Accurate abundance estimation of cliff-breeding Bounty Island shags using drone-based 2D and 3D photogrammetry

Abstract

Effective seabird management strategies rely on accurate population estimates, with previous methods typically employing ground counts of a target species. However, difficult and often inaccessible breeding habitats are now able to be explored due to recent technological advancements in Unoccupied Aerial Vehicles (UAVs). This study tested a novel approach by combining high-resolution orthomosaics and 3D models to provide population estimates of the remote cliff-breeding Bounty Island shag (Leucocarbo ranfurlyi) on the sub-Antarctic Bounty Islands in November 2022. Our results report 573 breeding pairs, estimating a total population of approximately 1733 birds, breeding on 13 of the 14 main islands. Given the topographical constraints of surveying the islands by boat, the most comparable assessment in 1978 shows a similar count of breeding pairs, proposing the Bounty Island shag population is stable. However, long-term monitoring and additional research surrounding foraging strategies is crucial for developing conservation efforts for one of the rarest and spatially restricted shag species in the world. Our study demonstrates a reproducible method for estimating elusive wildlife populations that can be used across species with wider applications.