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AUTHORS: Andrew D. Gilbert, Western Illinois University, Illinois Natural History Survey; Heath M. Hagy, Illinois Natural History Survey; Christopher N. Jacques, Western Illinois University; Aaron P. Yetter, Illinois Natural History Survey
ABSTRACT: Aerial waterfowl surveys have been conducted in the Illinois and Mississippi River floodplains since 1948. These traditional surveys provide an index of waterfowl population size and are used to track migration events, set harvest regulations, and for research purposes. New methods are being evaluated to estimate population size by randomizing survey locations and estimating count bias. We used double sampling to determine a correction factor for waterfowl estimates during fall aerial surveys. Immediately before an aerial survey, a ground observer surveyed waterfowl in predetermined locations from an elevated, unobstructed location where probability of detection was assumed to be 100%. Aerial counts were divided by ground counts for all common species and foraging guilds to determine count bias. Preliminary results indicate that mean detection rate for all waterfowl was 96.0% (SE=7%). Mean detection rate was 94.4% (SE=8%) for ducks, 105.2% (SE=11%) for dabbling ducks, 74.8% (SE=11%) for diving ducks, 53.3% (SE=8%) for mergansers, and 92.4% (SE=9%) for geese. Observers also documented disturbance to waterfowl caused by aerial surveys. Preliminary findings indicated 18.4% (SE=2%) of waterfowl, 12.2% (SE=2%) of ducks, 11.5% (SE=2%) of dabbling ducks, 4.5% (SE=1%) of diving ducks, 13.0% (SE=3%) of mergansers, and 28.6% (SE=4%) of geese exhibited negative responses (i.e., flew short distances, swam away, changed behavior significantly) to aerial surveys. Preliminary findings indicated that 5.5% (SE=2%) of waterfowl, 2.0% (SE=1%) of ducks, 1.2% (SE=1%) of dabbling ducks, 0.7% (SE=1%) of diving ducks, 4.3% (SE=1%) of mergansers, and 15.1% (SE=3%) of geese abandoned survey sites and did not return following aerial surveys. With our findings, traditional aerial surveys conducted in the Mississippi and Illinois River floodplains can be adjusted for count bias and compared with population estimates from randomized surveys to compare cost and time efficiency of aerial survey techniques.
