Encyclopedia
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 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 , 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 . 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 wintering grounds . Prior to migration, 55% of their bodyweight is stored fat to fuel this uninterrupted journey.
Seabirds
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 wintering areas. One Arctic Tern,
ringed 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 . 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 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 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 winters 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. Color 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 .
See also
...
- Flyway
- Mississippi Flyway
- Categories and
References
- Alerstam, T. . Detours in bird migration. Journal of Theoretical Biology, 209, 319-331.
- Berthold, Peter Bird Migration: A General Survey. Second Edition. Oxford University Press. ISBN 0-19-850787-9
- Weidensaul, Scott. Living On the Wind: Across the Hemisphere With Migratory Birds. Douglas & McIntyre, 1999.
- Dingle, Hugh. Migration: The Biology of Life on The Move. Oxford Univ. Press, 1996.
External links
- - includes several articles on bird migration
- - a review from the Science Creative Quarterly
- - satellite tagging of light-bellied brent geese
