arctic Terns phenomenal travels is their record of experiencing
more hours of daylight than any other living thing. Birds breeding
in the north of the arctic circle experience the 24 daylight that
occurs there in the summer months, and then, by travelling south
of the Antarctic circle, find the same conditions during the southern
The Quail and
Corncrake are lost distance migrants, something which can come as
a surprise considering how reluctant they are to break cover during
the breeding season. However these skulking birds cross large expanses
of sea and desert during their twice-yearly migration. Both species
favour the south west of Africa to winter, and most British breeding
birds would appear to cross the Mediterranean via Gibraltar.
Red Backed Shrikes
that nest in Western Europe take a lengthy route to their wintering
grounds in the southernmost parts of South Africa. All breeding
birds head over the eastern Mediterranean and down through the savannah
areas of the Great Rift Valley. One advantage the shrikes have over
most other migrant birds is that they are able to prey on their
fellow migrants, so they always have a food supply.
A lot of birds
that don't breed in the United Kingdom still use the country as
a stop over during migration. Many wading birds exhibit this behaviour.
Birds such as Knot, Curlew Sandpiper and Little Stint that breed
in the high Arctic and winter in South Africa stop over in Britain
en route to take advantage of the rich food source to be found in
our estuaries and marshes. These birds are not taking the obvious
route to their African wintering grounds so it really must be worth
their wile migrating via Britain to re-fuel.
Many birds less
characteristic of Britain regularly over shoot their usual breeding
grounds and appear in spring and autumn. Birds such as Wrynecks,
Hoopoes and Bee-Eaters typically appear at these times of year.
They are often blown west off their usual migration course, particularly
in spring and on occasions have stayed and bred.
phenomenon of migration truly is a wonder, how birds manage to navigate
and locate themselves, often in the case of young birds with no
prior knowledge, and how they can time their journey to take advantage
of good weather conditions as their bodies reach an optimum condition.