The Diversity Of Migration

Migration occurs to some degree in most bird species that live in seasonal environments, from arctic tundras to tropical savannahs and grasslands. It is in strongly seasonal environments that food supplies vary most markedly through the year, fluctuating between abundance and scarcity in each 12-month period. Generally speaking, birds time their migrations so as to be present during the periods of abundance and absent during the periods of scarcity. Only in the relatively stable conditions of tropical lowland rainforest, where food supplies remain fairly constant year-round, do the majority of bird species that breed there remain all year, but even these forest areas receive a seasonal influx of wintering migrants from higher latitudes. Worldwide, in response to seasonal changes in food supplies, more than 50 billion birds are thought to migrate every year on return journeys between breeding and non-breeding areas (Berthold 1993).

Because almost all migratory birds travel to milder climes for the non-breeding period, they move mainly on a north-south axis. However, many populations also have an easterly or westerly component in their movements, especially those that breed in the central parts of the northern landmasses and move to the warmer edges for the winter. Thus, the predominant autumn migration direction of intra-continental migrants in western Europe is southwestward, but the further east they breed within Europe, the stronger the westerly component in their autumn journeys. Western Europe is warmer in winter than equivalent latitudes anywhere else on the Eurasian landmass, so acts as a major wintering area for Eurasian migrants, including up to two million waterfowl. Nevertheless, some birds from eastern Europe move southeast to winter in the Middle East, East Africa or India. Similarly, in much of North America, many birds move southeastward in autumn, towards the warm southeastern States, or onward to the Caribbean Islands or South America (most of which lies in longitudes east of North America).

Some bird species move almost directly east-west on their migrations. For example, the Pochards Aythya ferina which breed in Siberia move up to 4000 km in autumn to winter in western Europe, in the process crossing up to 80° of longitude (M. Kershaw, in Wernham et al. 2002). Many species in southern Africa move from the arid west in summer to the wetter east in winter (Brooke 1994). Many seabirds, shorebirds and waterfowl of high latitudes fly east or west in spring along the northern edge of the continents before moving inland to nest on the open tundra to the south. In the autumn, they retrace their journeys along the northern coastline, until they reach the continental edges when they veer southwards towards their wintering areas (Alerstam & Gudmundsson 1999).

The Bald Eagles Haliaeetus leucocephalus that breed in southern North America show an unexpected pattern. The young are raised in winter or early spring, then move generally northwards for up to 2200 km, spending from May to September in Canada and Alaska, where they feed largely on salmon which fill the rivers at that time (Broley 1947). The young eagles therefore travel north in spring and south in autumn with the conventional migrants but, unlike them, they have been reared in the south before doing so. Many adult eagles also leave the south in spring, but it is not clear from ringing whether they travel as far as the juveniles. This migration was first established from ringing nestling eagles in Florida (Broley 1947), but more recently it has been confirmed in radio-tracked young from California (Hunt et al. 1992), and in colour-marked young from Texas (Mabie et al. 1994). This last study also showed that young returned to their natal areas to breed.

The young of several species of herons, raised in winter in the southern USA, disperse in various directions but mainly northward, again presumably to exploit the presence of fish in shallow water in the northern spring and avoid the effects of drought in more southern areas (Lincoln 1935a). Similar but less marked summer movements have been recorded among herons in Europe, and in the southern hemisphere some heron species in Australia also migrate to higher latitudes after breeding (Maddock 2000), as do flamingos in South America (Sick 1968b). In addition, the non-breeders of some seabird species, including the Little Auk Alle alle and several skua species, spread up to several hundred kilometres beyond their natal colonies in summer, exploiting the summer flush of food at higher latitudes, and some winter-breeding petrels and shearwaters also move to higher latitudes after breeding (see later).

Difficult journeys

Bird migrations may vary from a few tens to many thousands of kilometres, but it is the long and difficult journeys that best reveal the capabilities of migratory birds. Among landbirds, spectacularly long journeys are made by those species that fly regularly between northern Eurasia and southern Africa or Australasia,

Figure 1.1 Some long-distance migrations of birds. 1. Alaskan population of Pacific Golden Plover Pluvialis dominica; 2. Arctic Tern Sterna paradisaea; 3. Swainson's Hawk Buteo swainsoni; 4. Snow Goose Chen caerulescens;

5. Many North American breeding species that cross the Gulf of Mexico;

6. Ruff Philomachus pugnax; 7. Many European breeding species that cross the Mediterranean Sea and Sahara Desert; 8. Northern Wheatear Oenanthe oenanthe; 9. Amur Falcon Falco amurensis; 10. Arctic Warbler Phylloscopus borealis; 11. Short-tailed Shearwater Puffinus tenuirostris. Partly after Berthold (1993).

Figure 1.1 Some long-distance migrations of birds. 1. Alaskan population of Pacific Golden Plover Pluvialis dominica; 2. Arctic Tern Sterna paradisaea; 3. Swainson's Hawk Buteo swainsoni; 4. Snow Goose Chen caerulescens;

5. Many North American breeding species that cross the Gulf of Mexico;

6. Ruff Philomachus pugnax; 7. Many European breeding species that cross the Mediterranean Sea and Sahara Desert; 8. Northern Wheatear Oenanthe oenanthe; 9. Amur Falcon Falco amurensis; 10. Arctic Warbler Phylloscopus borealis; 11. Short-tailed Shearwater Puffinus tenuirostris. Partly after Berthold (1993).

or between northern North America and southern South America or Australasia (Figure 1.1). Such long movements are performed each year by many shorebirds, and some seabirds, passerines and others. Even on the shortest routes, this entails some individuals flying more than 25 000 km on return migration each year. Some of the participants are small enough to be held comfortably in the palm of your hand. The major advantage in migrating so far between the northern and southern hemispheres derives from the fact that the seasons are reversed. The species involved thus pass both breeding and non-breeding seasons in summer conditions when food is plentiful, although no such birds are known to breed regularly at both ends of their migration route (Chapter 13).

Most birds that migrate overland have plenty of places to stop and feed. They can therefore migrate, rest and feed almost every day, accomplishing their journeys by a series of short flights. Other birds cross mainly hostile areas, where they cannot stop and feed. They therefore have to accumulate larger body reserves, and make long flights between widely spaced stopping places (Chapter 5). For example, shorebirds typically complete their migrations in 2-4 long stages, refueling before each stage, and often travelling 1000-4000 km between suitable estuaries, even when mainly following coastlines. The flights themselves comprise long periods of muscular work without food or water, at great heights over inhospitable terrain, and usually require pinpoint navigation to widely separated refuelling areas. Flight paths and stopping sites of some shorebirds have been worked out in some detail from synchronised counts at different estuaries, from ring recoveries, and in some regions also from radar observations and studies of body weights.

To elaborate with one example, those Bar-tailed Godwits Limosa lapponica that winter in West Africa north of the equator have to face a journey of 10 000 km to their breeding grounds in Siberia (Piersma 1994a). Taking off from GuineaBissau at dusk, a bird could reach the next major mud flat, the Banc d'Arguin in Mauritania 1000 km away, by mid-day. From there, another 16 hours and 1000 km of flight would get the bird to the next suitable estuaries in Morocco and another 16 hours and 1000 km to the estuaries of the Loire and Gironde in western France, and yet another 10 hours and 600 km of flight to the Wadden Sea which fringes the northern Netherlands, Germany and Denmark. After a long period of refuelling on the Wadden Sea coast, most godwits seem to make the rest of their journey to Siberia in a single flight of 4000 km. In fact, most godwits also seem to fly from Banc d'Arguin to the Wadden Sea in one flight, as the Moroccan and French estuaries are used only by a small proportion of the population. Travelling at 60 km per hour (without the benefit of a tailwind), the entire journey translates to 167 hours of airtime, equivalent to a solid seven days and nights of flight, excluding breaks for refuelling (Piersma 1994a).

Some shorebird species that breed across the arctic show an astonishing array of migration routes. In the Ruddy Turnstone Arenaria interpres, for example, Alaskan birds migrate down the entire western seaboard of the Americas to winter as far south as Chile, while most of the Canadian birds head for the coasts of the Caribbean and beyond. The Greenland and eastern Canadian birds move to Britain and Ireland, and Scandinavian ones to West Africa. The central Siberian birds move south to the Middle East, the shores of the Indian Ocean and on to southern Africa, while the east Siberian/west Alaskan birds winter in southeast Australasia and Pacific Islands. Except at high ice-bound latitudes, few rocky shorelines anywhere in the world do not support wintering Ruddy Turnstones from one part of the breeding range or another.

Landbirds that migrate over oceans provide some of the most extreme examples of endurance flight and precise navigation. They travel without opportunity to feed, drink or rest, over vast stretches of open water devoid of helpful landmarks. They cannot stop, as birds do overland, when the weather turns against them. Yet millions of landbirds regularly cross the Mediterranean Sea and Gulf of Mexico at their widest points (about 1200 km), and smaller numbers regularly cross longer stretches, such as the western Atlantic between northeastern North America and northeastern South America (2400-3700 km), or the northern Pacific between Alaska and Hawaii and other central Pacific Islands (5000 km). However, the most impressive of all overwater migrations by a landbird is undertaken by the Bar-tailed Godwits Limosa lapponica from eastern Siberia and Alaska, which in autumn apparently accomplish an astonishing 175-hour non-stop 10400 km flight to New Zealand (Chapter 6). Apart from the length of the journey, imagine the navigational precision required. From the departure point in Siberia or Alaska, the target area of New Zealand subtends an angle of only 5°, extending over a relatively tiny part of the southern Pacific. To judge from their normal flight speeds, landbirds would take more than 100 hours of non-stop flight in still air to accomplish the longer of their overwater journeys, but by taking advantage of favourable winds, they can shorten their flight times, sometimes by as much as one half. Participants include many passerines and shorebirds, but also waterfowl which, unlike the others, can rest on the sea if need be.

Some overland journeys are also difficult. Long desert crossings are made by the many species (including passerines) that travel between Eurasia and tropical Africa. Most west European species cross at least 1500 km of the Sahara Desert immediately after crossing the Mediterranean Sea, an overwater journey of up to 1200 km. In autumn some species may make this Mediterranean-Saharan flight without a break, a total journey of 1500-2500 km, depending on the route taken (Chapter 6). Other birds from further east cross the central Asian deserts, and then another 1700 km of southern Arabia and its bordering gulfs, before reaching East Africa. In Australia, some waders cross the central desert in moving between southern and northern coasts, a journey of more than 2000 km.

Yet other birds cross high mountain ranges, including the Himalayas and Tibetan plateau. One such species is the Bar-headed Goose Anser indicus which in the process can rise to more than 8 km above sea level, where the air is thin and very cold (Chapter 6). Other species cross extensive areas of pack-ice that lie in spring between Siberia and Alaska or between Norway and Svalbard. A few species cross 2000 km of the 2-km-high Greenland ice cap on journeys between northeastern Canada and western Europe. No landbirds regularly cross the Southern Ocean to Antarctica (which holds only seabirds), and none is known to cross the North Pole, even though the tundras on either side are only 2000-3000 km apart (Gudmundsson & Alerstam 1998). There would probably be no advantage in trans-polar movements, which in any case might also present navigational problems (Chapter 9).

The various migrations mentioned above are among the longest and most impressive undertaken by landbirds. Most long journeys involve movement from one hemisphere to another, but this is not true of all. Brent Geese Branta berni-cla are restricted to northern latitudes, yet some of their breeding and wintering areas can be as much as 5000 km apart, involving substantial east-west shifts, as well as north-south ones. The journey from northeast Canada to Ireland involves these geese crossing polar seas, the Greenland ice cap and the North Atlantic.

At the other end of the spectrum, the shortest seasonal migrations by landbirds are undertaken by some altitudinal migrants, which move only a few kilometres from mountains to valleys for the winter, and by various gallinaceous birds which typically move over short distances (up to several tens of kilometres) between their breeding and wintering sites (Chapter 17). Such migrations often occur in various directions and intergrade with purely local movements.

Seabird movements are just as varied as landbird movements. Their study is hampered by the obvious improbability of getting ring recoveries from pelagic regions, but observations from ships and the satellite-based tracking of radio-marked individuals have helped to fill out the picture (Chapter 2). Some species, as well as moving north-south, cross from one side of an ocean to another. In the Atlantic, Manx Shearwaters Puffinus puffinus move from western Europe to eastern South America after breeding, and Arctic Terns Sterna paradis-aea from eastern Canada cross to West Africa, before continuing southward to the Southern Ocean. In addition, Sooty Terns S. fuscata cross the Atlantic from breeding colonies in the Caribbean to non-breeding areas off West Africa, but in the process make little or no latitudinal shift.

Like landbirds, many seabirds perform exceptionally long migrations, which are perhaps less demanding than the transoceanic flights of landbirds. This is partly because most seabirds are larger and more robust than the majority of landbirds, but also because many can more readily rest on the sea surface or feed en route. In moving between the Arctic and Southern Oceans, the Arctic Tern Sterna paradisaea may perform the longest migration of any tern, entailing a round trip of 30 000-50 000 km each year (this species is not known to rest on the sea). The birds move down the western coasts of South America and Africa, and on reaching the Southern Ocean, travel eastwards on the winds, some passing south of Australia and New Zealand. Many juveniles may continue eastwards, circling the Antarctic, before heading north again on their return flight, two or more years later (Salomonsen 1967b). These movements have been revealed by observations and by some striking ring recoveries, showing the presence in winter of North American birds off South Africa and of European birds off South Africa and Australia. Because some Arctic Terns may reach an age of 25 years, they might cover more than a million kilometres on migration during their lifetimes, about three times the distance between the earth and the moon. Some Common Terns Sterna hirundo from northern Europe have also been found off Australia, but this species apparently does not extend south towards Antarctica.

Conversely, some seabirds breeding in the southern oceans spend their non-breeding season in the northern hemisphere. Examples include the Sooty Shearwater Puffinus griseus which migrates between the South and North Atlantic, and between the South and North Pacific. Seventeen individuals from breeding colonies in New Zealand were tracked on migration using miniature archival tags to record geographical position, dive depth and ambient temperature (Shaffer et al. 2006). The tags revealed that these birds flew across the Pacific in a figure-eight pattern while travelling an average of 64 037 ± 9779 km round-trip. They took 198 ± 17 days over the journey, and reached speeds up to 910 ± 186 km per day. Each shearwater made prolonged stops in one of three discrete productive regions - off Japan, Alaska or California - before returning to New Zealand through a relatively narrow corridor in the central Pacific. The birds obtained food from the surface or down to depths of 68 m. Similar figure-eight migrations had previously been inferred from ring recoveries for the Short-tailed Shearwater P. tenuirostris which breeds on islands off southeast Australia and 'winters' in the north Pacific (Figure 1.1).

Several Antarctic seabirds, such as immature Southern Giant Petrels Macronectes giganteus, perform circumpolar migrations, flying eastward around the world in the Southern Ocean. Radio-tracking results from albatrosses have revealed the extraordinary distances travelled by some species in short time periods. For example, a Northern Royal Albatross Diomedia epomophora was found to fly up to

1800 km in 24 hours, and a Grey-headed Albatross D. chrysostoma circled the globe in just 46 days (Croxall et al. 2005). Albatrosses can cover long distances on routine foraging flights from their nesting islands, as well as on migration. Thirteen young Wandering Albatrosses Diomedia exulans, radio-tracked by satellite from their natal areas on the Crozet Islands, flew eastward in the southern Indian Ocean, where they foraged back and forth along a 2000 km strip of ocean, just north of the subtropical front. In their first year of life, they covered an average of 184 000 km (range 127 000-267 000 km), corresponding to a distance of 4.6 times round the earth at its widest part (Akesson & Weimerskirch 2005).

Not all seabirds migrate to lower latitudes in winter. Some species that breed in winter migrate to higher latitudes after breeding, like the Bald Eagles mentioned earlier. In the northern hemisphere, they include the Black-vented Shearwater Puffinus opisthomelas and Brown Pelican Pelecanus occidentalis off western North America and Bonin Petrel Pterodroma hypoleuca of eastern Asia, and in the southern hemisphere they include the Kerguelen Petrel Lugensa brevirostris and the King Penguin Aptenodytes patagonicus. For those penguins nesting on the Crozet Islands, this involves swimming about 1600 km (over 8° of latitude) to exploit the rich food supplies available at the edge of the winter pack-ice (Bost et al. 2004). Many other seabird species disperse eastward or westward after breeding to spend the winter away from their nesting colonies, concentrating at upwell-ings and other areas of abundant food, but remaining within the same oceanic zones year-round. Examples include the Northern Fulmar Fulmarus glacialis and Kittiwake Rissa tridactyla in the northern hemisphere, and various albatrosses and petrels in the southern hemisphere. Those albatross species whose breeding cycle lasts more than a year have an off-year after each successful breeding attempt. This applies to Wandering Albatrosses Diomedia exulans nesting on the Crozet Islands in the southern Indian Ocean. The adults leave the foraging areas that they frequent while breeding and spend their sabbatical years in sea areas 1500-8500 km away, elsewhere in the southern Indian Ocean or in the southwest Pacific. The ocean areas they occupy then range from tropical-subtropical waters (females) to sub-Antarctic-Antarctic waters (males), and individuals probably visit the same areas in each sabbatical (Weimerskirch & Wilson 2000). These albatrosses do not therefore perform a seasonal north-south movement like other birds but, as different sectors of the population breed in different years, by moving far from their breeding areas the sabbatical birds avoid competing with others that are breeding that year and thus operating from the colony. In this sense, their movements are again food-related.

While many landbirds face the hazards of ocean crossings, some seabirds make long overland journeys. The birds are seldom seen because they fly too high for human vision, and the evidence for their overland movements is based on unusual events and ring recoveries. In both Eurasia and North America, part of the migrations of the marine Sabine's Gull Xema sabini, Long-tailed Skua Stercorarius longi-caudus, Arctic Skua S. parasiticus, Arctic Tern Sterna paradisaea and several other seabirds may take place overland to and from their tundra nesting areas. Arctic Terns Sterna paradisaea may even cross the central parts of the Eurasian landmass en route between the Indian Ocean and the Siberian tundra. Occasional ringed birds were found dead more than 1000 km inland from any sea-coast (Bourne & Casement 1996).

The above examples give some idea of the variety of migration patterns found among birds from around the world, and of some of the more spectacular journeys.

Migration routes

For reasons of habitat and weather, birds do not always take the most direct (great circle) routes between their breeding and wintering areas; nor do they necessarily take the same routes in autumn and spring (Chapter 22). Thus, sea-ducks often migrate long distances around coastlines rather than taking overland shortcuts between breeding and moulting or wintering areas. For example, most Eiders Somateria mollissima breeding on islands far up the St Lawrence River in eastern Canada fly a coastal route of 2250 km to reach a point on the coast of Maine scarcely 640 km distant from their nesting islands, a shortcut which is taken by only a minority of birds (Reed 1975). Such detours may offer several benefits to migrating birds, such as continuous suitable habitat in which they can stop and feed, or reduced risk from adverse weather or predators. Mainly because of wind conditions, some roundabout routes may also offer reduced energy costs, despite the longer journey (Chapter 3; Alerstam 2001). The tradeoff between time-saving or energy-saving direct routes on the one hand and risk-reducing detours on the other may have favoured different patterns in different species, according to their flight capabilities and refuelling needs, as well as the habitat and seasonal weather patterns encountered en route.

Most bird species set off on a broad front between their breeding and wintering areas, but during the journey they may be concentrated to some extent along mountain ranges, sea-coasts and other 'leading lines'. Such streaming is particularly marked in waterbirds, which often migrate between specific sites, following river valleys or other routes offering wetland areas where they can rest and feed. It is also marked in raptors and other soaring birds that favour routes where ther-mals and other updrafts develop, and minimize any necessary water crossings. Because these features depend on geography and topography, such species tend to follow the same narrow traditional routes year upon year.

In taking roundabout routes, many birds divide their journey into distinct stages, each with a different main orientation. For example, most of the European passerines that migrate southwest to Iberia must then turn south or southeast if they are to reach West African wintering areas. Similarly, those that migrate from Europe southeast to the Middle East must then turn southwest to reach East African wintering areas. Their journeys are thus undertaken as two distinct stages, with different headings, although neither stage is necessarily accomplished non-stop. Many Eurasian-Afrotropical migrants make their return northward flight somewhat to the east of their autumn southward flight, probably in response to prevailing wind or feeding conditions (Chapter 22).

The tendency to take different routes in autumn and spring is especially marked in those species, such as the American Golden Plover Pluvialis dominica, that migrate between northeastern North America and southeastern South America (Figure 1.2). In autumn, when winds are favourable, these birds take the shortest route over the Atlantic, but in spring when winds over the Atlantic would be against them, they take the longer overland route through Central and North America. Such loop migrations are performed by many species in many

Figure 1.2 Loop migration of the American Golden Plover Pluvialis dominica between breeding areas in North America and wintering areas in South America. The outward route occurs largely over the Atlantic Ocean and the return route largely overland. Partly from Byrkjedal & Thompson (1998).

different parts of the world, illustrating the effects of seasonal local conditions on the development of migration routes (Chapter 22).

Costs of migration

While migration allows participants to exploit the resources of different regions at different times of year, the travel involved is not without costs. In addition to the energy required for the journey, migrants must travel through unfamiliar and sometimes hostile terrain, adjust to atypical habitats, face unfavourable weather and intense competition for the limited resources at staging sites, and suffer the consequences of navigation errors (Chapter 10). As explained above, some species perform long and hazardous journeys on which they can neither feed nor rest. Although the mortality associated with such journeys is hard to estimate, it may often be substantial, and storms have killed thousands or even millions of birds at a time (Chapter 28).

In addition, at some stopover sites, where birds replenish their fuel reserves, large numbers of individuals must often gather at one time. Local food supplies can then be greatly depleted, leading to intense competition, to the detriment of many. Avian predators often concentrate at the same places, giving a high ratio of predators to prey, which increases mortality and disrupts the feeding of many individuals (Chapter 27). Moreover, in passing through a wide range of areas, migratory birds are likely to encounter a greater range of parasites and pathogens than are resident birds that remain in the same areas year-round. The high densities that migrants experience at some stopover sites favour the transmission of certain kinds of parasites and pathogens, which in turn can compromise the migratory performance, survival and breeding success of infected individuals. Compared to residents, some migrants have larger immune defence organs, such as spleen and bursa of Fabricius (M0ller & Erritzoe 1998). The bursa is found only in sexually immature birds, but it is relatively larger in migrants, even before their first migration, than it is in residents. Disease agents may therefore play a greater role in the lives and deaths of migratory birds than in resident ones, and impose greater costs in terms of immunoprotection. In the process, migratory birds can also transport parasites and pathogens over long distances, as in recent years with the spread of the H5N1 strain of 'avian flu'. These are aspects of bird migration that have so far received little attention, and on which more research is needed. Overall, however, the benefits of migration to the participants, whether they accrue through increased survival, increased reproductive success or both, must on balance be greater than the mortality costs of the journey, for otherwise migration could not have evolved and could not persist.

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