Quantifying Shorebird Abundance and Distribution Through Aerial Surveys in the Illinois River Valley

The Illinois River Valley (IRV) serves as a crucial stopover area for migratory shorebirds in the midwestern United States despite the high prevalence of row crop agriculture and extensive rates of wetland loss and degradation in the region. Aerial surveys are commonly used to quantify waterfowl abundance and estimate population size, but few attempts have been made to evaluate aerial surveys for other guilds of wetland birds. We investigated whether aerial surveys may provide a good estimate of shorebird use of stopover sites. During July-September 2013-2017 we conducted concurrent ground and aerial surveys at 3-7 sites per week. Additionally, we undertook a complete survey of the IRV from Turner Lake to Meredosia Lake, where a single observer counted and assigned all shorebird detections encountered to either "big" or "small" size classes, and recorded wetland habitat characteristics at all 95 surveyed sites. The use of ground counts each survey allows for the calculation of aerial detection probability, while using habitat data of the specific count location from the ground as a correction factor for visibility bias. Aerial surveys detected 91% (N = 29, Range = 0%-178%) of individuals counted during ground surveys and detection rates appeared independent of effects of weather. The total number of shorebirds counted in the IRV each week ranged from 4,860 to 18,120, with an average of 12,024 birds. Aerial surveys appear to be an accurate and efficient method to quantifying shorebird abundance along large-river systems. Subsequently, we will replicate this study in multiple years, and test factors that affect detection rates and survey efficacy.Future plans include questions associated with stopover duration and which species are present in the IRV.