Abstracts

Presented by Andrew D. Gilbert - Email: agilb849@illinois.edu

Wetland managers commonly use aerial surveys to monitor the distribution and abundance of waterbirds to aid in population management and habitat conservation. However, most existing surveys only provide abundance indices that are uncorrected for visibility bias, which may limit their utility in accurately assessing local population size. Moreover, excessive disturbance from low-altitude aerial surveys may cause waterfowl to increase energy expenditure and exposure to hunting mortality contrary to the management objectives of many surveyed sanctuaries. We used concurrent ground and aerial surveys to estimate visibility bias and disturbance during cruise-style waterfowl surveys from September through January 20142017 in the Midwest, USA. We found the aerial observer had high detection rates and low count bias across waterfowl guilds, resulting in low overall visibility bias of ducks (11% 5% [SE], n = 124), geese (8% 3%, n = 70), and swans (5% 3%, n = 37). Group size, species prevalence, cloud cover, and temperature influenced visibility bias, but the direction and magnitude of effects were variable among taxa. We found disturbance and abandonment probabilities of waterfowl (x = 14% 2 and x = 3% 1, respectively) and other waterbirds (x = 13% 2% and x = 2% 1%, respectively) during aerial surveys to be low. Among waterfowl taxa, the odds of aircraft disturbance was 2.26.2 times greater at wetlands closed to waterfowl hunting than open wetlands, but wind speed and cloud cover did not impact disturbance rates of any taxa. The odds of disturbance for geese decreased 4.6% for every 1 increase in temperature. Low visibility bias, disturbance, and abandonment probabilities for most guilds indicate that well-designed, cruise-style aerial surveys provide reliable estimates of population size and periodic surveys are compatible with objectives of providing sanctuary conditions for most waterfowl.