Escape behaviour of feral horses during a helicopter count

Animal escape behaviour in response to aircraft could influence the accuracy and precision of aerial estimates of population size but is rarely investigated. Using independent observers on the ground and in the air, we recorded the behaviour of 17 groups, including 136 individually marked horses (Equus caballus), during a helicopter count in New Zealand's Kaimanawa Mountains and compared the helicopter count with a ground-based mark-resight estimate in the same area (20.5 km2). The helicopter induced running and changes in group size and composition in all horse groups that travelled from 0.1 up to 2.75 km before leaving the ground-observer's view. One-tenth of marked horses were not counted and a quarter counted twice. The possible double-counting of a further 23 (17%) could not be confirmed because only two of the three observers' records concurred. Thus, the helicopter count over-estimated the marked sub-population by at least 15% and possibly by up to 32%. The helicopter count (228 horses) was 16.9% larger than the mark-resight estimate (195, 95% CI = 157-234). We identify the characteristics of the helicopter count that stimulated horse escape behaviour and discuss how it should be considered in the design of aerial population-estimate methods.