Facultative movements are also shown by species that obtain their food from water or the ground, and which must therefore move in response to freezing temperatures, snow or drought. Such escape (or weather) movements occur as soon as waters freeze or snow covers the ground, cutting off food supplies. The dates of such movements therefore vary between years, depending on the weather, and in mild winters need not occur at all. Even in the arctic, some waterfowl and gulls remain in autumn to feed in remaining patches of open water until long after most other migrants have left. They are forced out as the last open water areas freeze in early winter, and must usually migrate south over habitats that have been ice- or snow-covered for some weeks.
Hard weather movements are often extremely obvious. Whenever cold weather strikes, thousands of birds can be counted as they stream past particular observation points, as large regions are evacuated within a matter of hours. Counts made at such times in Britain include the 20 000 Eurasian Skylarks Alauda arvensis that passed westward over the Axe Estuary in Devon on 28 December 1964, or the 8300 counted in 2 hours as they passed south over a site in northeast Scotland on
24 January 1976; the 4500 Northern Lapwings Vanellus vanellus that passed southwest over Tring in 50 minutes on 9 December 1967, and the 10 200 that passed in 2.5 hours over Portsmouth, Hampshire on 30 January 1972 (from Wernham et al. 2002). Over much of the world, ducks usually travel at night, so are seldom seen on migration, but their weather movements are marked by sudden massive overnight increases in the numbers on particular wetlands, following the onset of hard weather back along the migration route.
Many hard weather movers usually return soon after conditions improve again, sometimes less than a week later. One probable reason for their return over several hundred kilometres is the avoidance of competition for food, which is likely to be more intense in the overcrowded hard weather refuges than in the areas previously left. It is not that birds are necessarily driven back by competition, but this could be the ultimate factor involved. The proximate stimulus may be a favourable change in temperature or wind direction, for it would be advantageous for the birds to leave before they were weakened by food shortage. In Europe, winds from the south and west bring warmer temperatures and suitable tailwinds. It is almost as though such birds shuttle back and forth along part of their migration route, on average getting further from their breeding areas as winter advances. It seems that some facultative migrants have some sense of where they ought to be - that is, as near to the breeding areas as conditions allow - and can migrate effectively in either direction during the course of a winter.
Weather movements were found by radar to occur almost every day and night in November-February between Britain and continental Europe (Lack 1963). The Northern Lapwing Vanellus vanellus and Common Starling Sturnus vulgaris were the most frequent participants, but many other species were involved too, including finches, thrushes, larks, plovers, grebes and other waterfowl (Elkins 1988, Evans & Davidson 1990, Ridgill & Fox 1990). All these birds are partial migrants which leave in winter more or less in the same direction as they would normally migrate. Lapwings fleeing from hard winters may reach Spain, where they are known as avefria ('birds of the cold'). In the rare years when the cold weather extends to the usual hard-weather refuge areas (such as southwest Ireland), enormous mortality may occur among the huge numbers of migrants concentrated there (Clarke 1912). Needless to say, hard weather movements are much more pronounced in severe winters than in mild ones, and at mid-latitudes they have become less frequent in recent decades as winters have become milder.
Some wetland species perform the equivalent of hard weather movements in summer, often to higher latitudes, in their attempts to escape drought. Such movements have most often been recorded among various herons and ducks (in which they merge with moult migrations). In addition, at least one non-wetland species in Europe performs extensive weather movements in summer. The Common Swift Apus apus feeds on small high-flying insects which are available only in fine weather. As found by radar studies, these birds escape cold rainstorms by flying into the wind ahead of the rain then clockwise round the depression, and returning behind it (Lack 1956). In the process, they can travel up to 2000 km. It is mainly the non-breeders that participate in these movements, but also some breeders, forming flocks of up to 50 000 birds. Young Common Swifts can survive without food for periods of a week or more by becoming torpid, lowering their body temperature and metabolic rate, an adaptation lacking in most other birds.
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