Recounting bias can affect abundance estimates from intensive helicopter surveys of feral goats

Context Aerial surveys are widely used for estimating the abundance of wildlife over large areas. The failure to count all animals within survey transects is commonly acknowledged and there many techniques measure correct underestimation. However, possibility being counted more than once in intensive surveys, which leads overestimation, rarely examined. Animals can move response observers or vehicles, bias occur when before after detection. Movement immediately prior associated with observation processes methodologically accommodated distance sampling but attributable responsive movement platforms have passed requires investigation. Aims We sought investigate potential biases caused by animal during helicopter feral goats, quantify probability that available recounting because their movements. Methods Using ground-based behavioural studies simultaneous strip we measured extent movement, distances directions moved, design parameters, contrasted those random Key results Feral goats did not randomly helicopters. transect strips, therefore potentially visible from aircraft, were likely outside transect. Considerable (flushing) occurred between (64%, n = 448) moved towards unsampled already sampled. Because spatial separation transects, 21% adjacent leading overestimation. Conclusions Although most extensive macropods other Australia account overestimation design, sample intensively apply valid corrections undercounting produce positively biased estimates where flushing occurs. Likewise, thermal could be subject positive prone flushing. This routinely ignored management research close estimate abundance. Implications Responsive consideration designing aerial wildlife. Randomised without replacement larger will counteract bias.