Drones improve effectiveness and reduce disturbance of censusing Common Redshanks Tringa totanus breeding on salt marshes

Censusing breeding Common Redshanks Tringa totanus is hard, in view of the difficulty to access the breeding sites, find the highly camouflaged nests and avoid disturbing nesting birds. Drones are increasingly used to census waterbirds, owing to their capacity to overcome most of these difficulties. The present study aims to compare effectiveness, managerial efficiency and safety for birds of drone-conducted counts on a population of Redshanks breeding on a saltmarsh area of the Lagoon of Venice (NE Italy). Thirty-one drone flights on 29 marsh islands covering 6940 m, were conducted. We divided surveys into transects, each 20 m long and 10 m wide. In 2017-2018, drone flights counted 99 Redshank pairs, versus a count of 75 from ground censuses, in an area of 6.94 ha. Ninety-one breeding pairs of Redshanks were ascertained, combining results obtained with both methods. Drone surveys achieved a far better sensitivity and a slightly worse specificity than ground censuses. Overall, the average coefficient of agreement between methods was good, classified as substantial (Cohen's kappa = 0.76). Mean time spent to survey a transect of 20 m with a drone was far less than with the traditional approach: 4.2 +/- 0 vs. 137 +/- 29 s. Overall, 13.2 h were spent on censusing the whole area by the traditional approach vs. 0.46 h using the drone (-96.5%). Excluding the cost of the drone, this corresponded to an operational cost reduction of 94%. No apparent negative effects on nesting pairs or clutches were observed. Our study shows that drone flights constitute the least invasive option for studies on the breeding biology of Redshanks nesting at low densities on saltmarshes. Using a drone improved the effectiveness of nest finding and reduced disturbance to incubating birds. Integrating this new tool into the traditional ground searching approach would allow researchers to more rapidly identify nests, while at the same time reducing costs.