Waterfowl aerial inventories | Forbes Biological Station | Illinois Natural History Survey

Hen wood duck in flight over pale green field.

Waterfowl Aerial Inventories

The Illinois and Mississippi river valleys are major migration and wintering areas for nearly 30 species of waterfowl in the Mississippi Flyway. These areas also provide significant recreational opportunities (e.g., waterfowl hunting and bird watching) and economic support for the local and regional economies.

Approximately 214 miles of the Illinois River, from Spring Valley to Grafton, and 272 miles of the central Mississippi River, from Alton to Moline, are aerially inventoried for waterfowl weekly from September through early January.

The purpose of these aerial inventories is not to acquire complete counts of waterfowl numbers within specific geographic areas but to estimate the number of each species in order to provide an index of temporal changes within and among years and to document the distribution of the species throughout the monitored regions. Because of the efforts of INHS with the long-term support of the Illinois Department of Natural Resources, the annual inventory data on numbers and distribution of migratory waterfowl in Illinois is unequaled.

We aerially inventory 23 locations on the Illinois River and 18 locations on the central Mississippi River weekly. Flights are generally scheduled for each Monday beginning in early September, and once a flight is completed, results are posted as soon as possible.

Staff Lead: Josh Osborn

INHS Aerial Inventory Locations as of 2014.

INHS Waterfowl Aerial Inventory Data, 2025 – present

Illinois River	Central Mississippi River	Shorebird counts	Blog
1/12/26	1/12/26
1/7/26	Weathered Out
1/2/26	1/2/26
12/22/25	12/22/25
12/16/25	12/16/25
12/09/25	12/09/25
12/05/25	12/05/25
11/28/25	11/28/25
11/13/25	11/13/25		11/13/25
Weathered Out	Weathered Out
10/31/25	10/31/25
10/23/25	10/23/25		10/23/25
10/14/25	10/14/25		10/14/25
9/26/25	9/26/25
9/19/25	9/19/25		9/19/25
9/11/25	9/11/25	9/12/25	9/11/25
9/5/25	9/5/25	9/3/25
8/27/25	8/27/25	8/25/25	8/27/25
		8/21/25
		8/13/25
		8/6/25
		5/30/25
		5/22/25
		5/12/25
		5/6/25
		4/21/25
		4/16/25
4/8/25	4/8/25
3/26/25	3/26/25
3/12/25	3/12/25
3/6/25	3/6/25
2/20/25	2/20/25
2/10/25	2/10/25
1/7/25	1/7/25
1/3/25	1/3/25

IDNR’s archive of Waterfowl Aerial Surveys can now be found on Hunt Illinois. They also have older inventories from 2009–2020.

Send questions to Facilities Manager Josh Osborn.

Why Are Surveys Important?

Data from aerial inventories are used to direct waterfowl management, conservation planning for habitat acquisition, ecological research, and for public outreach. There are many important private, state, and federal waterfowl areas and refuges within these river floodplains, such as the Two Rivers and Great River National Wildlife Refuges (NWR), the Illinois River National Wildlife and Fish Refuges, and Keokuk Pool. Aerial survey results are frequently requested by and circulated among federal and state personnel. Specifically, the IDNR relies on these inventories to guide the establishment of hunting season dates, zones, and other regulations and to prioritize wetland habitat acquisitions. Previously, this database has been used by the Mississippi Flyway Technical Section and Council to monitor the abundance and distribution of migrating waterfowl, especially canvasback, mallard, and northern pintail. The Upper Mississippi and Great Lakes Region Joint Venture uses survey data to assist in parameterizing carrying capacity models and identifying priority regions for conservation. Requests for inventory information are received annually from state, federal, and private-sector entities to be used for projects such as Environmental Management Programs, scientific publications, theses and dissertations, formal presentations, education, newspaper and magazine articles, and many other uses.

Considering the varied uses of the data and the fluctuating population levels of many waterfowl species, continuation of the monitoring of waterfowl populations at a baseline level is desirable to maintain the consistency and extension of the long-term data set into the future. Monitoring the distribution and relative abundances of waterfowl and waterbirds over time is critical to detecting changes in migration corridors and can be used to identify research needs and indicate changes in habitat quality. Future threats to wildlife habitats and ecosystem services provided by rivers, lakes, and wetlands, which are surveyed annually (e.g., climate change modeling, habitat suitability modeling, flood prediction modeling), may be better understood and their effects predicted using this unique and long-term dataset.

How Are the Surveys Conducted?

Weekly during autumn, a highly skilled biologist and pilot use a fixed-wing aircraft to visit wetlands and lakes that historically hold more than 90% of the waterfowl and other waterbirds present in the Illinois and central Mississippi River valleys. Along the Illinois River, these sites are located between Hennepin and Grafton, IL, and from Grafton to New Boston, IL, along the central Mississippi River. Since 1948, only 4 different biologists have conducted the aerial surveys using standardized counting methods. The pilot often makes two passes around each site during each survey. During the first pass, the biologist estimates the number of birds present at each site. On the second pass, the biologist estimates species composition within each site. Sometimes, individual sites hold more than 100,000 waterbirds, and multiple passes may be required for the biologist to count birds and assign species composition. Because it is important to enumerate waterbirds by species, the pilot often circles between 200 and 300 feet above ground level, which requires skill and special permission from the Federal Aviation Administration.

What Should You Do If You See the Plane?

If you see our biologist counting ducks, feel free to watch and wave. If you notice our airplane while boating or driving near large groups of waterfowl, we would appreciate it if you could stop when it is safe to do so, to prevent birds from flushing in front of the airplane. After the count is completed, our biologist might even have time to wave to say “thanks!”

Do the Surveys Affect Waterfowl Distribution?

We have little information to indicate that ducks and geese leave areas en masse following our surveys. In fact, we have conducted ground surveys simultaneously with aerial surveys along the Illinois River and determined that approximately 13% of waterfowl exhibited a response to the airplane and that only 6% of waterfowl abandoned an area and did not immediately return during or following an aerial survey. Only 8% of ducks exhibited a response during aerial surveys, and <3% abandoned survey areas. Typically, aerial grid surveys last <4 minutes/site. Anecdotally, other sources of disturbance (e.g., raptors such as eagles, boats, hunting groups, barges, etc.) have a much greater effect on distributions of waterfowl and other waterbirds during autumn than do aerial surveys.

Questions or Concerns?

If you have questions or concerns regarding our aerial surveys, please contact Principal Investigators Auriel Fournier or Josh Osborn at the Forbes Biological Station (217-332-DUCK).