Page 28 - Complete Birds of Britain and Europe (DK - RSPB)

INTRODUCTION

MIGRATION

orthern Europe is full of insect food in summer,
Nwhen long days allow birds time to feed both
themselves and their young. Such resources are too
good to miss, but few birds can survive in the far north
in winter when the days become short and cold. Only
by huge movements of millions of birds can they exploit
feeding opportunities to the full.The regular rhythms of A SIGN OF THE SEASONS
migration are an essential part of many species’ existence. Migrating geese make a marvellous
spectacle and sound. In parts of northern
Europe they are a visible sign of the Canadian
INSTINCTIVE MOVE changing seasons as they migrate south Arctic islands
Birds migrate by instinct. In many species adults and in autumn and north in spring.
young birds migrate separately, but the youngsters are
somehow programmed to move at the right time and PUTTING ON WEIGHT
to follow the right routes. Some gather into large Small warblers such as the Sedge
flocks and migrate together. Others simply slip Warbler double their weight before
they migrate. Sedge Warblers eat
away one night alone, embarking on a journey aphids in reedbeds before flying
that remains one of nature’s most magical and across the Sahara in one flight that
dramatic undertakings. Before proceeding with may last four days. Others feast on North America
such a journey a bird needs to ensure that it is rich, sugary berries before they migrate.
in tiptop condition – this often requires Such birds quickly put on layers of
some careful preparation. fat: essential fuel for their journeys.
WHEN TO DEPART
Changing day length in spring and
autumn is more of a clue that the time
is right to migrate than changes in
temperature. A bird’s internal clock
takes note of the seasonal changes
and stimulates a restlessness at
migration time. It also starts off the
hormonal changes that make physical
adjustments, such as the accumulation
of extra fat for long-distance journeys.
OVERLAND FLIGHTS
Broad-winged birds such as White
MAKING THEIR WAY Storks use up too much energy in
flapping flight so must glide over
Birds navigate in a number of ways. long distances. To do this, they must
They certainly use the sun and the stars be able to gain height, which they
and may also see polarized light, which do by riding thermals or “bubbles”
allows them to judge where the sun is of warm, rising air. These only form
even on cloudy days.They probably have over land, so the birds must cross
a magnetic sense too, and may even be the sea at the narrowest points,
able to detect the position of the poles as such as Gibraltar and Istanbul.
they refer to the sky, which would indicate
their latitude.What we still don’t
understand is how they know
which way they should fly.
READY TO GO
Swallows and House
NIGHT MIGRANTS Martins migrate by day,
Many songbirds, such as Goldcrests, feeding on insects as they
migrate at night, relying on the stars to go. They gather in large
find their way. A sudden onset of cloud and fog flocks in autumn, before
may “ground” thousands of them along a coast, making a move together
making exciting birdwatching the next morning. towards Africa.

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