Flight heights obtained from GPS versus altimeters influence estimates of collision risk with offshore wind turbines in Lesser Black-backed Gulls Larus fuscus
Author(s): Johnston, D.T., Thaxter, C.B., Boersch-Supan, P.H., Davies, J.G., Clewley, G.D., Green, R.M.W., Shamoun-Baranes, J., Cook, A.S.C.P., Burton, N.H.K. & Humphreys, E.M.
Published: October 2023
Journal: Movement Ecology Volume: 11
Article No.: 66
Digital Identifier No. (DOI): 10.1186/s40462-023-00431-z
As previous BTO research has shown, bird-borne tracking devices are essential for shedding light on seabird flight behaviour. Different methods for gathering flight height information within tracking devices can influence the altitudes they produce, with implications for the collision estimates ultimately produced. This study compared the flight height altitudes produced from GPS and barometric altimeter data from tagged Lesser Black-backed Gulls to investigate this matter. Novel methods for calibrating GPS and altimeters were trialled to reduce error within the altitudes calculated by these methods as much as possible.
The data were collected by attaching tracking devices to adult Lesser Black-backed Gulls captured while they were incubating their eggs at two breeding colonies (the Isle of May in Scotland and Havergate Island in England) in 2019 and 2020. Tags contained both GPS, which estimates a bird’s position using a network of satellites, and altimeters, which use air pressure measurements to estimate flight height. These two systems enabled flight heights obtained from either method to be compared.
The results showed that altimeter data, when combined across years and colonies, produced higher flight heights than GPS data, and that this difference was more marked at higher sampling frequencies (e.g. one data point every 10 seconds) than at lower ones (e.g. one data point every five minutes). This in turn led to a greater number of collisions being predicted by CRMs for altimeter data than for GPS data. However, when GPS-altimeter data was split by sampling rate, year, and colony, flight heights from altimeters were not consistently higher, indicating local environmental conditions play a role influencing the accuracy of altitudes recorded.
Tracking devices can provide information on flight height across a range of behaviours and environmental conditions. Improvement of calibration steps to account for the influence of the environment on tracking devices will help improve the accuracy of flight height information collected in future. This will increase confidence in the use of tracking devices in future assessments of collision risk and help to better predict mortality associated with wind farms. Forming a better understanding of the impacts of wind farms on seabirds will help ensure their appropriate development in future throughout the North Atlantic.
NotesThis project was funded by the Department for Business, Energy and Industrial Strategy (BEIS)’s Offshore Energy Strategic Environmental Assessment research programme.
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