Bird migration
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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, 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.
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 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 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 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 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 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 (BTO News 258: 3, 2005). Prior to migration, 55% of their bodyweight is stored fat to fuel this uninterupted journey.
Seabird migration
Much 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 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, 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 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.
Migration in 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, Australasian birds tend to be sedantry 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 like 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.
See also
References
- Alerstam, T. (2001). Detours in bird migration. Journal of Theoretical Biology, 209, 319-331.da:Fugletræk
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