Northern Prairie Wildlife Research Center
The North American Prairie Pothole Region produces the bulk of North America's duck population. Smith et al. (1964) estimated that the region, comprising only 10% of the waterfowl breeding area of the continent, produces 50% of the ducks in an average year, and more than that when good water conditions prevail. About 23% of the Prairie Pothole Region lies in the Dakotas. Many wetlands in the region are also important fall staging and migration areas (Kantrud 1986b). The open, shallow wetlands found in cultivated fields are also heavily used by spring migrant waterfowl. For example, arctic-nesting geese stop in the Prairie Pothole Region to fatten while enroute to their breeding grounds (Alisauskus 1988). Availability of shallow wetlands in open, agricultural landscapes also allows the birds to disperse widely, thereby reducing or eliminating potential hazards from disease, particularly avian cholera (G. Krapu, pers. comm.).
Faanes and Stewart (1982) list 23 species of Anatidae that have nested at least once in North Dakota. The trumpeter swan (Cygnus buccinator), white-winged scoter (Melanitta fusca), and common merganser (Mergus merganser) have been extirpated as breeding species in the state. The snow goose (Chen caerulescens) is listed as an occasional breeder, but has nested only once in the state, and that under unusual circumstances. Rare breeders include the American black duck (Anas rubripes), cinnamon teal (Anas cyanoptera), common goldeneye (Bucephala clangula), and bufflehead (B. albeola).
The remaining 15 species are well established breeders in the Prairie Pothole Region of North Dakota. These include the Canada goose (Branta canadensis), wood duck (Aix sponsa), green-winged teal (Anas crecca), mallard (A. platyrhynchos), northern pintail (A. acuta), bluewinged teal (A. discors), northern shoveler (A. clypeata), gadwall (A. strepera), American wigeon (A. americana), canvasback (Aythya valisineria), redhead (A. americana), ring-necked duck (A. collaris), lesser scaup (A. affinis), hooded merganser (Lopodytes cucullatus), and ruddy duck (Oxyura jamaicensis). Bufflehead and common goldeneye do not breed in South Dakota (Whitney et al. 1978). The trumpeter swan has been restored as a breeding species in the state, but not in the Prairie Pothole Region. The American black duck, hooded merganser, and common merganser are rare, but probably breed in the state. Cinnamon teal breed regularly west of the Missouri River in South Dakota, but are rare in the Prairie Pothole Region, leaving 14 species of waterfowl as well established breeders in the Prairie Pothole Region of South Dakota.
In 1973-74, about 69%-79% of an estimated 439,600 to 1,067,500 pairs of breeding waterfowl in South Dakota were located in the pothole region (Brewster et al. 1976). The proportion of birds in the region was undoubtedly much higher before the construction of thousands of dams and dugouts for livestock watering facilities in the western part of the state.
Stewart and Kantrud (1973) estimated that about 84% of 2,281,000 pairs of breeding waterfowl were located in the pothole region of North Dakota in 1967. This region comprises 51.5% of the state area. They estimated that the breeding waterfowl population in the pothole region of the state averaged about 1,600,000 pairs (995,000-1,955,000 pairs) during 1967-69.
Because prairie wetlands are of extreme importance to an international resource, the numbers of breeding, molting, staging, and migrating waterfowl observed in these basins, and the foods consumed by these birds in various stages of their life cycle, have received considerable attention by wildlife biologists. A current review of these topics is available (Swanson and Duebbert 1989). The temporary, seasonal, and semipermanent wetlands dealt with in this report are by far the most important types of wetlands for most species of waterfowl that breed in the Dakotas and throughout the Prairie Pothole Region.
Wetland use by waterfowl can be expressed in various ways. Use expressed on an area basis results in high relative values for small basins, whereas use expressed on a wetland unit basis results in relatively high values for large basins, even though many large basins have large central areas of open water that may receive little use. Use of basins by breeding waterfowl has also been expressed in pairs per unit length of shoreline. This also tends to result in high values for small basins. Waterfowl use of basins has also sometimes been expressed in terms of basin area without regard to the presence or absence of surface water (Kantrud and Stewart 1977). This is done in attempts to evaluate an entire wetland class, not only those basins within a class that happen to contain ponded water at any particular time.
Temporary wetland basins. The primary function of these wetlands is to provide isolation for breeding pairs and supply invertebrate foods for waterfowl early in the nesting period. The rapid warming of these shallow wetlands in the spring results in early development of invertebrate populations (Swanson et al. 1974). Temporary basins, especially those containing flooded crop residues, receive considerable use by spring migrant waterfowl, but are usually dry during the fall migration period. Nevertheless, these basins are occasionally filled by fall rains, in which case they may be used to a considerable extent by fall migrant dabbling ducks.
Up to 6.8% of the South Dakota breeding population can be found on these basins when water conditions are relatively good, and the water ponded there makes up about 4% of the surface water available in all wetlands (Ruwaldt et al. 1979). About 2%-4% of the breeding waterfowl population was observed on temporary wetlands in North Dakota during the 1967-69 seasons (Stewart and Kantrud 1973). For both of these studies, the proportional use of the basins was underestimated because many intentively tilled wetlands, many undoubtedly of the temporarily flooded water regime, could not be classified as to water regime.
Cowardin et al. (1988 and unpubl. data) used wetland data from the late 1970's to early 1980's and waterfowl pair counts from the mid-1980's to estimate the density of five common species of breeding dabbling ducks in various classes of wetland basins during a wet year in the Dakotas. This analysis showed that 9.9%-13.4% of the mallards, 6.7%-10.6% of the gadwall, 4.3%-6.3% of the bluewinged teal, 7.7%-11.2% of the northern shoveler, and 13.3%17.1% of the northern pintail populations could be expected to be found on temporary wetlands. The relatively greater use of temporary wetlands by early-nesting species, mallard and northern pintail, is in agreement with the food-habit studies of Swanson et al. (1985) for the mallard and Krapu (1974a, b) for the northern pintail. These studies showed that temporary wetlands are a major source of the animal protein needed by nesting hens to produce clutches of viable eggs. Poor quality diets lead to reduced clutch and egg size, laying rate, and number of nesting attempts (Eldridge and Krapu 1988). That mallard breeding populations correlate better with acreages of water 15 cm or more deep than with acreages of water 30 or 46 cm or more deep provides further evidence of the importance of shallow water to early-nesting species (Stewart and Kantrud unpubl.).
Temporary basins are used only by breeding dabbling ducks (Anatini) in the Dakotas. Use of these basins decreases greatly during relatively dry springs, as ponded water can be absent or maintained for only a few weeks. In South Dakota, the greatest pair densities on these wetlands, on the basis of both basin area and basin unit, were, in decreasing order of magnitude, found for blue-winged teal (Anas discors), northern pintail (A. acuta), mallard (A. platyrhynchos), northern shoveler (A. clypeata), green-winged teal (A. crecca), and gadwall (A. strepera) (Ruwaldt et al. 1979).
Very similar results were reported by Kantrud and Stewart (1977) for temporary basins in North Dakota. They also observed use of these basins by small numbers of American wigeon (A. americana). They found that, on the basis of area of basins with surface water, temporary wetlands showed greater use by dabbling ducks (364.2 pairs/km2) than any other wetland class. But when all basins were considered without regard to the presence or absence of surface water, temporary basins supported 126.6 pairs/km2 . This was only about 47% and 79% of the dabbling duck density found on seasonal and semipermanent wetlands, respectively.
Seasonal wetland basins. These basins are a major source of invertebrate protein for laying female ducks early in the breeding season, and also provide isolation for pairs and sites for overwater nests. During wet years, these basins are also highly attractive as brood habitat (Talent et al. 1982) and molting areas. Seasonal wetlands usually receive considerable use by spring migrant waterfowl, but normally are dry by fall and little used by fall migrants unless unusually wet conditions prevail. Under these circumstances, seasonal wetlands can be heavily used by dabbling ducks, especially those species that regularly feed in upland grainfields.
During May and June of relatively wet years, 14.5%-16.2% of the breeding population can be found on seasonal basins in South Dakota. These basins contain 9.8%-11.0% of the surface water available in all wetlands under these conditions (Ruwaldt et al. 1979). In North Dakota, over 47% of the breeding duck population was observed on these basins over a three-year period (Stewart and Kantrud 1973). Again, the proportion of the waterfowl population that was observed on these basins is underestimated in both these studies, because of the difficulty of classifying intensively tilled wetlands early in the growing season.
Because of their greater average depth and water-holding ability, seasonal basins are used by breeding diving ducks (Aythyini) and stifftailed ducks (Oxyurini), as well as dabbling ducks. In recent years, use of seasonal wetland basins near wooded areas by wood ducks (Aix sponsa, tribe Cairinini) has also increased in the Dakotas.
On the basis of both basin area and basin unit, seasonal wetlands in South Dakota received the greatest use by blue-winged teal, followed in decreasing order by mallard, northern pintail, gadwall, northern shoveler, redhead (Aythya americana), green-winged teal, ruddy duck (Oxyura jamaicensis), and American wigeon (Ruwaldt et al. 1979).
Seasonal basins are used even more heavily in North Dakota, where diving ducks are more abundant and widespread than in South Dakota. In addition to the species recorded in seasonal basins in South Dakota by Ruwaldt et al. (1979), similar basins in North Dakota also supported lesser scaup (Aythya affinis), canvasback (A. valisineria), and ring-necked duck (A. collaris) (Kantrud and Stewart 1977). As in South Dakota, the main users of seasonal wetlands were blue-winged teal, northern pintail, mallard, gadwall, and northern shoveler. Kantrud and Stewart (1977) also found that, on the basis of basins with ponded water, classifiable seasonal wetlands in North Dakota supported 317.2 pairs of dabbling ducks per square kilometer and 16.6 pairs/km2 of combined diving and stiff-tailed ducks. Unclassifiable, intensively tilled wetland basins, many of which are also likely of the seasonal water regime, supported 193.2 pairs/km2 dabbling ducks and 6.8 pairs/km2 of combined diving and stiff-tailed ducks. They also observed that, when the presence or absence of surface water was not considered, seasonal wetland basins supported a greater density (287.2 pairs/km2) of breeding waterfowl than any other class of natural basin wetland.
Semipermanent wetland basins. These basins supply most of the needs of all common prairie-nesting waterfowl and their broods. Use of semipermanent wetlands by breeding waterfowl seems to be greatest when amounts of emergent cover and open water are approximately equal (Weller and Spatcher 1965). Schultz (1987) showed that livestock grazing of Typha stands in semipermanent wetlands could result in a tenfold increase in pairs of breeding ducks. Use by broods can increase greatly during years when seasonal wetlands are dry (Talent et al. 1982). A notable exception to the relatively high use of these wetlands occurs in early spring. Semipermanent wetlands are among the last to become icefree in the spring, and thus they do not provide an early source of invertebrate foods for nesting dabbling duck hens, particularly those that can begin nesting in late March or April.
Semipermanent wetlands are also the main general habitat for staging and fall migrant waterfowl in the Dakotas and the rest of the Prairie Pothole Region, although intermittently exposed and permanently flooded lacustrine wetlands are most heavily used by certain species.
Use of semipermanent wetland basins by dabbling ducks increases during the breeding season. In South Dakota during 1973-74, Ruwaldt et al. (1979) found 24.8%-26.6% and 41.7%-47.2%, respectively, of the May and June breeding duck population on these basins. During these months semipermanent basins accounted for 35%-42% of the surface-water area in the state. Semipermanent wetland. composed 18% of the area and 3% of the number of all wetland-, and 16% of the breeding duck population was found on these basins in the Prairie Pothole Region of North Dakota in 1967 (Stewart and Kantrud 1973). The following year was relatively dry, and 41% of the breeding duck population was observed on semipermanent wetlands.
Although the proportion of the breeding duck population found on semipermanent basins can increase greatly during relatively dry years, duck densities on these basins can decrease. Ruwaldt et al. (1979) observed 6.73 duck pairs per wet basin (2.57 pairs/ha) when water conditions were good and the water in the basins made up 42.3%-44.6% of the surface water area. At this time, 24.8%-41.7% of the total duck population was observed on these basins. During the following relatively dry year these basins still held 35.6% of the available surface water, but duck densities in them fell to 3.6 pairs per wet basin (2.07 pairs/ha). At this time, 26.6%-47.2% of the duck population was found on these basins. This phenomenon is presumably caused by the lack of enough water to produce foods in nearby temporary and seasonal basins (Krapu 1974a, Swanson et al. 1974) and the loss of surface water in the shallow wet meadow and shallow-marsh zones of the semipermanent wetlands themselves. Northern pintail, bluewinged teal, and northern shoveler seem particularly subject to such annual declines in density in semipermanent basins (Ruwaldt et al 1979). Annual population densities of these species have been shown to be highly correlated with the frequency and area of wetlands containing surface water in North Dakota (Stewart and Kantrud 1974).
When water conditions are good in South Dakota, semipermanent wetland basins support the highest densities of blue-winged teal, followed in decreasing order by the mallard, northern pintail, gadwall, redhead, northern shoveler, ruddy duck, green-winged teal, and American wigeon (Ruwaldt et al. 1979). In North Dakota, highest pair denaities on ponded semipermanent basins on the basis of area were also recorded for blue-winged teal, followed in decreasing order by redhead, gadwall, mallard, ruddy duck, northern pintail, northern shoveler, canvasback, green-winged teal, lesser scaup, ring-necked duck, and American wigeon (Kantrud and Stewart 1977). Use by all dabbling ducks and combined diving and stiff-tailed ducks was 189.1 pairs/km2 and 73.2 pairs/km2, respectively.
Semipermanent wetland basins are the principal breeding habitat for diving and stiff-tailed ducks in the Dakotas. During 1973-74, these basins supported 72.2%-100% of the populations of these birds in South Dakota (Ruwaldt et al. 1979). In North Dakota, 74.9% of these birds were found on semipermanent basins during 1967-69 (Kantrud and Stewart 1977). The redhead, canvasback, and ruddy duck nearly always nest over water in emergent vegetation in these wetlands, whereas nests of the lesser scaup and ring-necked duck are usually found in adjacent wet meadows. Redheads often lay their eggs in the nests of other overwater-nesting ducks.
Breeding waterfowl obtain most of their foods from wetlands, although field feeding in upland habitats is common during both spring and fall migration. Food consumption by adult and juvenile Anatini (dabbling ducks), Aythyinae (diving ducks), and Oxyurini (ruddy duck) during the breeding season in the Prairie Pothole Region was recently reviewed by Swanson and Duebbert (1989).
Food consumption varies greatly among species and between sexes of adults of a species; it also depends largely on relative availability of the food items as well as the wetland type, salinity, and vegetative zone of the wetlands where the birds were collected. For these reasons, only a superficial account of this complex subject can be presented here.
Consumption of invertebrates by the most common species of laying female Anatini is very high, varying from 72% by volume for the gadwall and mallard to 99% by volume for the northern shoveler and blue-winged teal. Foods of male Anatini during the breeding season vary from 70% seeds by volume for northern pintail to 97% invertebrates by volume for northern shoveler.
Juvenile Anatini of all species, especially during the first few weeks of growth, rely heavily on animal foods. All species investigated to date consume large amounts of either Insecta, Crustacea or Mollusca. Exceptions are older juvenile gadwall and American wigeon, which consume mostly vegetable material.
Adult female Aythyini and Oxyurini also rely heavily on invertebrate foods during the laying period. Consumption varies from a low of 70% by volume for the redhead to a high of 99% by volume for the ruddy duck (Woodin and Swanson 1989). Chironomidae (Diptera) are important for several species. Invertebrate use by most breeding male Anatini is similar to the females. An exception is the canvasback, which consumes about 98% vegetable foods by volume during the breeding period.
Nearly all juvenile Aythyini and Oxyurini studied to date consume 80% or more invertebrates, by either weight or volume, mostly Amphipoda, Trichoptera, and Dipteral The only exception is the redhead, which Bartonek and Hickey (1969) showed to consume 43% invertebrates by volume.
Other Marsh and Aquatic Birds
Approximately 39% of the 353 valid species on the North Dakota bird list (Faanes and Stewart 1982) use wetlands. Of the 223 species with known or inferred breeding status in North Dakota, 57 (26%) are marsh or aquatic birds other than waterfowl.
Of these 57 species, the spectacular whooping crane (Grus americana) has been extirpated as a breeding bird in the state. Distribution of nests and records of adults during the breeding season indicate this species once nested throughout the Prairie Pothole Region of North Dakota (Stewart 1975).
Of the 56 remaining species, 15 are generally not associated with the shallow-basin wetlands dealt with in this report, although many of them can be seen resting or feeding in or around these wetlands, both during the breeding season and in migration. This group is composed of the common loon (Gavia immer), red-necked grebe (Podiceps ariseaena), osprey (Pandion heliaetus), bald eagle (Haliaeetus leucocephalus), yellow rail (Coturnicops noveboracensis), common snipe (Gallinaao aallinago), American woodcock (Scolopax minor), ring-billed gull (Larus delawarensis), California gull (Larus californicus), Caspian tern (Sterna caspia), common tern (S. hirundo), least tern (S. antillarum), belted kingfisher (Cervle alcyon), willow flycatcher (Empidonax traillii), and swamp sparrow (Melospiza georaiana).
There are eight rare to uncommon species that likely nest, have nested, or are inferred to nest in vegetative zones found in wetlands dealt with in this report, but little is known of their habitat use. This group of birds includes the least bittern (Ixobrychus exilis), little blue heron (Earetta caerulea), tricolored heron (Egretta tricolor), cattle egret (Bubulcus ibis), green-backed heron (Butorides striatus) king rail (Rallus elegans), sandhill crane (Grus canadensis) and white-faced ibis (Plegadis chihi). The sandhill crane likely was a fairly common breeding species in North Dakota before the turn of the century (Stewart 1975).
An additional 11 fairly common to abundant species regularly breed or nest in various vegetative zones found in wetlands dealt with in this report, but little quantitative habitat use information is available for these birds. These species include the American white pelican (Pelecanus erythrorhynchos), doublecrested cormorant (Phalacrocorax auritus), great blue heron (Ardea herodias), Franklin's gull (Larus pipixican), spotted sandpiper (Actitis macularia), Forster's tern (Sterna forsteri), sedge wren (Cistothorus platensis), Le Conte's sparrow (Ammodramus leconteii), sharp-tailed sparrow (Ammodramus caudacutus), song sparrow (Melospiza melodia), and common grackle (Quiscalus quiscula).
For the remaining 22 species, a fair amount of information is available on use of wetland habitat in North Dakota. These 22 species are most characteristic of the wetlands dealt with in this report; the distribution of 21 of them among the subject wetlands is shown in Table 6. Nearly two-thirds of the total population were found on semipermanent wetlands, and all species used this wetland class. For 11 species, over half the population was found on semipermanent wetlands. Seasonal wetlands were used by 20 species and supported about one-third of the total population. From 53% to 64% of the population of seven of the species occupied seasonal wetlands. Temporary and undifferentiatedtillage wetlands were of little importance to the birds listed.
Densities are expressed in pairs per square kilometer in Table 7. These are crude measurements of density because they are based on the entire wetland, rather than on specific zone. or plant communities used by the birds. On this basis, the highest densities of birds occurred on semipermanent wetlands. These wetlands are especially attractive to American coot and black tern. Temporary wetlands supported high densities of redwinged blackbirds and savannah sparrows. Killdeer and marbled godwit were also found in greateat densities on these wetlands, although most of the use was of shoreline areas. Horned grebe, eared grebe, and willet were especially dense on seasonal wetlands. Agricultural use of wetlands obviously decreased their value to most species except for the American avocet and Wilson's phalarope, both of which reached highest densities on wetlands with frequently tilled soils.
Faanes (1982) censused breeding birds of all species in seasonal and semipermanent basin wetlands and other habitat types in a study area in central North Dakota during April-June 1980. He recorded 1357.8 pr/km2 in semipermanent wetlands. This density was exceeded only in shelterbelts (2658.7 pr/km2). Seasonal wetlands supported 574 pr/km2 . This density was exceeded only by shelterbelts, semipermanent wetlands' and prairie thickets (878.5 pr/km2 ).
Faanes (1982) also found the highest nesting or breeding densities of Forster's tern, red-winged blackbird, and brown-headed cowbird (Molothrus ater) in semipermanent wetlands, and noted that breeding song sparrows also used the periphery of this wetland class. He also observed breeding pairs of bobolink (Dolichonyx orzivorous), savannah sparrow, and sharp-tailed sparrow in seasonal wetlands.
Information on frequency of use of temporary, seasonal, and semipermanent wetlands was gathered for 13 selected nonwaterfowl species in South Dakota in 1975 and 1976 (Weber et al. 1982). Among the wetland classes listed (Table 8), the highest freauencies for eight species occurred in semipermanent wetlands that were used by 92% of the species. Seasonal wetlands were used by 85% of the species, and most freauently by the black-crowned night heron and Wilson's phalarope. Temporary wetlands were used by 54% of the species and most frequently by the American avocet, lesser yellowlegs and marbled godwit.
Very little information is available on use of prairie wetlands by migrants and summer visitors. Cultivated and non-cultivated seasonal and temporary wetlands provide habitat for millions of arctic- and subarctic-nesting shorebirds that pass through the Prairie Pothole Region each spring. Ten species of shorebirds studied in North Dakota foraged primarily on crustaceans and insects in these shallow basins during srping migration (Eldridge and Krapu in prep.). In one study area in central North Dakota, Faanes (1982) found use of semipermanent wetlands by white pelican, great blue heron, tundra swan (Olor columbianus), bufflehead, California gull, ring-billed gull, and rusty blackbird (Euphagus carolinus) to be eaual to or greater than other wetland types. Similar data for seasonal wetlands indicated high use of these basins by greater yellowlegs (Trinaa melanoleucus), solitary sandpiper (T. solitaria), shortbilled dowitcher (Limnodromus ariseus), Baird's sandpiper (Calidris bairdii), pectoral sandpiper (C. melanotos), stilt sandpiper (Micropalma himantopus), and water pipit (Anthus spinoletta). The highly variable water conditions that prevail across the region emphasize the need to maintain a widely dispersed and intact base of shallow basins to serve the function of spring migration habitat, as deep water does not provide an available invertebrate food source for birds that are morphologically adapted to feeding in shallow water.
A discussion of the use of wetlands by birds would not prairie be complete without mention of the importance of these basins to wintering populations of ring-necked pheasant (Phasianus colchicus). There are no quantitative data on this subject for the Dakotas, but it is obvious to anyone familiar with the habits of this species that it readily seeks the thermal cover and protection from predators and hunters afforded by the dense stands of emergent hydrophytes. There are large areas in both states where wetlands provide pheasants nearly the only protection available from the harsh winter winds common to the northern prairies. This species is an important game bird in both Dakotas. In 1986, 42,000 hunters harvested an estimated 122,000 birds in North Dakota (Grondahl 1987). An estimated 800,000 birds were harvested in South Dakota in 1985 (Ron Fowler, South Dakota Game, Fish and Parks Dept., pers. comm.).