Understanding Prairie Pothole Wetlands and Their Birds

A prairie pothole is a shallow, depressional wetland, usually a freshwater marsh, set into the grasslands of the northern Great Plains. Millions of these basins are scattered across the Upper Midwest of the United States and the southern Canadian prairie provinces. Together they form the Prairie Pothole Region, one of the most important breeding areas for ducks and other wetland birds in North America.

What Makes a Wetland a Prairie Pothole

The U.S. Environmental Protection Agency describes prairie potholes as depressional wetlands, primarily freshwater marshes. Depressional means the wetland sits in a low spot or basin in the land, where water collects, rather than along a flowing river or a coastline. Marsh means the wetland is dominated by soft-stemmed, non-woody plants rather than trees.

Potholes are not static ponds. Research on prairie wetland hydrology emphasizes that water levels vary widely within a landscape, and that variation shapes how birds use the wetlands. This is covered in more detail below.

Readers searching for which birds live in a swamp should note that the sources used here describe potholes as marshes. This reference covers pothole marshes only and does not describe the bird communities of other wetland types.

Where the Prairie Pothole Region Lies

On the U.S. side, prairie potholes are found mostly in the Upper Midwest, with North Dakota singled out as a core area. Conservation planning documents also place western Minnesota within the region, and Iowa once held an extensive prairie marsh and pothole complex. In Canada, the region spans southern Alberta, Saskatchewan and Manitoba.

Estimates of scale vary by source and by what is being counted. One Canadian conservation source gives a range of 2 to 8 million potholes rather than a single figure. A university research group cites 64 million acres of prairie potholes in the United States alone.

Why the Region Matters to Birds

The region is often called the duck factory. The U.S. Fish and Wildlife Service states that the Prairie Pothole Region produces nearly half of North America's waterfowl. Ducks Unlimited puts the importance of the wetlands themselves this way:

The prairie potholes are the engines of the duck factory, and no amount of secure grasslands or intensive management can make up for their loss.

Published species counts differ because sources group birds differently and cover different parts of the region. These figures should be read side by side rather than combined:

Measure | Figure | Scope

Wetland-dependent bird species | Nearly 120 species in 21 families | Whole North American region (U.S. Geological Survey)

Breeding waterbird species | 40 species | U.S. portion of the region (National Wildlife Federation)

Species supported by U.S. potholes | At least 18 waterfowl, 96 songbird and 36 waterbird species | 64 million acres of U.S. potholes (university research group)

Other migratory species | 225 species besides ducks, geese and swans | Region as a whole (a Minnesota county resource)

Bird Groups Found in Pothole Wetlands

The supplied sources name the following groups and species. This is not a complete checklist.

• Dabbling and diving ducks: Redhead, Canvasback, Northern Pintail and Northern Shoveler are all noted as breeding in the region. Waterfowl in general, including ducks, geese and swans, use pothole wetlands.
• Coots: listed among the aquatic species of pothole marshes.
• Rails: the Yellow Rail is named specifically, and rails generally are listed among breeding waterbirds.
• Large waterbirds: American White Pelican and herons are named among the region's breeding waterbirds.
• Songbirds: counted in the dozens of species, though individual species are not named in the supplied material.
• Grassland birds: mentioned as part of the region's bird life, without detail in the supplied material.

Two specialist terms come up often. Waterfowl means ducks, geese and swans. Waterbirds is a broader term that also covers birds such as pelicans, herons, rails and grebes. Wetland-dependent is broader still and includes any species that relies on wetlands for part of its life cycle.

How Water Levels Shape Bird Use

The best-supported relationships in the sources are these:

• More ponds, more breeding ducks: research on Iowa's prairie pothole wetlands reports a positive relationship between breeding duck numbers and pond densities in the prairie region.
• Variety in water levels supports the breeding cycle: a review of prairie wetland hydrology notes that when wetlands with highly variable water levels exist in the same landscape, waterfowl can use them at different stages of nesting and breeding.
• Water held into the season has value: the EPA notes that by holding water later into the growing season, prairie wetlands can supply water for livestock and soil moisture.
• The region serves both breeding and migration: pothole wetlands provide habitat for breeding waterfowl and for birds passing through on migration.

Some evidence is less certain. A long-running prairie marsh research program studied how water levels affect the main parts of the prairie wetland ecosystem, but the supplied material does not include its specific findings. Researchers are also modeling whether constructed water-quality wetlands, built to receive farm tile drainage and surface runoff, could provide resources for spring migratory waterbirds. That work is a proposed approach, not an established result.

How the Wetlands Have Changed

Loss has been severe in parts of the region. In Iowa, the prairie marsh and pothole complex historically covered about 7.6 million acres. By 1980 it had been reduced to fewer than 30,000 acres. The supplied summary of that figure does not state the causes, so they are not assigned here.

Explanations for impacts on birds remain partly open. A study of pothole wetlands in Alberta concluded that wetland birds in the northern part of the region may show sensitivity to agriculture. The authors present this as a possibility rather than a settled cause. The U.S. Geological Survey has examined what climate change may mean for the region's wetland-dependent birds, but that study's conclusions are not included in the supplied material.

Shorebirds are a related concern. A 2008 science news report quoted an estimate that some shorebird species have probably declined 50 to 60 percent over 30 to 40 years, under the headline that their migratory wetland habitat is declining fast. That report is not specific to prairie potholes, and the supplied excerpt cuts off before naming the primary cause.

Conservation and Monitoring

• Land protection and restoration: the Prairie Pothole Joint Venture has worked to protect and increase wetland acreage by buying land, securing conservation easements and restoring wetlands.
• Regional planning: the Joint Venture has published a conservation strategy covering native prairie, grasslands and wetlands in western Minnesota.
• Condition assessment: researchers have tested indicators of wetland condition for the region's U.S. portion in the north-central states.
• Satellite tracking: a university team built a tool called the Prairie Pothole Mapper, which uses extensive aerial surveys to train a model that detects water in satellite imagery. The team frames the project around concern that wetland protections are fading. This reference does not assess current legal protections.

Conservation groups also link pothole protection to people, describing it as protecting waterfowl, hunting traditions and rural economies across North America.

Applying This in the Field

• Read the water, not just the birds. Because pothole water levels vary, the same basin can look and function very differently from one visit to the next. Note whether a wetland is full, shrinking or nearly dry.
• Look at clusters of wetlands. The evidence on pond density and variable water levels suggests that a landscape with many potholes of different depths offers more to breeding birds than a single isolated basin.
• Think about the season. The region matters for both breeding and migration, so the birds present depend on timing.
• Use managed sites. Federal wetland management districts in the region publish birding information. One such district in North Dakota reports that all duck species common to the region use its lands.
• Check conditions before going. This reference gives no current water-level readings. Local land managers are the appropriate source for present conditions and access rules.

Gaps in This Reference

The supplied evidence does not explain how prairie potholes formed geologically. It also does not provide species-by-species identification marks, peak timing for breeding or migration, or the specific causes behind the Iowa wetland losses. Species counts differ between sources and should be checked against the full original studies before being cited as precise totals.