According to Peter Berthold, of the Max-Planck Institute for Ornithology, bird migration refers mainly to the annual movement from breeding grounds to resting areas and back again. Bird migration is fascinating because of the complexity of the behavioural patterns and the distances traversed. Some birds accomplish vast migratory voyages to locate familiar territory with apparent ease. The search for complete understanding of the reasons behind bird migration in all its forms is an ongoing one.
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Breeding habits tend to be unchanging, with large numbers of birds moving en masse from one place to another on a regular basis. According to The British Ornithologists’ Union, as of January 2013, there were 596 bird species recorded in Britain, about half of which are migrant breeders. Favourite breeding grounds are well established in the bird kingdom, often being near to places where the parent birds were hatched. Choughs, for example, which pair for life, use the same nest sites year after year, located on Ramsey Island, and other parts of Pembrokeshire and southwest Wales.
The main reason for bird migration is likely to be for feeding purposes, according to Berthold. For house martins and swallows, an annual autumn flight to Africa in search of insects for food, then back to Europe in spring, is routine. Autumn sees the arrival in England of redwings and fieldfares from Scandinavia, which come to eat the yew berries of trees in Kingley Vale National Nature Reserve. Brent geese from Siberia find their way to the salt marshes of Essex and elsewhere, where they feast on vegetation, especially eel-grass, according to the RSPB.
According to natural history writer Jonathan Elphick, climate and changes in weather patterns drive migratory cycles. Changing climate patterns may gradually influence the migratory behaviour of whole species, while local weather conditions affect the decisions of individual birds as to when it is auspicious to depart. As Arctic weather deteriorates, snow geese fly a distance of about 4,000 km, to the Gulf of Mexico and beyond. Wind direction, cloud cover, temperature and precipitation levels all play a part in triggering migratory behaviour.
Differential migration refers to short and medium-distance journeys where members of the same species exhibit different migratory patterns. Male ruffs winter in Europe, whereas females fly on to Africa. These more complex behaviours have different causes which are often age or gender dependent and may be related to a need to reduce competition within a species. Some species, such as waterfowl, take part in moult migration to allow the shedding and regrowth of feathers to take place. Dispersal of part of a flock to new territory, especially where numbers have grown rapidly, is also sometimes a reason for bird migration.
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