National Water Summary on Wetland Resources
United
States Geological Survey Water Supply Paper 2425
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One of the best known functions of wetlands is to provide a habitat for birds (fig. 28). Humans have known of the link between birds and wetlands for thousands of years. Prehistoric people drew pictures of birds and wetlands on cave walls, scratched them onto rocks, and used them in the design of artifacts (fig. 29); and Native American lore provides accounts of bird hunts in wetlands. Wetlands are important bird habitats, and birds use them for breeding, nesting, and rearing young (fig. 30). Birds also use wetlands as a source of drinking water and for feeding, resting, shelter, and social interactions. Some waterfowl, such as grebes, have adapted to wetlands to such an extent that their survival as individual species depends on the availability of certain types of wetlands within their geographic range. Other species, such as the northern pintail or the American widgeon, use wetlands only during some parts of their lives. |
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The value of a wetland to a specific bird species is affected by the presence of surface water and the duration and timing of flooding. |
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Wetlands occupy only a small part of the landscape that is now the conterminous United States--11 percent in 1780 and just 5 percent in 1980 (Dahl and others, 1991). Nonetheless, they are important to birds. During the past 20 years, policies and programs that encourage altering, draining, or filling of wetlands have decreased, and policies that encourage wetland conservation and restoration have increased. (See article "Wetland Protection Legislation" in this volume.) Among the wetland attributes society seeks to protect and conserve are those that benefit wildlife, particularly migratory birds. This article discusses the benefits that wetlands provide for birds and the effects of wetland losses on birds. |
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WETLAND FACTORS THAT AFFECT BIRDS | ||||||||
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The relation between wetlands and birds is shaped by many factors.
These include the availability, depth, and quality of
water; the availability of food and shelter; and the
presence or absence of predators. Birds that use wetlands
for breeding depend on the physical and biological
attributes of the wetland. Birds have daily and seasonal
dependencies on wetlands for food and other life-support
systems.
The value of a wetland to a specific bird species is affected by the presence of surface water or moist soils and the duration and timing of flooding. Water might be present during the entire year, during only one or more seasons, during tidal inundation, or only temporarily during and after rainfall or snowmelt. At times water might not be present at the land surface, but might be close enough to the land surface to maintain the vegetation and foods that are needed by birds. Birds may use wetlands located in depressions in an otherwise dry landscape, along streams, or in tidally influenced areas near shorelines. |
The availability or influence of water is a very important
wetland feature to birds. It is not, however, the only
feature that determines if birds will be present, how
birds use the wetland, or how many kinds or numbers of
birds may use the wetland. Other determining physical or
biological factors include water depth and temperature,
presence or absence of vegetation, patchiness or openness
of vegetation, type of vegetation, foods, water chemistry,
type of soils, and geographic or topographic location.
Any variations in any of these wetland features will cause
subtle, but distinct, differences in bird use. Wetlands provide food for birds in the form of plants, vertebrates, and invertebrates. Some feeders forage for food in the wetland soils, some find food in the water column, and some feed on the vertebrates and invertebrates that live on submersed and emergent plants. Vegetarian birds eat the fruits, tubers, and leaves of wetland plants. Water temperatures influence food production. Invertebrate production in the water column may ultimately depend on water temperature and the ability of a wetland to produce algae. Cold water might not be a hospitable environment for small animals and plants that some wetland birds eat. However, water that is too warm also might not produce foods that some birds prefer. |
The geographic location of a wetland may determine how and when birds will use it. | ||||||
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| Wetland vegetation provides shelter from predators and from the weather. The presence or absence of shelter may influence whether birds will inhabit a wetland or a nearby upland area. Predators are likely to abound where birds concentrate, breed, or raise their young. Wetlands form an important buffer or barrier to land-based predators and reduce the risk of predation to nesting or young birds. However, some predators, such as the raccoon (fig. 31), are well adapted to both wetland and upland environments, and take large numbers of both young and nesting birds. Mink forage for nesting or sleeping birds along the edges and interiors of wetlands. Other animals, such as the snapping turtle, the alligator (fig. 32), or the large-mouthed bass, are effective water-based predators of young birds, particularly young waterfowl. Snakes take their toll as well. Many bird species that are highly adapted to feeding in a wetlandenvironment also have genetic adaptations that lower their risk of becoming prey. One such example is the bittern (fig. 33), which has excellent protective coloration. The same vegetation that hides birds from predators also provides some shelter from severe weather. In spring, during cold and stormy weather, waterfowl such as canvasback ducks protect their young in the shelter of a marsh that is almost impenetrable to wind. |
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The geographic location of a wetland may determine how and when birds
will use it or use adjacent habitat. In the northern
latitudes or at high altitudes, some wetlands are covered
with ice in the winter and are temporarily "out of
service" for birds adapted to a water environment, but
emergent vegetation might still offer shelter and food for
some species. Birds that eat fish, aquatic invertebrates,
or submersed vegetation cannot forage for food because of
the ice cover. Some wetlands are on the migration path of
waterfowl and other migratory birds and provide stopover
locations for traveling birds (fig. 34). These birds
might feed in agricultural fields during the day and
return to the shelter of wetlands during the night.
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The "prairie potholes" are a special type of wetland, found in the
north-central part of the United States. These potholes
are an example of a wetland type that is important to
migrating waterfowl. Here the timing and duration of
inundation and the salinity of the water are important
factors in the production of plants and invertebrates used
by birds. These, and many other wetland characteristics,
are influenced by a number of things:
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THE IMPORTANCE OF WETLANDS TO BIRDS | ||||||||
| Because of the great variety of wetlands, bird adaptation to and use of wetland environments differs greatly from species to species. Birds' use of wetlands during breeding cycles ranges widely. Some birds depend on wetlands almost totally for breeding, nesting, feeding, or shelter during their breeding cycles. Birds that need functional access to a wetland or wetland products during their life cycle, especially during the breeding season, can be called "wetland dependent" (table 5). Other birds use wetlands only for some of their needs, or they might use both wetland and upland habitats. Of the more than 1,900 bird species that breed in North America, about 138 species in the conterminous United States are wetland dependent (American Ornithologists'Union, 1983). |
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Many bird species use forested wetlands as well as forested uplands, feeding on the abundant insects associated with trees (fig. 35). These birds are not dependent on wetlands because they use both habitats equally well. Some birds, such as wood ducks, are found primarily in forested wetlands and are dependent on this wetland type. Many migratory birds are wetland dependent, using wetlands during their migration and breeding seasons. Migratory birds may spend the winter in wetlands in the Southern United States, or farther south (fig. 34). Throughout winter, these birds use southern wetlands for food and nutrients to sustain them for their return trip north and the breeding season.
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Not all wetlands are of equal value to waterfowl and other birds. An inventory in the conterminous United States during the early 1950's showed that of 74.4 million acres of wetlands, 8.8 million acres had a high value for waterfowl, 13.6 million acres were of moderate value, 24.1 million acres were of low value, and 27.9 million acres were of negligible value (Shaw and Fredine, 1956, p. 17). These categories were identified on a State-by-State basis and were ranked according to use by waterfowl, with "high" being most used. The primary focus of this inventory was waterfowl; thus these rankings might not reflect wetland values for other birds. Also, the inventory was for only natural wetlands that had been little altered by human activities. The three areas of highest value are the Mississippi River corridor southward from Cairo, Ill., and westward along the Texas gulf coast; the entire east coast from Maine southward through most of Florida; and the northern Midwest. |
Widespread draining and altering of wet-lands has affected bird populations. | ||||||
THE INFLUENCE OF WETLANDS ON WATERFOWL POPULATIONS | ||||||||
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Considerable research has increased the understanding of wetlands' influence on the numbers of waterfowl that breed and their breeding success. However, the relation between wetlands and the population and propagation of various waterfowl species is not well understood. This relation depends on: (1) the number of wetlands in the area; (2) the wetlands' size and water depth; (3) whether the wetlands hold open water in the early spring or through late August; (4) the climate; and (5) the species of bird and the bird's adaptations to wetlands. In the prairie pothole region in the late 1970's, for example, as the number of wetlands in an area increased, populations of dabbling ducks increased, but at a ratio of less than 1:1 (fig. 36). In the past 20 years, the duck-pothole ratio has decreased, possibly due to decreases in upland cover and increases in predation. Bellrose (1977) also found waterfowl densities and propagation to be related to the number of wetlands per square mile; generally, waterfowl densities and propagation increased as the number of wetlands increased. However, he found that mallard production decreased when the number of wetlands exceeded 12 per square mile. |
Different waterfowl species adapt to different wetland types, inhabit different geographic areas, and nest at different times. The relation of many other species of birds to wetlands are undoubtedly just as complex. |
Widespread draining and altering of wetlands has affected bird populations. | ||||||
EFFECTS OF WETLAND LOSS AND DEGRADATION ON BIRDS | ||||||||
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About one-third of North American bird species use wetlands for food, shelter, and (or) breeding (Kroodsma, 1979). Thus, widespread draining and altering of wetlands has affected bird populations. Because most of the wetland drainage and alteration occurred between the 1930's and 1950, before scientific estimates of bird populations began, most estimates of population declines are inferred. Before the passage of the Migratory Bird Treaty Act in 1918, the reduction in waterfowl populations was blamed largely on excessive hunting and wetland drainage (Day, 1959). However, since 1930 most of the reduction has been attributed to the loss or degradation of wetlands (Bellrose and Trudeau, 1988) and the loss of suitable upland habitats that surround wetlands. |
For most wetland-dependent birds, habitat loss in breeding areas translates directly into population losses. As wetlands are destroyed, some birds may move to other less suitable habitats, but reproduction tends to be lower and mortality tends to be higher. Hence, the birds that breed in these poorer quality habitats will not contribute to a sustainable population through the years (Pulliam and Danielson, 1991). About one-half of the 188 animals that are federally designated as endangered or threatened are wetland dependent (Niering, 1988). Of these, 17 are bird species or subspecies (table 5). These birds are categorized as endangered or threatened because their populations are so low that the risk of their extinction is real and immediate. The circumstances that cause each species or subspecies to be endangered differ greatly. |
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Wetland loss due to draining, filling, or altering of surface-water
and ground-water flow is a concern to many people.
Wetland degradation also has a substantial effect on
birds. Although wetland degradation is a serious problem,
it is one that is more subtle and less understood than
wetland losses. Degradation can take many forms:
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An example of wetland degradation is found in the Chesapeake Bay region. Nutrients and sediments entering the bay from agricultural, urban, and industrial areas have caused increased algal blooms, decreased invertebrate production, and lowered oxygen levels. This degradation has reduced the acreage of seagrasses that form an important link in the food chain for invertebrates, fish, and wetland-dependent birds. The decline in the canvasback duck population in this area is thought to be directly related to the decline in seagrasses. Chemicals and sediments that move from agricultural areas into wetlands are two of the most pervasive sources of degradation. The shift in human populations from inland areas to coastal areas of the United States has caused problems in coastal wetlands through overloaded sewage treatment systems. The large and growing volume of industrial wastes that enter ground- and surface-water supplies also threatens to degrade wetlands. These threats, combined with habitat destruction, have a net negative effect on the population of wetland birds. Thus, if the amount and quality of wetland habitat is substantially reduced, populations of wetland-dependent birds in the area also can be expected to decrease. |
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SOME EFFORTS TO PRESERVE WETLAND BIRD HABITATS | ||||||||
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Many people believe that ownership or management of wetlands by
public
conservation agencies, such as the U.S. Fish and Wildlife Service, and by private organizations, such as the Nature Conservancy or the National Audubon Society, offers the best assurance that the highest value wetlands will be maintained for future generations. (A discussion of the agencies and organizations that participate in management and conservation of wetlands in each State can be found in the State Summaries section of this report.) A few early concerns for wetlands important to waterfowl are reflected in the creation of the first national wildlife refuge and in the establishment of the Federal Duck Stamp program. The first national wildlife refuge was created in 1903, by President Theodore Roosevelt, to protect a wetland--Pelican Island, Florida (U.S. Fish and Wildlife Service, [1995]). Concern for the loss of waterfowl led to the Federal Duck Stamp program that began in 1934 (Mitsch and Gosselink, 1993) and continues today. Duck stamps are sold to waterfowl hunters to provide money for the purchase or preservation of wetlands (fig. 37). |
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Several international treaties are partly responsible for much of the formal wetland protection in this country--the Migratory Bird Treaty and the Convention on Wetlands of International Importance especially as Waterfowl Habitat. "In 1918, the U[nited] S[tates] passed into law the Migratory Bird Treaty Act, ratifying a treaty with Great Britain, on behalf of Canada, that recognized the conservation responsibilities for more than 800 species of migratory birds shared by the two countries" (U.S. Fish and Wildlife Service, [1995]). Subsequent to that act, the United States developed the National Wildlife Refuge System consisting of 500 reserves--many of which are wetlands important to birds--comprising more than 90 million acres (fig. 38). The system has the highest ratio of wetlands to dry land in public ownership. The National Park Service manages the Everglades National Park and several preserves that also have high ratios of wetlands to dry lands. |
The Convention on Wetlands of International Importance especially as Waterfowl Habitat, more commonly known as the "Ramsar Convention" is an intergovernmental treaty for international cooperation for the conservation of wetland habitats. The U.S. Fish and Wildlife Service is responsible for implementation of the convention in the United States. A "List of Wetlands of International Importance" has been developed by the convention. Sites on this list are known as "Ramsar Sites" and are wetlands that convention members have a special obligation to preserve. There are 15 Ramsar sites in this country (fig. 38).
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About one-half of the 188 animals that are federally designated as endangered or threatened are wetland dependent. |
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SUMMARY AND CONCLUSIONS | ||||||||
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Human activities have caused shifts in wetland-dependent bird populations since European settlement of the United States, especially since the beginning of the 20th century. Many acres of wetlands were drained between the 1930's and 1950, well before any of the national bird surveys were begun. As a result, it is not possible to accurately determine the effects of habitat destruction on long-term wetland bird populations. It is apparent that there have been many changes in the distribution and numbers of wetland birds. Wetlands on breeding, migratory, or wintering areas are all important to sustain bird populations. As the wetland habitats in these areas are drained or altered, the ability of these areas to sustain bird populations decreases. Each species of wetland-dependent bird has a unique and complex set of needs for wetland habitats that makes it difficult to generalize about how loss or degradation of wetlands affects bird populations. It seems reasonable to expect, however, that as the numbers of wetlands in a region decline, so too will the numbers of wetland-dependent birds. |
In some parts of the United States, extensive wetland losses have displaced birds from large areas. Continued wetland losses probably will cause continued losses of wetland birds. However, recent recognition of the wetland values, and the effects of their losses, have provided incentives to maintain and restore wetlands. |
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REFERENCES CITED | ||||||||
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American Ornithologists' Union, 1983, Check-list of North American Birds: Lawrence, Kans., Allen Press, Inc., 6th edition, 877 p. Bellrose, F.C., 1977, Species distribution, habitats, and characteristics of breeding dabbling ducks in North America, in Bookhout, T.A., 1977, Waterfowl and wetlands--An integrated review: Proceedings of a symposium held at the 39th Midwest Fish and Wildlife Conference, Madison, Wis., La Crosse Printing Co., Inc, 152 p. Bellrose, F. C., and Trudeau, N.M., 1988, Wetlands and their relationship to migrating and winter populations of waterfowl, v. I: Portland, Oreg., Timber Press, p. 183-194. Dahl, T.E., and Johnson, C.E., 1991, Wetlands--Status and trends in the conterminous United States, mid-1970's to mid-1980's: Washington, D.C., U.S. Fish and Wildlife Service, 22 p. Day, A.M., 1959, North American waterfowl: Harrisburg, Pa., Stackpole Co., 363 p. Ehrlich, P.R., Dobkin, D.S., and Wheye, Darryl, 1992, Birds in jeopardy: Stanford, California, Stanford University Press, 260 p. Kroodsma, D. E., 1979, Habitat values for nongame wetland birds, in Greeson, P.E., Clark, J.R., and Clark, J.E. eds., 1979, Wetland functions and values--The state of our understanding: Minneapolis, Minn., American Water Resources Association, p. 320-343. |
Mitsch, W.J., and Gosselink, J.G., 1993, Wetlands: New York, Van Nostrand
Reinhold, 722 p.
Niering, W.A., 1988, Endangered, threatened and rare wetland plants and animals of the continental United States, in Hook, D.D., McKee, W.H., Jr., Smith, H.K., and others, 1988, The ecology and management of wetlands--Volume I--The ecology of wetlands: Portland, Oreg., Timber Press, 592 p. Pulliam, H.R., and Danielson, B.J., 1991, Sources, sinks and habitat selection--A landscape perspective on population dynamics: The American Naturalist, v. 137, p. 850-866. Shaw, S.P., and Fredine, C.G., 1956, Wetlands of the United States--Their extent and their value to waterfowl and other wildlife: U.S. Fish and Wildlife Service, Circular 39, 67 p. U.S. Fish and Wildlife Service, 1993, Annual report of lands under control of the U.S. Fish and Wildlife Service as of September 30, 1993: Division of Realty, 43 p. U.S. Fish and Wildlife Service, [1995], Wetlands of International Importance--United States Participation in the "Ramsar" Convention, Ramsar, Iran, 1971, 11 p. For Additional Information: Robert E. Stewart, Jr., National Biological Service, Southern Science Center, 700 Cajundome Boulevard, Lafayette, LA 70506 |
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