Feasibility of counting breeding Pied Avocets and Black-winged Stilts using drones

Drones are increasingly used for waterbird counting, but data on the accuracy of such counts are scarce for wader species. We compared both the effectiveness for bird detection and safety of adults and nests using drone survey methods as opposed to a traditional ground count method on breeding Black-winged Stilts Himantopus himantopus and Pied Avocets Recurvirostra avosetta in the Lagoon of Venice (north-eastern Italy). Drone flights found a significantly smaller number of breeding pairs than ground counts, with 18.1% and 20.5% of Black-winged Stilt and Pied Avocet pairs, respectively, being missed by drone surveys. As a consequence, the colony sizes estimated using a drone for both species were smaller than those estimated during ground counts. Furthermore, drone counts were inflated by 2% due to ‘false’ breeding pairs for both species – further biasing the accuracy of drone surveys in comparison with traditional ground counts. Restricting our analysis to Recurvirostridae nesting in small colonies greatly increased the accuracy of the detection rate using drones. Drone surveys caused far less disturbance than ground/boat intrusions, and the time spent off nests was considerably longer during ground counts than during drone surveys. As expected, the average time it took to survey a colony with a drone was lower than with the traditional approach. No apparent negative effects of either survey method on nesting pairs or clutches were observed. Our study shows that drones may be used as a powerful tool for counting small colonies of Recurvirostridae, but they are not an alternative method to ground surveys for census programmes as a whole.