Seabird flight height data collection at an offshore wind farm: final report

Executive Summary

This technical report presents analysed seabird flight height data collected within and around Beatrice Offshore Wind Farm (OWF) located in the Moray Firth. The aim of the study was to evaluate the impact on the flight height of sea birds in relation to the OWF.

Two aerial surveys were completed in June and July 2021. The first aerial survey took place across two days on the 15^th and 16^th June 2021 and the second aerial survey took place on the 15^th July 2021. Data were collected with a combined still image and aircraft-mounted Light Detection and Ranging (LiDAR) technology. Data were collected along 13 transects spaced two kilometres (km) apart across Survey Area 1 and 9 transects spaced two kilometres (km) apart across Survey Area 2. Additional flight lines were also captured onshore over Wick to validate the accuracy of the LiDAR data. Images were collected continuously (abutting digital still imagery) along the survey lines, at two-centimetre (cm) ground sample distance (GSD). The surveys were completed by one aircraft with approximately eight hours on-task per survey. Data were processed to allow each individual flying bird to be identified in the imagery and then matched with height measurement points collected from the LiDAR system.

These flight height data represent site-specific flight height measurements. For the majority of species, the birds were found flying less than 25 metres (m) above sea surface level, with the exception of herring gull (Larus argentatus) which were primarily recorded at flight heights of 50–100 m.

The differences in flight height of seabirds in the wind turbine generator (WTG) area (Survey Area 1) and a control area without WTGs (Survey Area 2) were studied. There was a significant difference in flight height between these areas. The birds recorded in Survey Area 1 were flying significantly lower (11.32m) than the birds in Survey Area 2 (20.03m) which could indicate an effect of the WTGs on the flight height in the study area.

Additionally, flight heights of sea birds were studied in relation to the distance to the nearest WTG, in Survey Area 1 only. There was no significant effect in the results of the linear models for most species. However, gannets and large gulls had significant results for flight height in relation to the distance to the nearest WTGs. The results for gannets show individuals flying lower closer to the WTGs but this result is influenced by one outlier. Large gulls flew significantly higher closer to the WTGs but this is based on a sample of eight individuals. Generally, the sample size of seabirds detected in this area was low which could have affected the statistical power. There were also no birds detected between 0 – 40 m of a WTG. The closest and second closest bird were at 44 m and 189 m, respectively. These could indicate avoidance behaviour of the detected sea birds and makes the analysis of flight height in close proximity to WTGs difficult.

The outputs of flight heights of seabirds produced from the combined imagery-LiDAR system could be incorporated into collision risk modelling (CRM). The benefits of using LiDAR would be the reduced uncertainty of the CRM. This will allow Environmental Impact Assessments (EIA) to be based on robust models and the impact of WTGs to be better understood.

The main limitation of this study was the low sample size, and therefore one recommendation would be to increase the coverage and number of surveys, as this would not only increase the number of flying birds detected, it can also allow seasonal variation to be investigated if surveys are completed at different times of the year. The survey design can be tailored to maximise detection of a specific species by using historic density for a site and peak abundance months.

In this study the number of birds in flight in Survey Area 1 was low and the linear models were mostly unsuccessful in showing any significant effect of WTGs on flight height. Therefore, to evaluate the effect of OWF on flight heights larger sample sizes are needed and surveys should be undertaken pre- and post-construction to allow any changes in flight heights to be investigated at the same location.