Evaluation of Aerial Surveys To Quantify Shorebird Abundance and Distribution 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 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 an accurate estimate of shorebird use of stopover sites in the IRV. During July-September 2017-2019, and April-May 2018-2019, we conducted concurrent ground and aerial surveys at 5-7 sites per week. Additionally, a single observer counted and assigned all shorebird detections to either "large" (Killdeer (Charadrius vociferous) and larger) or "small" (Pectoral sandpiper (Calidris melanotos) and smaller) size classes, and recorded wetland habitat characteristics at a total of 96 surveyed sites in the IRV. The use of ground counts each survey allows for the calculation of aerial detection probability count bias, while using habitat data of the specific count location from the ground as a correction factor. The aerial observer detected shorebirds at 114 out of 116 sites (98%) where shorebirds were present, and did not detect individuals at 8 out of 8 sites where no shorebirds were present. Preliminary results show aerial surveys detected 89% (N = 93, Range = 0%-250%) of individuals counted during ground surveys. We will present results of ongoing analyses in regards to quantifying count bias and occupancy in relationship to weekly habitat changes throughout the IRV. Aerial surveys appear to be an accurate and efficient method to quantifying shorebird abundance along large-river systems.