3.1.3.3      Rangefinder Flight Height Analysis

Analysis of the rangefinder data for gannet and kittiwake from the breeding season months (April-August) provided 1,327 observations for analysis.

3.1.3.3.1  Flight height and distance from boat

During examination of rangefinder flight heights and their distances from the boat, low flight heights (0-5 m) of gannet were observed up to distances of 800 m (2 data points were removed due to being detected outside rangefinder range; errors > 1,600 m) ( Figure 3.10   Open ▸ ), with visual inspection suggesting that rangefinder flight heights may be slightly influenced by the vessel. However, this would require further investigation utilising more complex modelling methods (Corbeau et al. 2019) to understand at what distances gannet foraging behaviour may become influenced by the presence of the boat. Our results showed that 30% of all rangefinder flight height records were recorded within 100 m from the boat.

The trendline within the graph below ( Figure 3.10   Open ▸ ) highlights that lower flight heights of gannet were more frequently observed at distances close to the boat. This contradicts the pattern seen in the observer-estimated flight heights where a higher proportion of birds were recorded in the lowest height band 0-5 m in Distance Band D than in closer Bands A, B and C.

Figure 3.10 Variation in rangefinder-derived flight heights of gannet against distance from the boat; data from the Berwick Bank 2020-2021 boat-based surveys. Trendline is represented by the redline.

Kittiwake were observed up until distances of 948 m ( Figure 3.11   Open ▸ ), with visual inspection suggesting that rangefinder flight heights measured within close proximity to the boat could be influenced by the vessel. 53.92% of all rangefinder flight height records of kittiwake were recorded within 100 m of the boat, with 72.47% of those records at flight heights of below 15 m. The extent of the influence of the boat on kittiwake flight heights would require further investigation utilising more complex modelling methods (and additional variables such as flight speed) to understand at what distances kittiwake flight behaviour may become influenced.

The trendline within the graph below ( Figure 3.11   Open ▸ ) highlights that lower flight heights of kittiwake were more frequently observed at distances close to the boat. This contradicts the pattern seen in the observer-estimated flight heights where a higher proportion of birds were recorded in the lowest height band 0-5 m in Distance Band D than in closer Bands A, B and C.

Figure 3.11 Variation in rangefinder-derived flight heights of kittiwake against distance from the boat; data from the Berwick Bank 2020-2021 boat-based surveys. Trendline is represented by the redline.

3.1.3.3.2  Gannet flight height distribution

Gannet observations ( Figure 3.12   Open ▸ ) ranged from 0 m to 78.8 m. Due to negative values in the data set (caused by variation in swell heights of up to 3.5 m influencing the eye height of the observer), a number of observations were corrected to 1 m above sea level for the analyses to ensure these low-flying birds were captured in the flight height distributions without yielding unrealistic negative flight heights (Gannet n = 86).

Figure 3.12 Proportion of gannet observations per month for each height band derived from rangefinder data.

Gannets appeared to fly at greater heights during August 2020, with 15.35% flying above 35 m (Table 3.12), whereas no records of gannet were observed above 30 m during April and May 2020 surveys.

Table 3.12 Summary of rangefinder records for Gannet (negative values corrected to 1 m above sea level)

3.1.3.3.3  Kittiwake flight height distribution

Kittiwake observations ranged between 0 m to 68.66 m Figure 3.13   Open ▸ ). Due to the presence of negative values, a number of observations were corrected to 1 m above sea level for the analyses (Kittiwake n = 12) to ensure these low-flying birds were captured in the flight height distributions without yielding unrealistic negative flight heights.

Figure 3.13 Proportion of kittiwake observations per month for each height band derived from rangefinder data.

Kittiwake appeared to fly at greater heights during the earlier breeding months (April, May and June) with 4.23% flying above 35 m during the April 2021 surveys (Table 3.13). There were no records of kittiwake flying greater than 35 m during the July and August 2020 surveys.

Table 3.13 Summary of rangefinder records for kittiwake (note errors removed and corrected according to eye height)

3.1.3.4      Flight Height Comparison

Figure 3.14   Open ▸ and Figure 3.15   Open ▸ illustrates the fitted distributions for gannet (log-normal) and kittiwake (normal) compared to the Johnston et al. (2014) and surveyor observational distributions. The best fitting distributions were based on visual assessment (Appendix 1).

The gannet fitted distribution from the rangefinder data for the combined Berwick Bank survey sites ( Figure 3.14   Open ▸ ) is similar to the Johnston et al. (2014) distributions, however does differ slightly, with the rangefinder data from the 2020-2021 boat surveys showing that a higher proportion of gannets were seen flying at greater heights, above 30 m. The visual observational surveyor flight height estimates show a lower proportion of birds observed to be flying above 30 m compared to Johnston et al. (2014) and the rangefinder data.

Figure 3.14 Variation in fitted flight height distributions for gannet based on the rangefinder data and visual observation data collected during boat-based surveys of the Berwick Bank project area in 2020 and 2021 and generic flight heights by Johnston et al. (2014).

Flight height comparisons for gannet are further shown within Table 3.14, highlighting the difference between the proportion of birds observed below and above 30 m and above 35 m. The observer estimated flight heights show similarities with Johnston et al. (2014), while the rangefinder flight height measurements are different, with a higher proportion of birds (8.36%) recorded flying above 30 m and 6.44% recorded flying above 35 m, corresponding approximately to collision risk height (assuming that the Berwick Bank turbine airgap is 37 m above LAT, with a tidal offset of approximately 2.5 m).

Table 3.14 Flight height percentage comparison for Gannet flying below/above 30 m. 2020-2021 data taken from Berwick Bank boat-based surveys.

The kittiwake fitted distribution from the rangefinder data ( Figure 3.15   Open ▸ ) differs considerably from the Johnston et al. (2014) flight height distributions, with the rangefinder detecting more birds in the airspace between 10 m to 30 m than below 10 m and overall a larger proportion of birds observed below 30 m. Surveyor flight height estimates were more similar to Johnston et al. (2014) estimates compared to the rangefinder data, however a larger proportion were again observed below 30 m compared to generic flight heights by Johnston et al. (2014).

Figure 3.15  Variation in fitted flight height distributions for kittiwake based on the rangefinder data and visual observational data collected during boat-based surveys of the Berwick Bank project area in 2020 and 2021 and generic flight heights by Johnston et al. (2014).

Flight height comparisons for kittiwake are further shown within Table 3.15, highlighting the difference between the proportion of birds observed below and above 30 m and above 35 m. The rangefinder data and observer estimated flight height data show a higher proportion of birds below 30 m (97.82 % and 98.80 % respectively) compared to Johnston et al. (2014) (94.77 %). There is a corresponding lower proportion of birds flying at collision risk height recorded by the rangefinder measurements (1.01%) and the observer flight height estimates (0.80%) compared to Johnston et al. (2004) (3.23 %; assuming that the Berwick Bank turbine airgap is 37 m above LAT, with a tidal offset of approximately 2.5 m).

Table 3.15 Flight height percentage comparison for Kittiwake flying below/above 30 m. 2020-2021 data taken from Berwick Bank boat-based surveys