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Bird Migration and Global Change: The Birds and the Issues
Birds are the most mobile of vertebrates. Whether they fly, swim, or run, their ability to cover great distances quickly enables many species to exploit different areas at different seasons in the annual cycle and at different stages in their life history. This is most evident in those that fly, but flightless penguins roam the southern oceans far from the islands and coasts that support their breeding colonies. Even a few flightless land birds have well-developed migrations. Emus in arid Western Australia move hundreds of miles in response to rainfall and vegetation conditions. In the forests of New Guinea, Dwarf Cassowaries track seasonal fruit production with altitudinal movements of a thousand or more meters.
Changing climates have special implications for migratory species. Migratory birds depend on specific habitats and resources in different geographical areas at different phases of their annual cycle. How are these complex patterns of habitat and resource use being affected by global climatic change? Is the ecological and evolutionary adaptability of migratory birds adequate to keep pace with changing climates and landscapes? Are future changes in climate likely to cause extinction of many of the world's most remarkable species? These are basic questions that we shall address.
Migratory Birds: Ecological Patterns
Migratory birds occupy all of the world's major environments, and their movements are extraordinarily diverse. Land, freshwater, and marine birds show well-developed migrations, as do birds of climatic zones ranging from polar regions to the tropics.
For land and freshwater birds, migrants vary enormously in the distance they travel, in the regularity of their schedules from year to year, and in the fraction of individuals that are migratory in different parts of their ranges. Short-distance migrants range from those that show local habitat shifts or altitudinal movements within a small geographical area to those that make intracontinental movements of up to a few hundred kilometers. Where I live in northern New Mexico, for example, the winter Dark-eyed Juncos at our feeder are a mix of latitudinal migrants from the Pacific Northwest and northern Rocky Mountains and gray-headed birds that are altitudinal migrants from nearby mountains. Weather conditions of the particular year influence many of these short-distance patterns. Frequently, only a portion of the population of a species in a given locality shows short-distance movements, a pattern termed partial migration. Over the geographical range of many species, local populations range from being completely resident to partially or fully migratory.
Birds of many different taxa and species at all latitudes exhibit short-distance movements. Birds of mountainous regions, from high latitudes to the tropics, show altitudinal shifts, with many such movements by tropical species yet to be documented. Many birds of semiarid regions in Africa, Asia, and Australia, characterized as "nomadic," show opportunistic movement patterns that enable them to utilize areas that have recently experienced favorable weather conditions.
Long-distance migrations of land and freshwater birds involve flights across major climatic zones, and often between continents or hemispheres. These movements are often more regular in timing than those of short-distance migrants. Strong-flying raptors and shorebirds perform some of the longest migrations. Radio tracking has shown, for example, that Bar-tailed Godwits fly nonstop for distances of 10,000 kilometers or more between breeding areas in Alaska and wintering areas in New Zealand and Australia. The Swainson's Hawks that occasionally fly over my home in New Mexico in spring spend the winter on the Argentinean pampas. Many small songbirds, however, make intercontinental migratory flights of thousands of kilometers.
Seabirds show diverse patterns of migration, as well. Some are short-distance migrants that disperse from coastal or insular breeding colonies to neighboring oceanic regions up to only a few hundred kilometers distant. Others, such as the Arctic Tern, are long-distance migrants that fly thousands of kilometers from breeding areas to nonbreeding ranges in distant oceans, sometimes in the opposite hemisphere.
Migratory Birds: Taxonomic Patterns
About 9930 species of birds exist worldwide, belonging to 204 families. From many sources in the literature, I have compiled a preliminary list of migratory birds, a task not as easy as it might at first seem. Many species that are commonly regarded as permanent residents are really partial migrants in some parts of their ranges. In northern New Mexico, for example, the Steller's Jay, considered by most reference books to be a permanent resident, is a partial migrant, with many birds moving to lower elevations in winter. Our understanding of the seasonal movements of tropical species, especially altitudinal movements of those of mountainous regions, is still very rudimentary. My survey of migratory species indicates that at least 2600 bird species of 141 families show some type of seasonal migration or substantial nomadism (Table 1.1). This corresponds to about 26.2 percent of all bird species, a figure that is sure to increase as we learn more about bird movements in regions such as eastern Asia, much of Africa, and mountain areas throughout the world. My estimate also substantially exceeds the estimate of 19 percent of migratory species presented by BirdLife International in 2008.
The frequency of migration varies widely among different groups of birds. Members of a few families of tropical birds, such as the family to which the ant thrushes and ant pittas belong (Formicariidae), are almost completely nonmigratory. Others, such as the New World and Old World warbler families (Parulidae and Sylviidae, respectively), contain some species that are permanent residents, others partial migrants, and still others long-distance temperate–tropical migrants. In most families of temperate zone songbirds, such as the titmice and chickadees (Paridae), some species are at least partial migrants, that is, with some populations that consist partly or largely of migratory individuals. Almost all seabirds are migratory, in the sense of spending nonbreeding periods at sea at feeding areas substantially distant from their nesting areas. Waterfowl that breed in the temperate zones or the Arctic are almost all migratory to varying degrees. Shorebirds breeding at high latitudes are nearly all long-distance migrants. Falcons and hawks that breed in the temperate zones or the Arctic are also mostly migratory—some only short-distance migrants but others showing intercontinental movements. Owls of these same latitudes, on the other hand, are in some cases migrants, in other cases permanent residents.
Migratory Birds: Geographical Patterns
The relative abundance of migratory species also differs markedly among major geographical areas (Table 1.2). The large landmasses of the North Temperate and Arctic zones contain many migrant species, most of which breed in the region. In tropical and subtropical regions of the continents, wintering migrants are numerous. Tropical island archipelagos such as the East and West Indies are also wintering areas for many migrants, as are smaller island areas throughout the Atlantic, Pacific, and Indian oceans. Many of the migrants to distant oceanic areas are nonbreeding shorebirds and seabirds. South Temperate Zone regions are also home to many breeding migrant species, and the birds that breed in the Antarctic are almost all migratory. Intratropical migration patterns are well developed in Africa, and to a lesser extent in Australia and the New World. Many birds of mountainous regions, regardless of latitude, show altitudinal movements.
Several major migration systems can be recognized in different world regions. In the New World, these include the Nearctic, Nearctic–Neotropical, Neotropical, and South American migration systems. The Nearctic migration system comprises land and freshwater birds that breed in Canada and the United States and winter primarily north of central Mexico. This migration complex involves about 423 species, including many species of waterfowl and other freshwater birds, hawks and owls, and small land birds. About 13 of the Nearctic species that breed at high latitudes migrate to Asia rather than to more-southern parts of North America.
The Nearctic–Neotropical migration system includes bird species that breed in North America and winter in southern Mexico, Central America, the West Indies, and South America. About 184 species of land, freshwater, and coastal marine birds are involved. About 76 of these species spend the nonbreeding season in Mexico, the West Indies, or Central America, but 108 species extend their winter ranges into South America. The Neotropical migration system comprises about 104 species, most of which are altitudinal migrants. About 5 species breed in the North American tropics and winter in South America. We know more about patterns of altitudinal migration in Mexico and Central America than anywhere else, and what we see here suggests that we have much to learn about altitudinal migration in mountainous regions elsewhere in the world.
The South American migration system comprises about 266 land, freshwater, and coastal marine birds. Most of these species breed in the temperate region of southern South America and migrate north in the austral winter, but at least 31 species show short-distance or altitudinal movements in the tropics. About 77 of the species breeding in temperate South America are fully migratory. Some 44 species, mostly tyrannid flycatchers and swallows, winter in the humid tropical region centered on the Amazon basin. For more than two-thirds of these species, ranges of non-breeding migrants overlap ranges of residents of the same species.
The Old World exhibits European, European–African, and Asian migration systems. In Europe, about 116 of 524 breeding species of land, freshwater, and coastal marine birds are short-distance migrants that winter within the region. The European–African migration system includes 185 species of land, freshwater, and shorebirds of thirty-two families that winter in Africa. These species are primarily hawks and falcons, waterfowl, shore-birds, Old World warblers, thrushes, swallows, pipits, wagtails, and shrikes. Of these, 62 species, particularly many hawks and falcons, shorebirds, Old World warblers, thrushes, and shrikes, winter exclusively in sub- Saharan Africa. Of the remaining species, some winter in Africa and some in areas of the Middle East.
The rich Asian migration system involves birds that move from breeding areas in eastern Eurasia to wintering areas in southern and southeastern Asia, the Philippines, and the East Indies. About 827 species of forty-four families show migratory movements within this region. About 35 species of sandpipers and plovers, 1 tern, and 10 species of land birds continue farther south to winter in the Australo–Papuan region. In Russia, China, and other parts of eastern Asia, at least 66 species of thrushes, flycatchers, Old World warblers, finches, and other passerines are short-distance or altitudinal migrants that do not reach the tropics.
Africa exhibits two well-defined migration systems. The southern African migration system involves 134 fully or partially migratory land and freshwater birds that breed in southern Africa and winter farther north. Waterfowl, rails, herons, kingfishers, cuckoos, swifts, swallows, and a variety of passerine birds are prominent in this system. Many birds breeding in Europe and Asia appear in southern Africa during the Northern Hemisphere winter. These include many species of sandpipers and plovers, gulls and terns, hawks, and Old World warblers.
The intratropical African migration system is centered on the equatorial region and the semiarid belts to the north and south. About 277 species, ranging from herons and plovers to larks and finches, move primarily north and south in this region, tracking favorable conditions related to wet and dry season weather patterns. Some cuckoos breed in the semiarid zones both north and south of the equator, switching places through migration when not breeding. In more-arid regions, movements of some species are best described as nomadic. Altitudinal migrants are frequent in the mountains of eastern Africa.
The Australo–Papuan region, comprising Australia and New Guinea, holds a largely self-contained migration system. This migration system consists of about 272 land and freshwater birds. Some are migrants between Australia and New Guinea, others move between Tasmania and mainland Australia, and still others show latitudinal and altitudinal movements within mainland Australia. In addition, many land birds and waterbirds perform irregular movements within interior mainland Australia. Some 169 species show partial migration patterns in Australia and Tasmania. Among Tasmanian birds, only 20 species are migratory, and only 4 are fully migratory, completely leaving the island for the Australian mainland.
Northern Hemisphere migrants also reach many Pacific islands, including New Zealand, although few species are involved. Long-distance migrants such as plovers and sandpipers most frequently visit these isolated regions.
Seabird migration systems relate less to latitudinal temperature patterns and more to the locations of suitable nesting areas and productive ocean waters. About 234 species of birds of eighteen families are largely or entirely pelagic in their distributions in the nonbreeding season (Table 1.3). Only very general migration patterns are apparent. In the eastern North Atlantic, about 31 species of loons, petrels, shearwaters, gannets, cormorants, sea ducks, skuas, gulls, and alcids breed in Arctic and subarctic areas and winter in pelagic or offshore coastal areas. In the western Atlantic, about 28 of the species of these groups breed in northern areas and winter in waters off the coasts of Canada and the United States. The North Pacific has a richer fauna. About 53 species of these seabirds winter southward through the Aleutian Islands and along the eastern and western coasts of the Pacific. Tropical and subtropical oceans host the richest and most diverse fauna of pelagic birds, including numerous petrels, shearwaters, terns, boobies, tropicbirds, cormorants, and frigatebirds.
In the southern oceans surrounding Antarctica, at least 94 species of penguins, albatrosses, petrels, cormorants, skuas, and related birds breed and forage to considerable distances from breeding localities. In tropical and warm temperate ocean areas, at least 97 species of seabirds of eleven families breed and wander widely over ocean areas distant from their nesting islands or coastal rookeries.
The Challenge of Climatic Change
Migratory birds face major challenges of survival in the face of rapid, human-induced global change. Over the past century, the earth's climate has warmed by 0.8°C, and by AD 2100, warming will likely be between 2.0°C and 4.5°C. Because they depend on habitats and resources in different areas at different stages of the annual cycle, populations of seasonal migrants in every world region will be affected by climatic change. The areas that migrants use seasonally include their breeding ranges, staging and stopover locations during migration, and areas occupied during the nonbreeding period. In one sense, because of their specialization for use of different habitats or geographical areas at different times, migratory species might seem to be at greater risk of extinction than permanent residents. A change in any one of the areas used during the annual cycle might cause their evolutionary strategy to fail. Some ornithologists conclude that such dependence on multiple geographical areas places migratory species at greater risk than resident species in the face of global climate change.
On the other hand, migratory capability is an extension of basic physiological and behavioral adaptations for local movements, homing, and the annual reproductive cycle, and it must possess a degree of flexibility. Climatic changes over long geological time have tested the ability of migrants to adjust breeding and nonbreeding ranges and alter migration routes. Perhaps this adaptability is adequate to respond to the rapid environmental changes now occurring. Or, of course, some migratory birds may be able to adjust quickly, while others may not.
Several factors could constrain the ability of migratory birds to respond to changing climate. Lack of genetic variability or phenotypic plasticity might limit the capacity of species to respond to changing conditions. Limited dispersal ability might also slow the ability of the species to shift their geographical ranges in response to changing conditions. An increase in frequency of extreme weather events, overall loss or degradation of one or more of the habitats on which they depend throughout the annual cycle, and changed influences of competitor species, predators, or diseases may also impact migratory species negatively.
Excerpted from Bird Migration and Global Change by George W. Cox. Copyright © 2010 George W. Cox. Excerpted by permission of ISLAND PRESS.
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