The Thief Lake IBA encompasses an extensive area of aspen parkland landscape in northwestern Minnesota, approximately 300 miles northwest of the Twin Cities. The IBA is located in northeastern Marshall County, extending into south-central Roseau County . Several small towns near the IBA include Gatzke, Newfolden, and Middle River to the south, Strandquist and Karlstad to the west, and Strathcona, Wannaska and Skime to the north.
Thief Lake IBA is situated near the edge of the Aspen Parkland region, a complex transition zone between the prairie and coniferous forest biomes, which extends from northwestern Minnesota through the Canadian prairie provinces. Much of the IBA overlies a series of higher beach ridges created by Glacial Lake Agassiz, which covered northwestern Minnesota, eastern North Dakota, and large portions of Ontario and Manitoba at the end of the last Wisconsin glaciation period. The Thief Lake IBA is located midway between the extensive Kittson-Roseau Aspen Parkland IBA, 4-5 km to the northwest, and Agassiz National Wildlife Refuge IBA, 3-4 km to the south.
In the United States, tallgrass aspen parkland occurs only in Minnesota. The Thief Lake IBA encompasses the second largest area of aspen parkland remaining in the state (the Kittson-Roseau Aspen Parkland IBA, just to the northwest, is the largest). This IBA lies within the Aspen Parkland Physiographic Area (Partners In Flight Area 30), which harbors the highest number of breeding birds of any physiographic area on the continent. Thief Lake is one of the most important marshy lakes in the entire Aspen Parkland region in Minnesota.
Thief Lake IBA, like the nearby Kittson-Roseau Aspen Parkland IBA, is located in a relatively remote area, infrequently visited by most birders. Surveys for breeding birds, particularly nongame species, have been very limited. Most information on numbers of birds is based on surveys by the Minnesota Department of Natural Resources? Minnesota County Biological Survey (MCBS). More than 192 bird surveys (point counts and species lists) were conducted within the IBA by MCBS. The maximum numbers listed represent the number of MCBS surveys at which a species was detected. However, many areas have not been surveyed. Actual numbers of breeding pairs are undoubtedly much higher than indicated.
Historically, frequent fire was an important natural process essential in maintaining the open character of the aspen parkland landscape that dominates much of this IBA. In the absence of fire, open grassland and sedge habitats have become overgrown with undesirable levels of woody vegetation such as willow shrubs and aspen. Some lands, particularly those owned by the Minnesota DNR (Wildlife Management Areas), are being managed through prescribed burning in an effort to maintain and/or enhance the open character of the aspen parkland landscape. This is an ongoing effort, often contingent on available funds for management activities. However, many such areas are not being burned, and have succeeded to aspen forest and other tree and shrub-dominated habitats which typically are of much lower value to priority bird species in this IBA.
Approximately 43 percent of this IBA (99,030 acres), including most of the important tracts, is owned and managed by the Minnesota DNR Section of Wildlife.