5.10. Herring gull
- The Firth of Forth is known to support one of the largest coastal roosting populations of herring gulls in Scotland (NatureScot, 2020b). Many wintering herring gulls present along the east coast of Scotland are migrants from further afield, such as Norway and north Russia, boosting local populations between September and February (Wernham et al., 2002; Furness, 2015). Generally, the species uses inshore areas primarily for roosting, although foraging in intertidal areas is also likely to occur (Rome and Ellis, 2004). The species is currently Red-listed on the UK Birds of Conservation Concern List (Stanbury et al., 2021).
- Herring gulls were the most abundant large gull species in the Offshore Ornithology Study Area during the survey period, present in relatively moderate densities from mid to late summer and again in winter. Few birds were present throughout the rest of the year. This pattern occurred in both years of surveys, with an overall greater abundance of birds in the second year. Design-based density estimates ranged between 0.00 birds/km2 (95%CI 0.00 – 0.01) and 0.51 birds/km2 (95%CI 0.34 – 0.73) in 2019/20 compared to 0.00 birds/km2 (95%CI 0.00 – 0.01) and 1.28 birds/km2 (95%CI 0.39 – 2.29) in 2020/21 (
- Table 5.58 Open ▸ ). Mean-peak population estimates were similar in the non-breeding season, calculated at 3,382 birds (95%CI 957 – 6,294), compared to 3,356 (95%CI 2,246 – 4,733) birds in the breeding season ( Table 5.61 Open ▸ ).
- Boat-based surveys of Seagreen Alpha and Bravo recorded herring gulls within both sites in regionally important numbers in the breeding season, likely to be linked to nearby SPAs. Taking into account urban nesting birds[1]as well as those within SPAs, the total count of herring gulls within the foraging range (mean max distance +1 sd from Woodward et al. 2019) of the Project approximates the regional population and is estimated at 29,600 breeding adults. Generally, herring gulls were distributed to the west of the Offshore Ornithology Study Area with many birds distributed in the western buffer (e.g., November 2019 and December 2020), and in the northwest buffer, such as in July and August 2019. Generally, fewer birds were observed within the Project, except for June and July 2019 and July 2020.
- Habitat use varied between seasons, with most birds recorded as sitting on the water during the breeding season, while during the non-breeding season the proportion of sitting and flying birds was relatively similar. Peaks in proportions of flying birds fluctuated within bio-seasons, such as the marked increase of flying birds between September and November 2020 ( Figure 5.54 Open ▸ ). Herring gulls generally feed by making shallow dives from the surface of the water, and the high proportion of sitting birds during the breeding season suggests the use of the Offshore Ornithology Study Area during foraging. The increased proportion of flying birds during the non-breeding period may be explained by the passage of migrant birds from further afield or the movement of successful breeders away from the colony to other areas (Furness, 2015).
- Of the birds that could be aged, most were recorded as adults. The highest average proportions of immature and juvenile birds occurred in the non-breeding season, at 32% and 2% respectively. The presence of more immature birds may be explained by increased movement of this life history stage compared to other demographics, since younger birds are known to disperse across larger distances compared to adults which generally remain closer to colonies (Wernham et al., 2002).
- Flight direction varied between surveys, with many birds flying northwest and southeast in July 2020. In December 2020, when abundance of herring gulls peaked, birds flew in all directions, but a large proportion were recorded flying southwards. This may be attributed to movement to colonies to the south of the Offshore Ornithology Study Area such as those located at St. Abb’s Head, which currently supports approximately 172 AON’s (SMP, 2021).
Table 5.57: Herring gull bio-seasons taken from NatureScot (2020a)
Table 5.61: Mean seasonal peak (MSP) population and density (birds/km2) of all herring gulls in the Offshore Ornithology Study Area across the two years of surveying (March 2019 to April 2021) estimated using design-based analysis. Data include “no-identification” birds apportioned to species
Figure 5.53: Summarised flight direction of herring gulls across the Offshore Ornithology Study Area
Figure 5.54: Percentage of flying herring gulls per survey across the Offshore Ornithology Study Area
5.11. Lesser black-backed gull
- Lesser black-backed gulls are distributed throughout the UK, with breeding populations being a qualifying feature of ten UK SPAs (Stroud et al., 2001). National populations have fluctuated, with increases observed between the 1960s and early 2000s and decreases over the following decade (JNCC, 2014) which may be associated with redistribution of birds or changes to fishery discards policies (Ross-Smith et al., 2014; Furness et al., 1992). Specific to the Offshore Ornithology Study Area, lesser black-backed gulls are a qualifying species for the Forth Islands SPA, with 1,684 AON, 131 AOT and 97 AOT in 2018 on the Isle of May, Fidra and Craigleith respectively (NatureScot, 2018; SMP, 2021). The species is currently Amber-listed on the UK Birds of Conservation Concern List (Stanbury et al., 2021).
- Relatively high abundances observed throughout the aerial surveys may be due to the presence of several colonies in the vicinity of the survey area. Taking into urban nesting pairs[2], as well as those within SPA’s, the total count of herring gulls within the foraging range (mean max distance +1 sd from Woodward et al. 2019) of the Project approximates the regional population and is estimated at 13,994 breeding adults.
- Lesser black-backed gulls were recorded intermittently throughout the survey programme, mainly occurring during the breeding season, peaking in July 2019 and 2020 ( Figure 5.55 Open ▸ ). Design-based density estimates ranged between 0.00 birds/km2 and 0.06 birds/km2 (95%CI 0.04 – 0.09) in 2019/20 compared to 0.00 birds/km2 and 0.23 birds/km2 (95%CI 0.17 – 0.29) in 2020/21 ( Table 5.64 Open ▸ ). Between years, peaks in abundance varied substantially, with nearly three times as many birds recorded in July 2020 compared to July 2019. Much lower numbers of lesser black-backed gulls were recorded during the non-breeding season, with no birds between January and February in 2019 or 2020, as most birds move south for the winter months.
- The mean seasonal peak estimated population in the Offshore Ornithology Study Area for the breeding season was 580 birds (95%CI 427 – 741; Table 5.67 Open ▸ ).
- Boat-based surveys of Berwick Bank recorded 179 lesser black-backed gulls during the breeding season, similar to raw data collected during digital aerial surveys of the Offshore Ornithology Study Area. Lesser black-backed gulls were also recorded on Seagreen boat-based surveys and highlighted as a species likely to be present in regionally important numbers in the region, likely linked to breeding birds from the nearby Forth Islands SPA.
- Birds were distributed throughout the Offshore Ornithology Study Area in June and July 2019 and July 2020 ( Figure 5.56 Open ▸ ; Table 5.50 Open ▸ ). In 2019 and 2020, lesser black-backed gulls were widespread across the Offshore Ornithology Study Area, such as in July 2019, with a more concentrated distribution towards the northwest of the study area in August 2019. The species typically exhibits long foraging flights offshore, feeding on fish and discards from commercial fisheries (Camphuysen, 2013). It is likely that fisheries discards currently make up a much smaller percentage of diet than previously, following the discards ban at sea which was fully implemented in 2019 (Ulhmann et al., 2019). The species has, like many gulls, increased in urban areas feeding on human discards and litter.
- Of the birds that could be aged, most were recorded as adults. The highest average proportion of immature birds was recorded in the breeding season, at 9% of all recorded birds (Table 5.83). No juvenile lesser black-backed gulls were recorded.
- Flight direction was variable, with many birds flying east between June and August 2019 and July 2020 ( Figure 5.58 Open ▸ ). Birds also flew west in July and August 2020.
Table 5.63: Lesser black-backed gull bio-seasons taken from NatureScot (2020a)
Table 5.64: Monthly density and population estimates of all lesser black-backed gulls across the Offshore Ornithology Study Area using design-based analysis. Data include “no-identification” birds apportioned to species
Figure 5.55: Estimated densities (birds/km2) of all lesser black-backed gulls across the Offshore Ornithology Study Area using design-based analysis. Data include “no-identification” birds apportioned to species
Table 5.67: Mean seasonal peak (MSP) population and density (birds/km2) of all lesser black-backed gulls in the Offshore Ornithology Study Area across the two years of surveying (March 2019 to April 2021) estimated using design-based analysis. Data include “no-identification” birds apportioned to species
Figure 5.56: Distribution of lesser black-backed gulls across the Offshore Ornithology Study Area between March 2019 and March 2020
Table 5.68: Mean count, SD and proportion of lesser black-backed gulls in each age class averaged across bio-season
Figure 5.58: Summarised flight direction of lesser black-backed gulls across the Offshore Ornithology Study Area
5.12. Common tern
- Located in both temperate and subtropical areas in the northern hemisphere, common terns are migratory seabirds found in coastal and inland areas (Mitchell et al., 2004). Within the UK, population trends have been relatively stable although regional fluctuations do occur, following changes in prey availability, nesting sites or predation (Mitchell et al., 2004). Common terns are a qualifying species for the nearby Forth Islands SPA, which is estimated to hold around 3% of the GB population, corresponding to 334 pairs (mean 1997 – 2001; NatureScot, 2018). Leith docks, located in Edinburgh also supports a large breeding population, estimated to be at around 514 and 246 AON in 2018 and 2019 respectively (SMP, 2021), although the Offshore Ornithology Study Area is out with the mean maximum foraging range (+1SD) for birds from this colony. The species is currently Amber-listed on the UK Birds of Conservation Concern List (Stanbury et al., 2021).
- Common terns return to UK waters between April and September to breed ( Table 5.69 Open ▸ ), spending their winters across the southern hemisphere. Birds were observed in both the 2019/20 and 2020/21 survey periods, with over 10 times more observations occurring in Year 2 compared to Year 1. Across all surveys, design-based density estimates ranged between 0.00 birds/km2 and 0.13 birds/km2 (95%CI 0.10 – 0.16) in 2019/20 compared to 0.00 birds/km2 and 1.49 birds/km2 (95%CI 1.02 – 2.02) in 2020/21 ( Table 5.70 Open ▸ ).
- Birds were primarily observed during the late breeding season, with peaks occurring in August 2019 and 2020 ( Table 5.70 Open ▸ ). Peaks at this time can likely be attributed to post-breeding movements of adults and juveniles away from coastal breeding colonies. Mean seasonal peak abundance was highest in the breeding season, with a population estimate of 3,225 birds (95%CI 2,224 – 4,332) ( Table 5.73 Open ▸ ). Lower abundance towards the start of the breeding season, such as between May and June can be attributed to birds beginning egg-laying and nest attendance, in which they are more closely associated with their nest sites until chicks have fledged.
- Berwick Bank boat-based surveys recorded one common tern, present in the July 2020 survey, with boat-based surveys for Seagreen Alpha and Bravo recording the species in regionally important numbers in the Alpha site only. No individuals were recorded within the Bravo site. Data summed from ESAS surveys conducted between 1980 and 1996 clipped to the Offshore Ornithology Study Area recorded four common terns throughout the 16-year period.
- Birds were widespread throughout the Offshore Ornithology Study Area, such as in August 2019 and 2020, located both within the Proposed Development Array area and the buffer. In August 2019 and 2020, there appeared to be no clear patterns in distribution, however in September 2020, common terns were concentrated to the south of the Offshore Ornithology Study Area.
- Bar one individual, all birds were recorded as flying. Common terns feed almost exclusively while on the wing, predominantly by either plunge diving or contact dipping, hawking or even intraspecific kleptoparasitism (Garcia et al., 2011 and references therein), therefore flight behaviour cannot be used to distinguish between foraging and passage. Generally, common terns flew eastwards and westwards, with most birds flying west in August 2020 and east in September 2020. In many months, so few birds were recorded flying that no conclusions regarding flight direction could be determined.
Table 5.69: Common tern bio-seasons taken from NatureScot (2020a)
Table 5.70: Monthly density and population estimates of all common terns across the Offshore Ornithology Study Area using design-based analysis. Data include “no-identification” birds apportioned to species
Table 5.73: Mean seasonal peak (MSP) population and density (birds/km2) of all common terns in the Offshore Ornithology Study Area across the two years of surveying (March 2019 to April 2021) estimated using design-based analysis. Data include “no-identification” birds apportioned to species
Figure 5.62: Summarised flight direction of common terns across Offshore Ornithology Study Area
5.13. Arctic tern
- The most common breeding tern species in the UK, Arctic terns are present over the summer breeding season, forming coastal breeding colonies on vegetated rock and sand (Mitchell et al., 2004; Forrester et al., 2007). A significant proportion of Arctic terns breed in Scotland, which supports 84% of the British Isles breeding population and 10% of the world population (Forrester et al., 2007). Large variation in colony size between years is common, with some colonies only present for a few years at a time (Devlin et al., 2008). Arctic terns are a qualifying species for the nearby Forth Islands SPA, which supports 1.2% of the population of Great Britain, equating to approximately 540 pairs at time of designation (mean 1992 – 1996; NatureScot, 2018), compared to 832 pairs in 2017 (SMP, 2021). Post breeding, Arctic terns migrate south through the North Sea and down the eastern seaboard of the Atlantic, with many juvenile birds wintering in western and southern Africa (Hatch, 2002). The species is currently Amber-listed on the UK Birds of Conservation Concern List (Stanbury et al., 2021).
- Arctic terns spend a shorter period in UK waters than common terns, returning to UK shores to breed between late April and early September. Individuals were present during this period in both years of surveying. During the breeding season, the mean seasonal peak density was calculated at 1.03 birds/km2, equating to a mean seasonal peak population estimate for the Offshore Ornithology Study Area of 4,074 birds (95%CI 3,188 – 5,088; Table 5.79 Open ▸ ).
- Across all surveys, design-based density estimates ranged between 0.00 birds/km2 to 1.61 birds/km2 (95%CI 1.32 – 1.93) in 2019/20 compared to 0.00 birds/km2 to 0.44 birds/km2 (95%CI 0.29 – 0.63) in 2020/21 ( Table 5.76 Open ▸ ). Arctic terns were distributed throughout the Offshore Ornithology Study Area ( Figure 5.64 Open ▸ to Figure 5.66 Open ▸ ). In July 2019 and August 2020, birds were found congregated in the southeast of the Offshore Ornithology Study Area, while in August 2019, birds were spread between west and northeast.
- Peak abundances were recorded in late summer, such as in August 2019 and 2020, which can likely be attributed to adults and juveniles moving through the Offshore Ornithology Study Area away from breeding colonies. The origins of these birds are unknown but are likely to be from nearby colonies as well as elsewhere in the country, such as Shetland.
- Berwick Bank boat-based surveys in 2020-2021 recorded relatively high numbers of Arctic terns, recorded in all surveys apart from in April 2021. Boat-based surveys of Seagreen Alpha and Bravo estimated that regionally important numbers of the species were likely to be present within both sites with the species also recorded during pre-construction digital aerial surveys.
- Flight direction varied between surveys, with a large proportion of birds flying east and west in July and August 2019, while in August 2020 a substantial number of birds flew north ( Figure 5.67 Open ▸ ).
- Very few birds were recorded as sitting on the water on surveys, with 90% of total birds recorded as flying. As with common terns, birds feed almost entirely whilst on the wing; surface feeding, plunge-diving or hawking in the air. Due to this, flight behaviour cannot be used to distinguish between those foraging and passing through the survey area.
Table 5.75: Arctic tern bio-seasons taken from NatureScot (2020a)
Table 5.76: Monthly density and population estimates of all Arctic terns across Offshore Ornithology Study Area using design-based analysis. Data include “no-identification” birds apportioned to species
Table 5.79: Mean seasonal peak (MSP) population and density (birds/km2) of all Arctic terns in the Offshore Ornithology Study Area across the two years of surveying (March 2019 to April 2021) estimated using design-based analysis. Data include “no-identification” birds apportioned to species
Figure 5.64: Distribution of Arctic terns across Offshore Ornithology Study Area between March 2019 and March 2020
Figure 5.66: Distribution of Arctic terns across Offshore Ornithology Study Area April S02 2021
Figure 5.67: Summarised flight direction of Arctic terns across Offshore Ornithology Study Area