Drone-induced flight initiation distances for shorebirds in mixed-species flocks.
Abstract
Drone use has increased in the last decade prompting management efforts to regulate flights near vulnerable wildlife; however, these efforts are hindered by a lack of data characterising drone-induced wildlife disturbance. Many shorebird populations are rapidly declining and efforts to survey them can be hampered by the inaccessibility of their habitat. Drone surveys might overcome this barrier, although there is a risk that increased drone use near shorebirds may cause disturbance and exacerbate declines. We characterise drone-induced disturbance for mixed-species shorebird flocks to inform the management of drones near shorebirds. We conducted 240 drone approaches of mixed-species flocks containing roosting non-breeding shorebirds in Moreton Bay, eastern Australia. We performed approaches horizontally at varying vertical distances and recorded metrics characterising the drone, flock and environment. This enabled us to estimate the factors influencing disturbance as well as the distance of an approaching drone at which the flock is likely to take flight. We determined the drone-induced flight initiation distance for 12 species of waterbird, including seven shorebird species. We could not approach Eastern Curlew Numenius madagascariensis at any vertical distance below the recreational drone limit of 120 m without inducing flight; however, for all other species, there was less than 20% probability that they would take flight when approached by a drone at vertical distances above 60 m. We do not recommend approaches below 30 m. We also found that flight initiation of mixed-species flocks is largely dictated by the most sensitive species present, and that disturbance generally increases with approach velocity, drone noise/size, decreasing distance, and if the drone is obscured by trees. Policy Implications. We conclude that (i) drone use needs to be carefully regulated to ensure roosting shorebird flocks are not approached within distances that will disturb the most sensitive species likely to be present, (ii) researchers contemplating drone surveys need to carefully evaluate the risk of disturbance, especially where there are mixed-species flocks, and (iii) that alternatives to drone surveys should be sought wherever disturbance data for the species potentially encountered are unavailable.