Modelling flight heights of Lesser Black-backed Gulls and Great Skuas from GPS: a Bayesian approach

Offshore wind farms are now operating or under construction in many locations, but while spinning turbine blades are crucial for generating renewable energy, they also represent a potential threat for flying animals, particularly seabirds, which can be injured and killed if they collide with them. In order to correctly characterise the extent of this threat, we need accurate measurements of the height at which seabirds fly. New research by the BTO has demonstrated a novel way to do this, using state-of-the-art GPS tags and cutting edge statistics.

The work, led by Viola Ross-Smith and Ali Johnston, used data downloaded from long-life GPS tags fitted to Lesser Black-backed Gulls and Great Skuas breeding at protected colonies in Suffolk, Orkney and Shetland. GPS tags record flight height with error, which varies depending on the configuration of satellites in the sky at the time each measurement is taken. The study solved this error problem using a powerful statistical technique known as a Bayesian state-space model. The analysis also examined the association between a bird’s location and its flight height, as well as how flight behaviour varies between day and night.

The results showed that Lesser Black-backed Gulls flew significantly lower at sea than over land, and significantly lower at night, when turbines might be harder to detect and avoid, than during the day. Great Skuas flew lower than Lesser Black-backed Gulls, but did not show significant relationships between flight height, location and time of day. Importantly, the study found that both species, but particularly Great Skuas, primarily flew beneath the height of offshore wind turbine blades. The results also support previous BTO research in suggesting that Lesser Black-backed Gulls and Great Skuas are safer when turbines are designed so that the lower edge of the blades is 30 m above sea level (as opposed to some current turbine designs, where the blade tips reach 22 m above sea level). 

This study describes an effective way of resolving the problem of error associated with GPS tracking data, which can be used to provide important information on seabirds’ risk of collision with offshore wind turbines. This approach could be applied more widely, to reliably model animal movement at a time when tracking is increasingly being used as a tool to inform management and conservation.

For further information on the modelling used in this study, please see this blog for the Journal of Applied Ecology.

Notes

This work was funded by the UK Department of Energy and Climate Change (DECC), and the Marine Renewable Energy and the Environment (MaREE) project (funded by Highlands and Islands Enterprise, the European Regional Development Fund and the Scottish Funding Council).