Use of an optical rangefinder to assess the reliability of seabird flight heights from boat‐based surveyors: implications for collision risk at offshore wind farms

Abstract

Visual estimates of the distribution of the flight heights of birds are often critical for collision risk assessments for wind‐energy projects. However, the reliability of such data is largely unknown. The results of trials where rangefinder height estimates were compared with those of an Unmanned Aerial Vehicle (UAV) used to mimic a flying bird at varying distances and heights from the operator suggested that the rangefinder performed well, with an overall mean deviation from the actual UAV height of −0.4 m. Accuracy and precision were influenced by an interaction between UAV height and distance to the surveyor. We also compared measurements of the flight heights of 16 species of seabirds made with an optical rangefinder to surveyor estimates made in 5‐m bands during boat‐based surveys off the east coast of Scotland UK from May to October 2017. Overall agreement between the rangefinder and surveyor estimates (N = 1235) was 58%, but increased to 92% when including the adjacent 5‐m bands. We found that surveyors were more likely to underestimate bird heights, potentially influencing collision‐risk assessments for wind farm projects. Use of rangefinders may be limited to days with good visibility, but their accuracy can be excellent within equipment limitations. However, suitability of rangefinders for the species or task in question should be verified before application. Further use of rangefinders is recommended for training, assessment of surveyor accuracy and consistency, calibration of visual flight height distributions, or replacement of visual observations where appropriate. Such applications could increase confidence in impact assessments and inform appropriate mitigation for wind farms.

Methods to quantify avian airspace use in relation to wind energy development

“…However, while developing a framework around sensor‐based methods is likely to improve data accuracy and precision (Becker 2016, Fijn & Gyimesi, 2018, Harwood et al . 2018), it is unlikely to be the most cost‐effective means of data collection. This is likely to create challenges for the consenting process that may be overcome when such improvements in data accuracy and precision can be quantifiably demonstrated.…”

Estimating bird flight height using 3‐D photogrammetry

“…To assess collision risk, accurate estimates of flight heights are required (Stantial & Cohen, 2015; Harwood, Perrow & Berridge, 2018). Collision risk predictive models could be misrepresented without flight height data or compounded by inaccurate measurements, especially for rarer species (Stewart, Pullin & Coles, 2007; Ferrer et al ., 2012).…”

“…Collision risk predictive models could be misrepresented without flight height data or compounded by inaccurate measurements, especially for rarer species (Stewart, Pullin & Coles, 2007; Ferrer et al ., 2012). Unfortunately, flight height data are often insufficient in quantity and quality, and many studies simply default to estimates by surveyors (Band, 2012; Harwood et al ., 2018). Surveyors can be trained to estimate flight heights relative to fixed structures, and then allocate bird flight to height bands (e.g.…”

“…Surveyors can be trained to estimate flight heights relative to fixed structures, and then allocate bird flight to height bands (e.g. Osborn & Dieter, 1998; Stantial & Cohen, 2015; Harwood et al ., 2018). However, this method is subjective and prone to underestimation, especially with fatigued observers and when measurements are taken before infrastructure construction when no comparative heights are available in the field of reference (Stantial & Cohen, 2015; Harwood et al ., 2018).…”

“…Osborn & Dieter, 1998; Stantial & Cohen, 2015; Harwood et al ., 2018). However, this method is subjective and prone to underestimation, especially with fatigued observers and when measurements are taken before infrastructure construction when no comparative heights are available in the field of reference (Stantial & Cohen, 2015; Harwood et al ., 2018). Optical laser rangefinders are accurate for single large birds up to 100 m away and 50 m high but yield inaccurate results or fail to yield any measures at all for multiple small, distant, irregular, and fast‐flying birds (Desholm et al ., 2006; Stantial & Cohen, 2015; Wulff et al ., 2016; Borkenhagen, Corman & Garthe, 2018; Harwood et al ., 2018).…”