Abstract
Estimating the population of burrow-nesting seabirds is a challenging task, as human presence in the colony creates disturbances and can damage burrows and occupants. Here, we present a novel method using aerial photographs taken with Unmanned Aerial Vehicles (UAVs) to estimate the population size of a burrow-nesting seabird, the Black-vented Shearwater (Puffinus opisthomelas), on Natividad Island, Mexico. Our results provide a census of burrows in the colony, with very low detection error (5.6%). This is greater accuracy compared to other methods based on extrapolating results from sample plots to total colony area. We then combined this burrow census with ground truth data on occupancy to estimate population size. We obtained a population estimate of 37,858 and 46,322 breeding pairs for 2016 and 2017 respectively. The proposed method provides a cost effective and repeatable approach for monitoring numbers of burrows occupied in a colony, thereby enabling easier and faster estimates of population trends. We suggest this method can be valid for other burrow-nesting species in habitats without dense vegetation cover.
Highlights
Seabirds are among the most threatened of bird taxa, with 42% of 365 species listed as threatened on the IUCN Red List [1]
Accurate population sizes and trends are challenging given they often breed in remote sites, are long-lived life-history and have vast at-sea distributions (2,3,4)
Robust colony and population estimates are critical for understanding trends and informing conservation management of the status of burrowing seabird populations
Summary
Seabirds are among the most threatened of bird taxa, with 42% of 365 species listed as threatened on the IUCN Red List [1]. Accurate population sizes and trends are challenging given they often breed in remote sites, are long-lived life-history and have vast at-sea distributions (2,3,4). For burrowing petrels, estimating colony sizes is even more difficult than for surfacenesting seabirds because it requires accurate identification of usually cryptic burrows and nocturnal behavior, where nest occupancy requires ground truth [2, 3]. Robust colony and population estimates are critical for understanding trends and informing conservation management of the status of burrowing seabird populations. Improving the accuracy and repeatability of burrow counting methods, plus lowering the cost and likelihood of disturbance to colonies, remains important to improve population size estimates and understanding population trends.
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