Wetland Quantity and Quality for Waterbirds in Illinois

Waterbirds use a variety of wetland types during critical periods, such as spring migration. However, the amount wetland area in the Midwest has decreased, and remnant wetlands are often degraded and may not meet habitat needs for waterbirds. The National Wetland Inventory (NWI) is currently the most comprehensive measure of wetland quantity in this region, but unfortunately, NWI lacks data regarding the timing, depth, and persistence of surface water inundation. Therefore, current information regarding wetland quantity may overestimate wetland availability for waterbirds because many wetlands are not inundated and accessible to waterbirds during the appropriate periods. We estimated wetland quantity for waterbirds during critical periods in Illinois and modeled factors that affect wetland quality. We surveyed wetland vegetation characteristics, potential hydrologic stressors, waterbird occupancy, and management intensity on approximately 100 wetland plots (0.25 km2) during three sample periods critical to focal waterbirds (i.e., spring [migrating dabbling ducks], early summer [nesting marsh birds], late summer [migrating shorebirds]) across the state of Illinois. Within each plot, we mapped inundated area of NWI polygons (Forested, Emergent, Pond, Lake, River). Water depth and vegetation cover were assessed for each polygon type. Additionally, high-resolution aerial photographs of all plots and the surrounding area during each sample period were obtained. During spring, less than 30% of any wetland type was flooded shallowly and accessible for foraging dabbling ducks. Likewise, emergent vegetation (<10%) and overall vegetation (<25%) were rarely inundated. Summer inundation rates were greater than spring, especially in forested (34%) and emergent (61%) polygons. During fall, flooded area was similar to spring, but shallow inundation was slightly greater. However, mudflats, critical for shorebirds, comprised less than 20% of any wetland type. Based on preliminary results, most wetland area assessed by the NWI in Illinois is not flooded at suitable depths for waterbirds during critical periods.