still capable of returning rapidly to nest sites. Such phenomenal powers are the product of
computing several sophisticated cues, including an inborn map of the night sky and the pull of
the earth’s magnetic field. How the birds use their ‘instruments’ remains unknown, but one
thing is clear: they see the world with a superior sensory perception to ours. Most small birds
migrate at night and take their direction from the position of the setting sun. However, as well
as seeing the sun go down, they also seem to see the plane of polarized light caused by it,
which calibrates their compass. Travelling at night provides other benefits. Daytime predators
are avoided and the danger of dehydration due to flying for long periods in warm, sunlit skies is
reduced. Furthermore, at night the air is generally cool and less turbulent and so conducive to
sustained, stable flight.
F. Nevertheless, all journeys involve considerable risk, and part of the skill in arriving safely is
setting off at the right time. This means accurate weather forecasting and utilizing favourable
winds. Birds are adept at both, and, in laboratory tests, some have been shown to detect the
minute difference in barometric pressure between the floor and ceiling of a room. Often birds
react to weather changes before there is any visible sign of them. Lapwings, which feed on
grassland, flee west from the Netherlands to the British Isles, France, and Spain at the onset of
a cold snap. When the ground surface freezes, the birds could starve. Yet they return to Holland
ahead of a thaw, their arrival linked to a pressure change presaging an improvement in the
weather.
G. In one instance a Welsh Manx shearwater carried to America and released was back in its
burrow on Skokholm Island, off the Pembrokeshire coast, one day before a letter announcing its
release! Conversely, each autumn a small number of North American birds are blown across the
Atlantic by fast-moving westerly tailwinds. Not only do they arrive safely in Europe, but, based
on ringing evidence, some make it back to North America the following spring, after probably
spending the winter with European migrants in sunny African climes.
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