Waterfowl, marsh birds, shorebirds, and other waterbirds rely on wetlands to meet dynamic physiological needs. However, the midwestern United States has undergone extensive wetland loss, and extant wetlands are often degraded. Currently, there is limited information available to estimate the quantity of suitable habitat for wetland-dependent birds to prioritize wetland management and inform conservation planning processes. Furthermore, the hydrology and other physical characteristics of wetlands that determine their suitability for waterbirds often differ among species, foraging guilds, and temporally. The suitability of stopover wetlands used by spring migrants influences survival and body condition of some species as they prepare for breeding, and most secretive marsh birds species have experienced population declines presumably linked to losses in quantity and suitability of habitat during the breeding season.
Therefore, I identified factors that influenced the availability of suitable waterbird habitat and provided baseline data on the spatial distribution and extent of wetlands capable of supporting migratory and breeding bird populations in Illinois. I assessed habitat conditions in Illinois wetlands during spring, summer, and autumn 2016 β 2017 across Illinois Natural Divisions. Each spring, I surveyed wetland sites for dabbling ducks and other waterbirds (e.g., geese, diving ducks, herons) using aerial surveys, and conducted call-broadcast surveys for secretive marsh birds at sites with emergent vegetation spanning the marsh bird breeding and migration chronology in Illinois. I assessed wetland suitability for dabbling ducks, marsh birds, and shorebirds; waterfowl and other waterbird abundances; and marsh bird occupancy as a function of local wetland characteristics, landscape context and integrity, and wetland management practices and characteristics to assist conservation planners prioritize wetland restoration and enhancement in the Midwest, USA.
Across all survey periods, suitable vegetative and hydrological conditions for wetland-dependent bird guides comprised a small portion of the total area of wetlands and deepwater habitats within the National Wetlands Inventory. Furthermore, important vegetation cover and inundation levels considered suitable for waterbirds varied among NWI classes and Illinois Natural Divisions. Suitable emergent vegetation for migrating and breeding marsh birds and mudflats and shallowly inundated foraging habitats for migrating shorebirds were particularly limited during survey periods, and wetlands with greater complexity and connectivity to other wetland types offered the greatest proportion of suitable habitat resources. Surface water inundation and vegetation coverage were the most important predictors of waterbird density during spring migration; for every 10% increase in the proportion of inundated vegetation, dabbling duck density increased 34.8%. Similarly, local wetland characteristics such as inundated persistent emergent vegetation and forested cover, and to a lesser extent, persistent emergent vegetation, were important components predicting marsh bird occupancy in Illinois.
My study highlights a poor representation by the National Wetlands Inventory of the quantity of suitable habitat conditions for migratory wetland-dependent birds, specifically wetland inundation and vegetation cover. Future emphasis should be placed on identifying variables appropriate for predicting wetland suitability for waterbirds by combining National Wetlands Inventory data with other available spatial data and addressing demographic responses (e.g., survival, nest success, breeding propensity) of waterbirds to suitable habitat in Illinois since suitable habitat appears to be very limited for some waterbird guilds
