Human activities are modifying the chemistry and physics of the atmosphere, as well as the characteristics of the land surface, freshwater lakes and streams, and oceans. These changes are altering the earth’s energy balance, its basic patterns of atmospheric and ocean circulation, and the climatic conditions of all of the earth’s ecosystems. In turn, these effects are altering the habitats and resources critical for migratory birds. Direct human impacts on the earth’s ecosystems are interacting with these relationships, creating a complex pattern of global environmental change.
Many biologists believe that migratory birds are at a greater average risk of extinction due to changing climate than are resident species. This is based on the fact that migratory birds depend on different habitats and resources in different locations during their annual cycle and on the fact that the failure of any of these could be fatal. A second general belief is that the climate is now changing faster than birds have experienced in the past and that evolutionary adjustments by birds may not be able to keep pace with this change.
Migratory birds exhibit a high degree of ecological and evolutionary adaptability, and many are now showing rapid adjustment to climatic changes.
We begin by characterizing the patterns of migration shown by birds of different ecological and taxonomic groups and the frequency of migration in different world environments, from polar regions to the tropics. The nature of climate change throughout the world is examined, and the impacts of this change on bird habitats are evaluated.
The Birds and the Issues
Migratory Birds: Ecological Patterns
Migratory birds occupy 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.
Land and freshwater birds
Short-distance migrants
For land and freshwater birds, migrants vary enormously in
the distance they travel,
the regularity of their schedules from year to year,
and 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. 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.
For example, the winter Dark-eyed Juncos at our feeder are a mix of latitudinal migrants from the Pacific Northwest and the northern Rocky Mountains and gray-headed birds that are altitudinal migrants from nearby mountains.
Long-distance migrations
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.
For example, Bar-tailed Godwits fly nonstop for 10,000 kilometers or more be- tween breeding areas in Alaska and wintering areas in New Zealand and Australia.
Seabirds migration:
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 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. Many species that are commonly regarded as permanent residents are 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, the seasonal movements of tropical species, especially altitudinal movements of those of mountainous regions, is still very rudimentary.
A survey of migratory species indicates that at least 2600 bird species of 141 families show some seasonal migration. 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.
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 antpittas belong (Formicariidae), are almost completely nonmigratory.
Others, such as the New World and Old World warbler, 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, 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 is 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.
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.
Nearctic migration system
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 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 Central America, Mexico, 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.
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 the 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, the 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 a 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. Or, of course, some migratory birds may be able to adjust quickly, while others may not.
Response to Climate Change
Changing climates are altering migratory behavior in several ways.
These include:
- Change in the relative numbers of migrant and resident individuals in local populations.
- Change in the distance between areas occupied at different stages in the annual cycle.
- Change in the direction of migratory movements.
- Change in timing and speed of migratory movements.
Some of these responses appear to be simple phenotypic responses of individuals to altered environmental conditions, whereas others are genotypic or microevolutionary responses.
Changes in the proportion of migrants and residents can occur if ameliorated climatic conditions during the nonbreeding season in a particular location favor more individuals’ becoming permanent residents. Or, if weather conditions and resource availability patterns become more strongly seasonal, an increase in the migratory fraction of the population can occur. Since patterns of global climate change vary from region to region, both of these changes are expected.
Migration birds stopover
Long-distance migrants also face problems with habitat availability and quality along their migration routes. The energy they consume is supplied by fat that is deposited before and during migration. Predeparture staging areas and migration stopover areas where they can refuel are essential during migration to and from breeding areas. Removal and fragmentation of natural vegetation along migration routes can be detrimental to migrants even if breeding and nonbreeding areas remain favorable. Numerous studies document the use of individual stopover areas, but understanding their overall importance requires analysis of regional landscape conditions. In turn, this requires understanding the patterns of orientation and capability for flights between stopover locations for diverse migratory species.
Chemical Pollution
Acid precipitation remains a serious form of pollution in areas downwind from large urban and industrial areas throughout the world. In some regions, such as eastern Asia, acid precipitation is becoming worse.
In North America and Europe, damage to high-elevation forests by acid precipitation is extensive. Declining populations of migratory birds are associated with several of these areas. The most serious population declines of passerine birds in North America over the period 1966–1992, for example, were concentrated in areas of the Adirondack Mountains, the Blue Ridge Mountains, and the Cumberland Plateau subject to acid precipitation.
Ultraviolet Radiation
Depletion of the stratospheric ozone layer over polar regions has led to concern that high levels of ultraviolet radiation could cause catastrophic destruction of plankton-based marine food chains, especially in Antarctic waters. Populations of penguins and other seabirds, which depend on these food chains, might therefore be affected. Planktonic organisms show acclimation to ultraviolet, however, and damage from current levels of radiation may be much less than feared. Nevertheless, the possibility exists that migratory seabirds might be affected by the disruption of food chains in ocean areas receiving intense ultraviolet radiation.
Summary
Climatic change interacts with many other influences of human activity on the global environment.
A central issue of climatic change for all migratory species is their de- gree of vulnerability to extinction. Migratory birds have dealt successfully with major climatic shifts during Pleistocene glaciation, although some extinctions almost certainly did occur. Now, however, the climate is changing more rapidly than during the Pleistocene.