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Long-distance land bird migration
Many species of land 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 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, summarized 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
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 Eleanora'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
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 specialists
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 species 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
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 therefore presents 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 the 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, like the Wallcreeper and
White-throated Dipper may move only altitudinally to escape the
cold higher ground. Other species like the 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. Interesting, in
Scandinavia, the female of this species migrates, but not the
male, giving rise to the specific name coelebs, a batchelor.
Short distance passerine migrants have two evolutionary origins.
Those which have long distance migrants in the same family, like
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 relative,
like the waxwings, are effectively moving in response to winter
weather, rather than enhanced breeding opportunities.
Wildfowl and wader migration
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
like 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 site 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
Some Alaskan Bar-tailed Godwits are reputed to have the longest
non-stop flight of any migrant, flying 6,800 miles to their
Pacific island wintering grounds. prior to migration, 55% of
their bodyweight is stored fat to fuel this uninterupted journey.
Arctic TernsMuch of what has been said in the previous section
applies to many seabirds. Some, like 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
wintering 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
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, like Wilson's Petrel, and Great Shearwater are
amongst the few species that breed in the southern hemisphere and
migrate north in the southern winter.
Migration of birds in 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 winters in Africa.
In the high mountains, such as the Himalayas and the Andes, there
are of course also altitudinal movements of greater or lesser
extent by many species
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