Visibility Bias and Disturbance of Waterfowl During Aerial Surveys

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 develop 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 visibility bias. Preliminary results indicate that mean detection rate for all waterfowl was 93% (SE=5%). Mean detection rate was 91% (SE=6%) for ducks, 96% (SE=7%) for dabbling ducks, 88% (SE=14%) for diving ducks, and 92% (SE=4%) for geese. Observers also documented disturbance to waterfowl caused by aerial surveys. Preliminary findings indicated 14% (SE=2%) of waterfowl, 10% (SE=1%) of ducks, 10% (SE=1%) of dabbling ducks, 6% (SE=1%) of diving ducks, and 21% (SE=3%) of geese exhibited negative responses (i.e., flew short distances, swam away, changed behavior significantly) to aerial surveys. Preliminary findings indicated that 4% (SE=1%) of waterfowl, 2% (SE=1%) of ducks, 1% (SE=1%) of dabbling ducks, 3% (SE=1%) of diving ducks, and 9% (SE=2%) of geese abandoned survey sites and did not return following aerial surveys. Traditional aerial surveys conducted in the Mississippi and Illinois River floodplains can be adjusted for visibility bias and compared with population estimates from randomized surveys to compare cost and time efficiency of aerial survey techniques.