Estimating occupancy and abundance of shorebirds through aerial surveys in the Illinois river valley
"Shorebirds are one of multiple guilds of wetland birds that have been experiencing population declines over the last 50 years. These species migrate long distances between northern breeding grounds and southern wintering grounds, and many need to stop and refuel along the way. 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 can be used to accurately 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 vociferus) and larger) or ""small"" (Pectoral sandpiper (Calidris melanotos) and smaller) size classes, and recorded wetland habitat characteristics at a total of 96 sites in the IRV weekly. Dynamic occupancy analyses showed the prevalence of wet mud drove site occupancy, and higher occupancy rates were observed in the fall than the spring. Abundance analyses also found mud availability was also the driving factor in site abundance. Overall abundance and wet mud availability varied by season, with 15 times more shorebirds and more than twice the amount of wet mud available in the fall. Managers should focus on progressively exposing wet mud for migrating shorebirds especially during July–August, and also in May if the Illinois River level is low enough for managers to manipulate water levels."
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign