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Bird migration

2007 Schools Wikipedia Selection. Related subjects: Birds

   Flock of Barnacle Geese during autumn migration
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   Flock of Barnacle Geese during autumn migration

   Many species of birds undertake seasonal journeys of various lengths, a
   phenomenon known as Bird migration. The different strategies followed
   by bird groups are detailed below.

Long-distance land bird migration

   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 spend the non-breeding season 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.

Short-distance land bird migration

   The long-distance migrants in the previous section are effectively
   genetically programmed to respond to changing lengths of days. However
   many species move shorter distances, but may do so only in response to
   harsh weather conditions.

   Thus mountain and moorland breeders, such as Wallcreeper and
   White-throated Dipper, may move only altitudinally to escape the cold
   higher ground. Other species such as Merlin and Skylark will move
   further to the coast or to a more southerly region.

   Species like the Chaffinch are not migratory in Britain, but will move
   south or to Ireland in very cold weather. Interestingly, in
   Scandinavia, the female of this species migrates, but not the male,
   giving rise to the specific name coelebs, a bachelor.

   Short-distance passerine migrants have two evolutionary origins. Those
   which have long-distance migrants in the same family, such as the
   Chiffchaff, are species of southern hemisphere origins which have
   progressively shortened their return migration so that they stay in the
   northern hemisphere.

   Those species which have no long-distance migratory relatives, such as
   the waxwings, are effectively moving in response to winter weather,
   rather than enhanced breeding opportunities.

Wildfowl and waders

   The typical image of migration is of northern landbirds such as
   swallows and birds of prey making long flights to the tropics. Many
   northern-breeding ducks, geese and swans are also long-distance
   migrants, but need only to move from their arctic breeding grounds far
   enough south to escape frozen waters.

   This means that most wildfowl remain in the Northern hemisphere, but in
   milder countries. For example, the Pink-footed Goose migrates from
   Iceland to Britain and neighbouring countries. Usually wintering
   grounds are traditional and learned by the young when they migrate with
   their parents.

   Some ducks, such as the Garganey, do move completely or partially into
   the tropics.

   A similar situation occurs with waders (called "shorebirds" in North
   America). Many species, such as Dunlin and Western Sandpiper, undertake
   long movements from their arctic breeding grounds to warmer locations
   in the same hemisphere, but others such as Semipalmated Sandpiper
   travel huge distances to the tropics.

   Most of the wildfowl are large and powerful, and even the waders are
   strong fliers. This means that birds wintering in temperate regions
   have the capacity to make further shorter movements in the event of
   particularly inclement weather.

   The same considerations about barriers and detours that apply to
   long-distance land-bird migration apply to water birds, but in reverse:
   a large area of land without bodies of water that offer feeding sites
   is a barrier to a water bird. Open sea may also be a barrier to a bird
   that feeds in coastal waters. Detours avoiding such barriers are
   observed: for example, Brent Geese migrating from the Taymyr Peninsula
   to the Wadden Sea travel via the White Sea coast and the Baltic Sea
   rather than directly across the Arctic Ocean and northern Scandinavia.

   For some species of waders, migration success depends on the
   availability of certain key food resources at stopover points along the
   migration route. This gives the migrants an opportunity to "refuel" for
   the next leg of the voyage. Some examples of important stopover
   locations are the Bay of Fundy and Delaware Bay.

   Some Alaskan Bar-tailed Godwits have the longest non-stop flight of any
   migrant, flying 11,000 km to their New Zealand non-breeding areas (BTO
   News 258: 3, 2005). Prior to migration, 55% of their bodyweight is
   stored fat to fuel this uninterrupted journey.

Seabirds

   Arctic Terns
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   Arctic Terns

   Much of what has been said in the previous section applies to many
   seabirds. Some, such as the Black Guillemot and some gulls, are quite
   sedentary; others, such as most of the terns and auks breeding in the
   temperate northern hemisphere, move south varying distances in winter.
   The Arctic Tern has the longest-distance migration of any bird, and
   sees more daylight than any other, moving from its arctic breeding
   grounds to the antarctic non-breeding areas. One Arctic Tern, ringed
   (banded) as a chick on the Farne Islands off the British east coast,
   reached Melbourne, Australia in just three months from fledging, a sea
   journey of over 22,000 km (14,000 miles). Seabirds, of course, have the
   advantage that they can feed on migration.

   The most pelagic species, mainly in the 'tubenose' order
   Procellariiformes, are great wanderers, and the albatrosses of the
   southern oceans may circle the globe as they ride the "roaring forties"
   outside the breeding season. The tubenoses in general spread thinly
   over large areas of open ocean, but congregate when food becomes
   available. Many of them are also among the longest-distance migrants;
   Sooty Shearwaters nesting on the Falkland Islands migrate 14,000 km
   (9,000 miles) between the breeding colony and the North Atlantic Ocean
   off Norway, and some Manx Shearwaters do the same journey in reverse.
   As they are long-lived birds, they may cover enormous distances during
   their lives; one record-breaking Manx Shearwater is calculated to have
   flown 8 million km (5 million miles) during its over-50 year lifespan.

   Pelagic birding trips attract petrels and other procellarids by tipping
   "chum", a mixture of fish oil and offal, into the sea. Within minutes,
   a previously apparently empty ocean is full of petrels, fulmars and
   shearwaters attracted by the food.

   A few seabirds, such as Wilson's Petrel and Great Shearwater, breed in
   the southern hemisphere and migrate north in the southern winter.

The tropics

   In the tropics there is little variation in the length of day
   throughout the year, and it is always warm enough for an adequate food
   supply. Apart from the seasonal movements of northern hemisphere
   wintering species, most species are in the broadest sense resident.
   However many species undergo movements of varying distances depending
   on the rainfall.

   Many tropical regions have wet and dry seasons, the monsoons of India
   being perhaps the best known example. An example of a bird whose
   distribution is rain associated is the Woodland Kingfisher of west
   Africa.

   There are a few species, notably cuckoos, which are genuine
   long-distance migrants within the tropics. An example is the Lesser
   Cuckoo, which breeds in India and spends the non-breeding season in
   Africa.

   In the high mountains, such as the Himalayas and the Andes, there are
   also seasonal altitudinal movements in many species.

Australasia

   Bird migration is primarily, but not entirely, a Northern-Hemisphere
   phenomenon. In the Southern Hemisphere, seasonal migration tends to be
   much less marked. There are several reasons for this.

   First, the largely uninterrupted expanses of land mass or ocean tend
   not to funnel migrations into narrow and obvious pathways, making them
   less obvious to the human observer. Second, at least for terrestrial
   birds, climatic regions tend to fade into one another over a long
   distance rather than be entirely separate: this means that rather than
   make long trips over unsuitable habitat to reach particular
   destinations, migrant species can usually travel at a relaxed pace,
   feeding as they go. Short of banding studies it is often not obvious
   that the birds seen in any particular locality as the seasons change
   are in fact different members of the same species passing through,
   gradually working their way north or south.

   Relatively few Australasian birds migrate in the way that so many
   European and North American species do. This is largely a matter of
   geography: the Australasian climate has seasonal extremes no less
   compelling than those of Europe; however, they are far less predictable
   and tend to take place over periods both shorter and longer. A couple
   of weeks of heavy rain in one part or another of the usually dry centre
   of Australia, for example, produces dramatic plant and invertebrate
   growth, attracting birds from all directions. This can happen at any
   time of year, summer or winter and, in any given area, may not happen
   again for a decade or more.

   Broader climatic extremes are highly unpredictable also: expected
   seasonal heat or rain arrives or does not arrive, depending on the
   vagaries of El Niño. It is commonplace to have stretches of five or ten
   years at a time when winter rains do not eventuate during the El Niño
   cycle, and equally common to have La Niña periods which turn arid zones
   into areas of lush grass and shallow lakes. Long distance migration
   requires a heavy investment in time and body mass—and, given the random
   nature of El Niño, an investment with an uncertain return.

   In broad terms, Australasian birds tend to be sedentary or nomadic,
   moving on whenever conditions become unfavourable to whichever area
   happens to be more suitable at the time.

   There are many exceptions, however. Some species make the long haul to
   breed in far distant northern climes every year, notably swifts, and a
   great many wading birds that breed in the Arctic Circle during the
   southern winter.

   Many others arrive for the southern spring and summer to breed, then
   fly to tropical northern Australia, New Guinea, or the islands of South
   East Asia for the Southern winter. Examples include cuckoos, the Satin
   Flycatcher, the Dollarbird, and the Rainbow Bee-eater.

   Others again are altitudinal migrants, moving to higher country during
   summer, returning to warmer areas in winter such as several robins, or
   travel north and south with the seasons but within a relatively
   restricted range. The tiny 10 cm Silvereye is an example: most of the
   southernmost Tasmanian race crosses the 200 miles of Bass Strait after
   breeding to disperse into Victoria, South Australia, New South Wales
   and even southern Queensland, replacing the normal residents who fly
   still further north, following the band of fertile country along the
   coast, feeding through the day and travelling mostly at night. The
   northernmost populations, however, are nomadic rather than migratory,
   as are the Silvereyes of southern Western Australia, which is bounded
   by thousands of miles of desert to the north and east, and sea to the
   south and west.

Study techniques

   Bird migration has been studied by a variety of techniques of which
   ringing is the oldest. Colour marking, use of radar, satellite tracking
   and stable hydrogen isotopes are some of the other techniques being
   used to study the migration of birds.

Migration conditioning

   It has been possible to teach a new migration route to a flock of
   birds, for example in re-introduction schmes. After a trial with Canada
   Geese, microlites were used in the US to teach safe migration routes to
   reintroduced Whooping Cranes .

   Retrieved from " http://en.wikipedia.org/wiki/Bird_migration"
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   with only minor checks and changes (see www.wikipedia.org for details
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