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Migration of Birds

Future Directions

The mystery that formerly cloaked the periodic travels of birds for the writer of Jeremiah has been partly dispelled through our discovering the extent and times of seasonal journeys of most species. Even though we understand more, we still share with the ancients the awe and wonder as we contemplate these twice annual global movements. Indeed, in some ways, what we have learned only makes the phenomenon of migration more amazing! But many gaps still remain in our knowledge. Some of which, like the nature of the bicoordinate system that birds use to identify geographic positions, have barely been explored. Other hypotheses, like the use of olfactory cues for orientation need further verification.

We study migration because we are curious. That suffices as a rationalization for our efforts. Yet there are practical implications of what we discover. Clearly the regulation of hunting pressure on many game animals is dependent upon knowing the patterns and intensities of migratory movements. The protection of nongame bird populations for which our society has recognized its responsibility must similarly rely upon understanding migration. There are even direct economic aspects. Studies have indicated, for example, that local, nonmigratory populations of various blackbirds cause nearly all of the rice damage in southern States, and that "hordes from the North" contribute very little to the losses. In addition, the transport of arborviruses by long distance migrants has direct implications for human health.

While the habitat requirements sought by migratory species on their breeding areas have been the focus of much research and some attention also has been paid to the habitat selection of these species on their winter ranges, we are largely ignorant of the habitat requirements of these species during their migratory journeys. To what extent are the migratory pathways described in this book a reflection of continental habitat patterns that provide the resources necessary at stop-over sites used during migration? Is habitat selection during migration based on the same criteria the species uses on the wintering range or the breeding area? How important to the success of migrating passerines are the forested corridors along major river systems? How important is the density and spatial distribution of wetlands in the migration of waders and waterfowl? If important, how large an area is required to sustain these species during the periods of passage?

The nature and extent of environmental modifications wrought by humans throughout the world are readily apparent, and yet we have little understanding of how migrants are affected by changes in land use or habitat degradation. Extensive forests have been burned or cut away. Rolling prairies have been turned over by the plow and planted in monocultures of tame grasses and row crops. Natural landscapes continue to be obliterated by urban sprawl. Air pollutants carried by continent-spanning winds rain acid depositions on fields, mountains, lakes and forests. Wetlands are drained or filled. Riparian vegetation is lost as rivers disappear under the intensive mining of ground water by irrigators in the arid West. The once slow but now rapidly increasing concentrations of carbon dioxide, methane, and other greenhouse gases generated by human activities will alter global climates, even if we cannot predict with assurance the directions these changes will take.

Migratory pathways evolved over the eons in expectation of a moderately stable environment with sufficient food and cover along appropriate corridors that connected sustaining winter ranges with suitable breeding areas. Still, the environment has always been changing. Except for catastrophic events that punctuated the history of life from time to time, change occurred at gradual rates. This rate of change was slow enough that the processes of evolution allowed bird populations to make compensatory modifications that ensured continued existence. But human impacts on the environment generate rates of change that exceed many species' ability to adapt. A wetland long-used by shorebirds as a critical foraging site on their extended journey from South America to the arctic tundra is drained and cultivated during the short interval between spring and fall passages. Warblers return from the tropics to find clear-cut mountainsides with no other suitable habitat open for their use. Weedy fields that persisted in the floodplains of major rivers and offered cover and food for communities of wintering sparrows down from the north now greet their fall arrival with orderly rows of stuble or the asphalt and lawns of industrial parks. We know that the birds cannot use these altered habitats, but we do not know the consequences of these events on migrant populations. If we decide that we must ameliorate the impacts of these changes, we must first of all know the consequences of these changes.

Not all species are negatively affected by our impact on the environment. While warblers and thrushes breeding in forest interiors may decrease with increased forest fragmentation, edge species will take advantage of the more open habitat and increase. As towns form tree-dominated islands across the Great Plains, many eastern species like the Baltimore Oriole expanded their ranges westward. Feed-grains sustain larger numbers of blackbirds than would not have naturally survived the stress of winter before the development of a large cattle feedlot industry. Dredge spoil offers dependable nesting sites for terns along the Atlantic and Gulf coasts. Airports in the northeast provide perennial grasslands for Upland Sandpipers that traditionally had to rely on limited larger patches on the coastal plain or the ephemeral meadows of forest succession. Yet, the general trend of our effect on the environment is toward uniform sameness; we have reduced the heterogeneity of the landscape. This, in turn, reduces the richness of bird species we experience in our daily lives. And since we as a species have thrived on the diversity in our environment, our quality of life suffers.

The Federal Government of the United States has recognized its responsibility to migratory birds under these changing conditions. Enabling acts allow for carrying out migratory bird treaty obligations in cooperation with other countries, and now most species have legal protection under regulations administered by the U.S. Fish and Wildlife Service. Refuges have been established to foster migratory species. Environmental laws enacted during the latter half of the twentieth century have helped to retard and even thwart environmental degradation. Nongovernmental organizations and state agencies have come to play an increasing role in the protection of migratory birds. Yet the effectiveness of conservation efforts is increased in the same measure that we, the people, become acquainted with migratory bird resource and interest ourselves personally in the well-being of the various species. We are faced with a two-fold challenge. Firstly, our challenge is to develop an ethic that recognizes our stewardship of these resources and that motivates the economic and political choices we make so that we may balance our immediate needs against a sustainable quality of life for future generations. And secondly, we are challenged to gain the understanding that is necessary to implement the good stewardship we desire.

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