Surveys of colonial-nesting waterbirds are needed to assess population trends and gain insight into the health of wetland ecosystems. Use of unmanned aerial systems (UAS) for such surveys has increased over the past decade, but possible sources of bias in surveys conducted with UAS have not been examined. We examined possible visibility biases associated with using a UAS to survey waterbird colonies in cypress-tupelo watersheds and coastal island habitats in Texas in 2016. We used known numbers of four waterbird decoy types, including Black Skimmers (Rynchops niger), terns, and white-and dark-plumaged herons, to estimate their detectability in each habitat. Six observers independently counted decoys from aerial imagery mosaics taken with a consumer-grade, off-the-shelf quadcopter drone. We used generalized linear mixed-effects models to estimate detection probabilities of each decoy type. Black Skimmers at the coastal island had a detection probability of just 53%. Detectability of both white-and dark-plumaged herons was lower in the canopied cypress-tupelo habitat than the coastal island. In addition, cloud cover > 50% further reduced detectability of white heron decoys in cypress-tupelo habitat. Use of the double-count method yielded biased-low abundance estimates for white-and dark-plumaged herons in canopied sites, suggesting that habitat differences were a greater source of bias than observer error. Black Skimmers were the only decoy type to be imperfectly detected at the coastal island, a surprising result given the stark contrast of their plumage with their sand and shell nesting substrate. Our results indicate that UAS-derived photographic surveys are prone to low detection probabilities at sites where vegetation occludes nests. In habitats without canopy, however, UAS surveys show promise for obtaining accurate counts of terns, white herons, and dark herons. 4 Corresponding author. monitoreos utilizando UAS son prometedores para obtener conteos precisos de gaviotas y garzas blancas y oscuras.

Aerial surveys can also disturb nesting colonial waterbirds, sometimes causing severe panic responses, nest abandonment, and delayed return times to the nest 13-15. It is common for birds to look up, scan more, or remain vigilant during aircraft overflights 16. Furthermore, prolonged vigilance can divert time and energy away from activities that are needed to increase individual fitness 17,18. However, some mixed-species nesting colonies are not significantly affected by aerial surveys, behaviourally or reproductively 19,20. Certain species, e.g., Least Tern (Sternula antillarum), Common Tern (Sterna hirundo), Gull-billed Tern (Gelochelidon nilotica), and Black Skimmer (Rynchops niger), do not react noticeably to manned aircraft, suggesting that overflights do not detract from their incubation behaviour 21. This is not always the case, as Adélie Penguins (Pygoscelis adeliae) are more prone to abandon nests after helicopter and fixed-wing aircraft surveys 15. Unmanned aerial vehicles (UAVs) have recently garnered attention for surveying wildlife. They have been used to survey a variety of avifauna, including Common Terns 22 , Black-headed Gulls (Chroicocephalus ridibundus) 23 , cliff-nesting seabirds 24 , and other species that lend themselves well to aerial photography. UAVs provide benefits that make them appealing for surveying wildlife, such as suitability for fine spatial resolution, researcher safety, transportability, cost, and the ability to easily switch out sensors or payloads 25. Perhaps the most important potential benefit UAVs provide is a small and quiet platform, which could potentially be less disturbing to wildlife than manned aircraft. Recent studies have found that some species of waterbirds are not significantly disturbed by UAV surveys when flown in horizontal transects 22,26-28. However, for Adélie and Gentoo Penguins (Pygoscelis papua), lower survey altitudes can exacerbate disturbance behaviour even when the UAV is flown horizontally 29. Due to the recent popularity and availability of UAVs among researchers, managers, and recreationists, there is a need to continue studying their effects on waterbirds. Knowledge gaps remain, especially regarding methodological best practices and operating thresholds in wildlife research and management. Although recent research has addressed disturbance caused by approach angle 24,26 , hovering 24,30 , and low altitude flyovers 30,31 , few studies have taken into consideration the impact of UAV surveys intended for obtaining high-resolution mosaicked orthophotos of large nesting sites. This type of survey is generally flown in horizontal strip transects and at reasonable heights, which enable photogrammetric ease and image stitching 32. For waterbirds in particular, few studies have investigated disturbance caused by horizontal UAV surveys at heights greater than 50 m 22,33 , even though it has been suggested that count accuracy does not significantly increase for imagery below 90 m 34. Safe operating thresholds are needed not only for wildlife researchers,...