Many species of land migratory birds migrate very long distances, the most common pattern being for birds to breed in the temperate or arctic northern hemisphere and winter in warmer regions, often in the tropics or the temperate zones of the southern hemisphere.
There is a strong genetic component to migration in terms of timing and route, but this may be modified by environmental influences. An interesting example where a change of migration route has occurred because of such a geographical barrier is the trend for some Blackcaps in central Europe to migrate west and winter in Britain rather than cross the Alps. Theoretical analyses, summarised by Alerstam (2001), show that detours that increase flight distance by up to 20% will often be adaptive on aerodynamic grounds – a bird that loads itself with food in order to cross a long barrier flies less efficiently. However some species show circuitous migratory routes that reflect historical range expansions and are far from optimal in ecological terms. An example is the migration of continental populations of Swainson’s Thrush, which fly far east across North America before turning south via Florida to reach northern South America; this route is believed to be the consequence of a range expansion that occurred about 10,000 years ago. Detours may also be caused by differential wind conditions, predation risk, or other factors.
The advantage of the migration strategy is that, in the long days of the northern summer, breeding birds have more hours to feed their young on often abundant food supplies, particularly insects. As the days shorten in autumn and food supplies become scarce, the birds can return to warmer regions where the length of the day varies less and there is an all year round food supply. Most of the passerine migrants fly by night in small flocks. During dusk prior to migration, they show a restlessness which is termed zugunruhe. They may also sing at night during this period of pre-migration restlessness.
The downside of migration is the hazards of the journey, especially when difficult habitats such as deserts and oceans must be crossed, and weather conditions may be adverse.
The risks of predation are also high. The Eleonora’s Falcon which breeds on Mediterranean islands has a very late breeding season, timed so that autumn passerine migrants can be hunted to feed its young.
Whether a particular species migrates depends on a number of factors. The climate of the breeding area is important, and few species can cope with the harsh winters of inland Canada or northern Eurasia. Thus the Blackbird Turdus merula is migratory in Scandinavia, but not in the milder climate of southern Europe.
The nature of the staple food is also important. Most specialist insect eaters are long-distance migrants, and have little choice but to head south in winter.
Sometimes the factors are finely balanced. The Whinchat Saxicola rubetra of Europe and the Siberian Stonechat Saxicola maura of Asia are a long-distance migrants wintering in the tropics, whereas their close relative, the European Stonechat Saxicola rubicola is a resident bird in most of its range, and moves only short distances from the colder north and east.
Certain areas, because of their location, have become famous as watchpoints for migrating birds. Examples are the Point Pelee National Park in Canada, and Spurn in England. Drift migration of birds blown off course by the wind can result in “falls” of large numbers of migrants at coastal sites.
Another cause of birds occurring outside their normal ranges is the “spring overshoot” in which birds returning to their breeding areas overshoot and end up further north than intended.
A mechanism which can lead to great rarities turning up as vagrants thousands of kilometres out of range is reverse migration, where the genetic programming of young birds fails to work properly.
Recent research suggests that long-distance passerine migrants are of South American and African, rather than northern hemisphere, evolutionary origins. They are effectively southern species coming north to breed rather than northern species going south to winter.
Broad-winged long distance migrants
Some large broad-winged birds rely on thermal columns of rising hot air to enable them to soar. These include many birds of prey such as vultures, eagles and buzzards, but also storks.
Migratory species in these groups have great difficulty crossing large bodies of water, since thermals can only form over land, and these birds cannot maintain active flight for long distances.
The Mediterranean and other seas therefore present a major obstacle to soaring birds, which are forced to cross at the narrowest points. This means that massive numbers of large raptors and storks pass through areas such as Gibraltar, Falsterbo and the Bosphorus at migration times. Commoner species, such as the Honey Buzzard, can be counted in hundreds of thousands in autumn.
Other barriers, such as mountain ranges, can also cause funnelling, particularly of large diurnal migrants.
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