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 are often used to quantify sizes of waterbird colonies; however, these surveys would benefit from a better understanding of associated biases. We compared estimates of breeding pairs of waterbirds, in colonies across southern Louisiana, USA, made from the ground, fixed‐wing aircraft, and a helicopter. We used a marked‐subsample method for ground‐counting colonies to obtain estimates of error and visibility bias. We made comparisons over 2 sampling periods: 1) surveys conducted on the same colonies using all 3 methods during 3–11 May 2005 and 2) an expanded fixed‐wing and ground‐survey comparison conducted over 4 periods (May and Jun, 2004–2005). Estimates from fixed‐wing aircraft were approximately 65% higher than those from ground counts for overall estimated number of breeding pairs and for both dark and white‐plumaged species. The coefficient of determination between estimates based on ground and fixed‐wing aircraft was ≤0.40 for most species, and based on the assumption that estimates from the ground were closer to the true count, fixed‐wing aerial surveys appeared to overestimate numbers of nesting birds of some species; this bias often increased with the size of the colony. Unlike estimates from fixed‐wing aircraft, numbers of nesting pairs made from ground and helicopter surveys were very similar for all species we observed. Ground counts by one observer resulted in underestimated number of breeding pairs by 20% on average. The marked‐subsample method provided an estimate of the number of missed nests as well as an estimate of precision. These estimates represent a major advantage of marked‐subsample ground counts over aerial methods; however, ground counts are difficult in large or remote colonies. Helicopter surveys and ground counts provide less biased, more precise estimates of breeding pairs than do surveys made from fixed‐wing aircraft. We recommend managers employ ground counts using double observers for surveying waterbird colonies when feasible. Fixed‐wing aerial surveys may be suitable to determine colony activity and composition of common waterbird species. The most appropriate combination of survey approaches will be based on the need for precise and unbiased estimates, balanced with financial and logistical constraints. (JOURNAL OF WILDLIFE MANAGEMENT 72(3):697–706; 2008)

Abstract. Artificial light is increasing in coverage across the surface of our planet, impacting the behavioral ecology of many organisms. Attraction to sources of artificial light is a significant threat to certain fledgling shearwaters, petrels (Procellariidae), and storm-petrels (Hydrobatidae) on their first nocturnal flights to the sea. Disorientation by light can cause these birds to crash into vegetation or manmade structures, potentially resulting in death from physical injury, starvation, dehydration, predation by introduced predators, or collisions with vehicles. We developed a GIS-based method to model the intensity of artificial light that fledgling procellariids and hydrobatids could view en route to the ocean (to estimate the degree of threat that artificial light poses to these birds) and present two models for the island of Kauai as examples. These models are particularly relevant to the federally threatened Newell's Shearwater, or`A`o (Puffinus newelli ), of which .30,000 fledglings have been collected in response to disorientation by lights on Kauai during the past 30 years. Our models suggest that there are few to no portions of Kauai from which young birds could fledge and not view light on their post-natal nocturnal flights, which is concerning given evidence of a Newell's Shearwater population decline. In future work using this technique, night light intensity layers could be altered to model the effects of modified coastal light conditions on known and potential procellariid and hydrobatid breeding locations. Furthermore, certain methods presented herein may be applicable to other seabirds and additional taxa in which attraction to anthropogenic light poses a serious threat, including migratory passerines and hatchling marine turtles. Components of this modeling approach could potentially be used to spatially estimate effects of other point-source threats to ecological systems, including sound and air pollution.

Eastern black rails (Laterallus jamaicensis jamaicensis) are of conservation concern. These birds vocalize infrequently and inhabit dense vegetation, making them difficult to detect. We conducted the first large-scale study of black rail occupancy and abundance in Texas, USA. We conducted point count surveys that were 6-minute call-playback broadcasts and detected rails acoustically. We fit 19 occupancy and 19 abundance models while accounting for imperfect detection. We used the top occupancy model to estimate the required survey effort to precisely estimate black rail occupancy with and without detection correction methods. We found that detection increased with moon phase but decreased with wind speed and ambient noise. Occupancy and abundance increased with cordgrass (Spartina spp.) and intermediate marsh cover. Estimates of mean occupancy (0.27 and 0.26) and abundance (1.08 and 1.00 rails/point) were similar between 2015 and 2016. Estimated survey efforts under mean (wind ¼ 6-10 km/hr, lunar phase ¼ half-moon) and optimal (wind ¼ 0-5 km/hr, lunar phase ¼ full moon) environmental conditions were less for detection correction methods than for non-correction methods. We also concluded that efforts to predict black rail presence from na€ ıve occupancy estimates are impractical. Managers and researchers who wish to monitor black rails populations should use detection correction techniques when estimating occupancy and abundance. Ó

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,...