Drone assessment of habitat selection and breeding success of Gull-billed Tern Gelochelidon nilotica nesting on low-accessibility sites: a case study

Abstract

The use of drones in the study of waterbird breeding biology has received considerable attention in the last years, but very few studies were made along the Mediterranean. We studied habitat selection and breeding success of the Gull-billed Tern Gelocheli-don nilotica in two colonies inside fish farms along the Italian Adri-atic coastline, using a small, commercial drone. Both colonies were located on small islets surrounded by very shallow water and clayey bottoms, thus being difficult to access. Compared with availability, the Gull-billed Terns selected quadrats higher above the water level, with a higher vegetation cover, which lay farther from the water edge, con-tain less water, and which are more frequently located in the center of the islands. 147 of 178 clutches (mean clutch size ± SD: 2.61 ± 0.58) hatched 383 chicks (82.5%; 2.15 ± 1.09 chicks per nest), with large differences between the two colonies. Hatching success was 95.0% and 69.1%; the most common cause of egg loss was flooding (97.0%). Nest attendance could easily be ascertained by 30-m above ground level drone-derived imagery. The use of a drone allowed the study of some aspects of the breeding biology of the Gull Billed Tern in two breeding sites where the traditional field approach, i.e. researchers reaching the sites by boat, would have been very difficult, causing unavoidable and prolonged disturbance to the nesting adults.