Descending from migration

When landing from migration, birds do not always lose height steadily, like an aeroplane, but instead drop almost vertically from high altitude into habitat below, as described long ago on Heligoland Island by Gatke (1895). Similarly, on Cyprus small nocturnal migrants were seen to 'drop like stones from the sky at first light and immediately dive into cover' (Bourne 1959); while on the Louisiana coast in spring, migrant passerines were seen to dive nearly vertically down from more than 1 km, producing 'a whizzing sound as they pulled out of the dive just above the trees' (Gauthreaux 1972). Such 'fallout' occurs in other circumstances, as when migrating birds are confronted by a sudden headwind or downpour, and literally drop from the sky to seek refuge. Thousands of birds seem to come from nowhere, ladening trees and bushes or carpeting the ground, presenting an amazing spectacle to any onlooker. One well-known fallout site comprises 5 ha of trees at the small town of High Island, situated in the open coastal plain of Texas, which attracts hundreds of bird-watchers every spring. Geese have a particular form of flight, known as whiffling, through which they lose height rapidly when over their destination, and this behaviour is seen not only on migration but also as geese return to their winter roosts at night.

Apart from emergency landings, little attention has centred on the question of when is the optimal time of day or night for migrating birds to land. While dusk provides the latest time that a diurnal migrant is likely to land, and dawn for a strictly nocturnal migrant, both groups extend their migrations if they find themselves over water. However, radar studies have shown that many diurnal migrants stop in the middle of the day, and many nocturnal migrants stop well before the night is ended, showing that other factors influence when they settle. In both groups, the time of landing and the flight duration are highly variable, probably depending on the internal state of the bird with respect to energy reserves and tiredness, or the influence of other individuals, as well as on external factors, such as weather and habitat. Birds tend to land as they approach a coastline or other barrier late in the day, or late in the night, and generally land rather than risk flying in bad weather. As freshly arrived nocturnal migrants are found in suitable habitats at dawn, they must be able to recognise suitable habitats at night while flying overhead. Apart from the limitations imposed by overall energy stores, the need for water or sleep may also limit flight duration. What is uncertain is the extent to which an endogenous (internal) rhythm of flight and rest may influence the birds' behaviour.

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