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North American Ducks


by
David F. Caithamer
U.S. Fish and Wildlife Service
Graham W. Smith
U.S. Fish and Wildlife Service
Increased predation and habitat degradation and destruction coupled with drought, especially on breeding grounds, have caused the declines of some duck populations. More than 30 species of ducks breed in North America, in areas as diverse as the arctic tundra and the subtropics of Florida and Mexico. For many of these species, however, the Prairie Pothole region of the north-central United States and south-central Canada is the most important breeding area (Fig. 1), although migratory behavior and the life histories of different species lead them to use many wetland habitats.
Fig. 1. The Prairie Pothole region and areas sampled in the Breeding Population and Habitat Survey.
Numerous sources of information are available on the status of duck populations in North America. The two most comprehensive and reliable sources are the Breeding Population and Habitat Survey, conducted since 1955 and encompassing the Prairie Pothole region, boreal forests, and tundra habitats from South Dakota to Alaska (Caithamer et al. 1993; Fig. 1), and the Midwinter Survey, encompassing the United States and portions of Canada and Mexico at regular intervals. Results from these surveys are the basis for this article.
The Breeding Population and Habitat Survey is conducted during May and June when most species occupy their breeding ranges. Pilot-biologists and observers in airplanes identify and count ducks on a sample of transects. Not all ducks are visible from the air, so some transects are resurveyed more thoroughly with a helicopter or from the ground to obtain complete counts. These data are used to correct the air counts and obtain unbiased estimates of duck densities in these areas. Estimates of number of pairs of ducks are expanded to provide population estimates for the entire surveyed area. This survey, conducted by the Canadian Wildlife Service and the U.S. Fish and Wildlife Service (USFWS), is among the most extensive and comprehensive surveys conducted annually for any group of animals anywhere in the world. Survey estimates are the major determinant governing the regulation-setting process for the sport harvesting of ducks by both Canadian and United States provincial, state, and federal governments.
The Breeding Population and Habitat Survey is most reliable for mallards (Anas platyrhynchos), gadwall (A. strepera), American wigeon (A. americana), green-winged teal (A. crecca), blue-winged teal (A. discors), northern shoveler (A. clypeata), redhead (Aythya americana), canvasback (A. valisineria), and scaup (A. affinis and A. marila). Researchers and managers are trying to expand the geographic range of this survey in the Pacific Flyway, eastern Canada, and the northwestern United States.
The breeding survey, however, poorly monitors species such as whistling ducks (Dendrocygna spp.), mottled ducks (Anas fulvigula), American black ducks (A. rubripes), most sea ducks and mergansers (Lophodytes cucullatus, Mergus merganser, M. serrator), and wood ducks (Aix sponsa).
The Midwinter Survey has been conducted annually in early January since the mid-1940's. It is not as reliable as the breeding survey because of methodological shortcomings and because winter is a poor time to survey population abundance (Eggeman and Johnson 1989). Despite its limitations, this survey does provide useful information on such species as the black duck that are not well surveyed by the breeding survey (Conroy et al. 1988).

Status and Trends
Population estimates of all ducks from the breeding survey have varied from 26.5 to 42.8 million since 1955 (Fig. 2). Generally, breeding populations were high in the 1950's and 70's and low in the 60's, 80's, and 90's. The 1993 estimate of 28.0 million was 20% below the 1955-92 average.

Fig. 2. Duck populations in North America, 1955-93, from the Breeding Population and Habitat Survey and the Midwinter Survey.
Estimates of ducks from the Midwinter Survey also have varied since 1955 (Fig. 2). The 1993 estimate of 10.3 million ducks was the lowest recorded, and 44% below the 1955-92 average.
The Breeding Population and Habitat Survey provides reliable estimates for seven species of dabbling ducks, while the Midwinter Survey provides estimates for eight. The breeding population of total dabbling ducks in 1993 was 20% below the 1955-92 average. Compared with the 1955-92 average, 1993 breeding population estimates suggest population declines for mallards, American wigeon, blue-winged teal, and northern pintail. Population estimates were unchanged for green-winged teal and increased for gadwall and northern shoveler (Figs. 3-5). During the most recent 10-year period, the breeding population of northern pintail decreased, gadwall populations increased, and populations of six other species were stable (Table). Midwinter estimates of all species of dabbling ducks were stable or increased during 1984-93 (Table).

Fig. 3. Mallard, northern pintail, and green-winged teal breeding population estimates, 1955-93.
Midwinter estimates are the only long-term data available for black ducks. Apparent differences in population trends between the breeding and midwinter surveys (Table) are a function of differences in the quality of the surveys and in the populations monitored by the surveys. For example, breeding mallards have increased in recent years in the Atlantic Flyway, which is outside the breeding survey area. The breeding survey indicates a stable trend for mallards while the winter survey indicates an increasing trend; the two surveys monitor different portions of the total continental population. Table. Estimated annual numbers (in thousands) and recent trends (1984-93) of ducks based on the survey areas monitored by breeding and midwinter surveys.  
Tribe and species Breeding
 Wintering
  No. Trend No. Trend

Perching ducks
Wood duck     33 Stable

Dabbling ducks
American black duck     278 Stable
American wigeon 2,053 Stable 1,088 Stable
Blue-winged teal 3,192 Stable    
Blue-winged and cinnamon teal     166 Increasing
Gadwall 1,755 Increasing 1,168 Increasing
Green-winged teal 1,694 Stable 2,086 Increasing
Mallard 5,708 Stable 4,994 Increasing
Mottled duck     129 Increasing
Northern pintail 2,053 Decreasing 2,241 Stable
Northern shoveler 2,046 Stable 638 Stable

Diving ducks
Canvasback 472 Stable 298 Stable
Greater and lesser scaup 4,080 Decreasing 1,070 Stable
Redhead 485 Stable 336 Stable
Ring-necked duck 868 Increasing 421 Increasing

Sea ducks and mergansers
Bufflehead 869 Increasing 126 Increasing
Eidersa 8 Decreasing 132 Stable
Goldeneyeb 592 Stable 122 Stable
Harlequin duck     <1 Stable
Mergansersc 528 Stable 264 Increasing
Oldsquaw 174 Decreasing 10 Decreasing
Scotersd 1,006 Decreasing 160 Stable

Stifftails
Ruddy duck 387 Stable 110 Decreasing
a Eiders include common eider (Somateria mollissima), king eider (S. spectabilis), spectacled eider (S. fischeri), and Steller's eider (Polysticta stelleri). 
b Goldeneye include Barrow's goldeneye (Bucephala islandica) and common goldeneye (B. clangula). 
c Mergansers include hooded merganser (Laphodytes cucallatus), red-breasted merganser (Mergus serrata), and common merganser (M. merganser). 
d Scoters include black scoter (Melanitta nigra), surf scoter (M. perspcillata), and white-winged scoter (M. fusca).

Five species of diving ducks are monitored by breeding and winter surveys. Because lesser scaup are not distinguished from greater scaup in the surveys, these species have been combined. Breeding populations of diving ducks in 1993 were 18% below the 1955-92 average. Redhead and scaup breeding populations were lower than average, whereas the canvasback population was near average, and the ring-necked duck (Aythya collaris) population was above average (Figs. 4, 6). From 1984 to 1993, the breeding population of scaup declined while the breeding population of ring-necked ducks increased (Table). The Midwinter Survey also indicated an increasing population of ring-necks during this period (Table).

Fig. 4. Scaup, blue-winged teal, and gadwall breeding population estimates, 1955-93.
Fourteen species of sea ducks, mergansers, and their allies were monitored by the breeding survey. These 14 species plus the harlequin duck (Histrionicus histrionicus) were monitored during the Midwinter Survey. Because some of these species are difficult to identify during aerial surveys, or are encountered rarely, they are combined with related species (see Table).

Fig. 5. American wigeon and northern shoveler breeding population estimates, 1955-93.
Collectively, breeding populations of mergansers and their allies were 9% lower in 1993 compared to the 1955-92 average. Merganser, oldsquaw (Clangula hyemalis), eider, and scoter breeding populations in 1993 were all lower than their 1955-92 averages (see Table for species). The breeding population of goldeneye in 1993 was similar to the 1955-92 average, whereas the bufflehead (Bucephala albeola) breeding population was higher than the long-term average. During the last 10 years, breeding populations of eiders, oldsquaw, and scoters decreased, bufflehead increased, and goldeneye and mergansers were stable (Table). Winter population estimates during 1983-92 decreased for oldsquaw, increased for bufflehead and mergansers, and were stable for other species in the sea duck tribe (Table).

Fig. 6. Redhead and canvasback breeding population estimates, 1955-93
In the United States and Canada, wood ducks are the only representative of the tribe Cairinina and ruddy ducks (Oxyura jamaicensis) are the only representative of the Oxyurini tribe. Wood ducks are hard to survey because they inhabit forested wetlands where it is difficult to obtain reliable counts. Their current population, however, is greater than in the early 1900's (Bellrose 1980). Midwinter counts of wood ducks during 1983-92 indicated a stable population (Table). Ruddy duck breeding populations in 1993 were similar to the 1955-92 average.

Factors Affecting Population Status
Duck population changes occur on breeding, staging, and wintering habitats, with the changes on breeding habitats having the greatest effect on populations. Degradation and destruction of wetlands over the last 200 years have diminished duck populations; wetland alteration and degradation continue. The rate of wetland loss has been greatest in prime agricultural areas such as the Prairie Pothole region (Fig. 1), and lowest in northern boreal forests and tundra. Thus, species such as dabbling ducks that mostly nest in the severely altered Prairie Potholes have been harmed more than species like sea ducks and mergansers that nest farther north (Bellrose 1980; Johnson and Grier 1988).
Because most dabbling ducks need grassy cover for nesting (Kaminski and Weller 1992), conversion of native grasslands to agricultural production, including pastures, has reduced available nesting cover and contributed to a reduced nesting success for dabblers. This condition is especially true in the Prairie Pothole region of the United States and Canada (Fig. 1). In addition, highly variable precipitation in the Prairie Potholes has changed the number of wetlands available for nesting. For example, in 1979 there were 6.3 million wetlands in the surveyed portion of the Prairie Pothole region, but by the next spring, wetlands in the same area had decreased 55% to 2.9 million. Two years later they increased more than 100% to 4.2 million. These annual changes can temporarily mask the long-term declining trend in wetland abundance across the Prairie Pothole region.
The changing availability of wetland habitats in the Prairie Potholes region causes substantial fluctuations in some duck populations. During periods of high precipitation, larger wetland basins are full or overflowing, and shallow wetlands are abundant. Species such as the northern pintail, which tend to use shallow or ephemeral wetlands for feeding, produce more young when wetland numbers increase (Smith 1970; Hochbaum and Bossenmaier 1972). Consequently, population numbers increase as they did during the 1970's.
During the driest periods, however, such as those in the 1980's, only the deepest and most permanent wetlands retain water, causing population declines in species such as pintails that rely primarily on shallow wetlands. Population numbers are more stable for species like the canvasback, which rely on deeper marshes, and are therefore less affected by annual changes in wetland numbers because deeper marshes consistently retain water, providing ample habitat in most years (Stewart and Kantrud 1973).
Nest success in the Prairie Pothole region has declined in recent years largely because of increased nest predation caused by the range expansion of some predators and by reduced nesting habitat (Sargeant and Raveling 1992). Fewer and smaller areas of nesting habitat concentrate duck nests, enhancing the ability of predators to find nests. Predators such as raccoons (Procyon lotor) have expanded their range northward, probably because they can den in buildings, rock piles, and other human-made sites during winter.
Although wetland drainage, urbanization, and other human-caused changes have resulted in wintering habitat losses, these losses have been offset, at least for dabbling ducks, by increased fall and winter food from waste grain left in stubble fields. In addition, the national wildlife refuge system has protected and managed many staging and wintering areas for the benefit of waterfowl.
Modern duck-hunting regulations are believed to keep recreational harvest at levels compatible with the long-term welfare of duck populations. The proportion of ducks harvested varies regionally and by species, age, and sex. In 1992, 2%-12% of the adult mallards from the Prairie Pothole region were killed by hunters. Harvest rates of other species were generally lower. These conservative harvest rates are unlikely to cause population declines (Blohm 1989).

Conclusions
Changes in duck populations reflect changes in quality and quantity of waterfowl habitats. Long-term declines in populations have been caused by extensive habitat alterations. By contrast, short-term changes primarily reflect weather and resultant availability of wetland habitats. Maintenance of the current monitoring system and initiatives to improve our monitoring capability are essential for effective duck management.
Maintaining or increasing the quality and quantity of waterfowl habitat is needed to stabilize or increase duck populations. Agricultural policies and practices can profoundly affect habitat availability in Canada and the United States. For example, the Conservation Reserve Program, in which certain agricultural areas were set aside and planted in grasses, has added much-needed dabbling duck nesting habitat and therefore has improved their productivity in the U.S. portion of the Prairie Pothole region (R.E. Reynolds, USFWS, personal communication). The North American Waterfowl Management Plan, through its regional joint ventures, is striving to increase the habitat available for waterfowl and to improve monitoring of some populations.
For further information:
David F. Caithamer
U.S. Fish and Wildlife Service
Office of Migratory Bird Management
Henshaw Laboratory
11500 American Holly Dr.
Laurel, MD 20708
References
Bellrose, F.C. 1980. Ducks, geese and swans of North America. 3rd ed. Stackpole Books, Harrisburg, PA. 540 pp.

Blohm, R.J. 1989. Introduction to harvest: understanding surveys and season setting. Pages 118-129 in K.H. Beattie, ed. Sixth International Waterfowl Symposium, Washington, DC.

Caithamer, D.F, J.A. Dubovsky, F.A. Johnson, J.R. Kelley, Jr., and G.W. Smith. 1993. Waterfowl: status and fall flight forecast. Administrative Rep., U.S. Fish and Wildlife Service, Washington, DC. 37 pp.

Conroy M.J., J.R. Goldsberry, J.E. Hines, and D.B. Stotts. 1988. Evaluation of aerial transect surveys for wintering American black ducks. Journal of Wildlife Management 52:694-703.

Eggeman, D.R., and F.A. Johnson. 1989. Variation in effort and methodology for the midwinter waterfowl inventory in the Atlantic Flyway. Wildlife Society Bull. 17:227-233.

 Hochbaum, G.S., and E.F. Bossenmaier. 1972. Response of pintail to improved breeding habitat in southern Manitoba. Canadian Field-Naturalist 86:79-81.

Johnson, D.H., and J.W. Grier. 1988. Determinants of breeding distributions of ducks. Wildlife Monograph 100:1-37.

Kaminski, R.M., and M.W. Weller. 1992. Breeding habitats of nearctic waterfowl. Pages 568-589 in B.J. Batt, A.D. Afton, M.G. Anderson, C.D. Ankney, D.H. Johnson, J.A. Kadlec, and G.L. Krapu, eds. Ecology and management of breeding waterfowl. University of Minnesota Press, Minneapolis.

Sargeant, A.B., and D.G. Raveling. 1992. Mortality during the breeding season. Pages 396-422 in B.J. Batt, A.D. Afton, M.G. Anderson, C.D. Ankney, D.H. Johnson, J.A. Kadlec, and G.L. Krapu, eds. Ecology and management of breeding waterfowl. University of Minnesota Press, Minneapolis.

Smith, R.I. 1970. Response of pintail breeding populations to drought. Journal of Wildlife Management 34:943-946.

Stewart, R.E., and H.A. Kantrud. 1973. Ecological distribution of breeding waterfowl populations in North Dakota. Journal of Wildlife Management 37:39-50.


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