Great black-backed gull (Larus marinus)

Figure 1. Trends in abundance index of great black-backed gull 1986–2019 for the UK (red) and Scotland (blue). Further details, including 95% confidence limits, can be found in the sections below. Based on SMP data; view the methods of analysis.

A large proportion of the great black-backed gull nests monitored for abundance are located in Scotland and the UK and Scottish trends are, therefore, closely matched (Figure 1). The UK and Scotland great black-backed gull trend indices increased during the 1990s but declined thereafter, with index values remaining below the baseline for the remaining monitoring period, more markedly so for Scotland. More detailed information can be found in the individual country sections below. The number of colonies monitored frequently are only sufficient to allow meaningful abundance trends to be produced for the UK and Scotland.

Productivity

Figure 2. Trends in productivity (no. of chicks fledged per pair) of great black-backed gull 1986–2019 for UK (red) and Scotland (blue). Further details can be found in the sections below. Based on SMP data; view the methods of analysis.

A large proportion of the great black-backed gull nests monitored for productivity are located in Scotland, so the UK and Scottish trends are closely matched. Productivity in both the UK and Scotland declined until 2005, increased thereafter until 2018 but fell slightly in 2019. The number of great black-backed gull colonies monitored within the SMP only allows meaningful productivity trends to be produced for the UK and Scotland.

Figure 3. Trend in UK abundance index (solid line) of great black-backed gull 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

The great black-backed gull UK population changed relatively little between the three censuses. However, data submitted annually to the SMP suggest the index peaked around the time of the most recent census (1998–2002) then declined (though with a brief recovery between 2014 and 2016) to reach its lowest point in 2018 at 36% below the baseline. In 2019, it increased to 14% below the baseline (Figure 3).

It has been suggested that great black-backed gulls have competitive advantage over other seabird species when scavenging at sea for fishery discards and offal¹, and hence have not undergone (at least, until recently) the declines that other scavengers (fulmar, herring and lesser black-backed gull) have in recent decades which may be due to a reduction in discards and offal². The species also forages on natural prey (e.g. rabbits and other seabirds) and appears to be quite adaptable to changing dietary opportunities.

Productivity

Figure 4. Trend in UK productivity (no. of chicks fledged per pair) of great black-backed gull 1986–2019. Based on SMP data; view the methods of analysis.

Productivity of great black-backed gulls at UK colonies declined between 1991 and 2005, when it reached its lowest point since monitoring began, at 0.64 chicks fledged per pair. Productivity has increased since then and, in 2019 was 0.97 chicks fledged per pair. The effects of predation by American mink Neovison vison on this large and aggressive gull appear to be less severe than for its smaller relatives. However, in some years, it does appear that mink can impact on productivity; in 2014, a comparison of islands off west Scotland with mink control, against those with none, found that productivity for each group was 3.00 and 0.94 chicks fledged per pair, respectively – a difference of 69%³.

Figure 5. Trend in abundance index (solid line) of great black-backed gull in Scotland, 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

National census data show that great black-backed gull numbers in Scotland changed relatively little between the Operation Seafarer (1969-70) and Seabird 2000 (1998–2002) censuses, although the trend from SMP annual sampling (Figure 5), shows a decline from 1986 to 1990 followed by an increase until Seabird 2000. Since then, there appears to have been a prolonged decline in the breeding population, with the index in 2018 reaching its lowest ever value, at 69% below the 1986 baseline. In 2018, 112 colonies were surveyed (accounting for 10% of the great black-backed gull population in Scotland) which held a total of 907 AON, a 76% decline since Seabird 2000 (3,859 AON). Numbers at Scotland’s largest colony, Calf of Eday, decreased by 97% between Seabird 2000 and 2018 (from 1,350 AON to 43 AON). In 2019, the index had recovered slightly to 59% below the baseline.

Productivity

Figure 6. Trend in productivity (no. of chicks fledged per pair) of great black-backed gull in Scotland, 1991–2019. Based on SMP data; view the methods of analysis.

The great black-backed gull productivity trend in Scotland closely matches that of the UK because 87% of data have been collected at Scottish colonies. Productivity of great black-backed gulls at Scottish colonies declined between 1991 and 2005, when it reached its lowest point to date, at 0.54 chicks fledged per pair. Productivity has increased since then and, in 2018, was 1.26 chicks fledged per pair. In 2019, however, it decreased to 0.87. The decline in productivity between 1991 and 2005 may have contributed to the downward population trend noted since 1999. As with common and herring gulls, most data come from a long-term study of islands on the west coast of Scotland where American mink, an introduced mammal, can reduce productivity by preying on the eggs and chicks of gulls, terns and other seabirds. In 2014, a comparison of islands where American mink were controlled against those with no, or unsuccessful, mink control found a difference of 69% (success of 3.00 and 0.94 chicks per pair in each group, respectively)³.

Figure 7. Number of great black-backed gull AON on Annet and Lundy, 1986–2019.

Between Operation Seafarer (1969-70) and the Seabird 2000 census (1998–2002), great black-backed gull numbers in England decreased slightly. The largest in England is in the south-west, with the Isles of Scilly holding 807 AON during Seabird 2000, over 50% of the English population. A complete survey of the Isles of Scilly in 2015 found 1,017 AON, an increase of 26% since Seabird 2000. Annet, the second largest island in the Isles of Scilly, holds the highest number of breeding great black-backed gulls, with numbers there increasing from 168 AON during Seabird 2000 to 199 AON in 2019. On Lundy, where rats Rattus sp. were eradicated between 2002 and 2004, numbers have begun to increase, from 35 AON during Seabird 2000 to 46 AON in 2018 (Figure 7).

Productivity

Few data are available on great black-backed gull productivity in England and an annual trend cannot, therefore, be provided. Data that have been submitted to the SMP have all been collected since 1998, most of which come from colonies holding less than 10 pairs. The exception being Brownsea Island where the number of nests has increased to double figures in recent years. Great black-backed gulls at this colony between 2010 and 2016 fledged an average of 1.25 chicks per pair. However, a decline has been observed, with as low as 0.36 chicks fledged per pair being recorded in 2017.

Figure 8. Trend in abundance index (solid line) of great black-backed gull in Wales, 1986–2019 with 95% confidence limits (dotted lines; drawing of upper limit restricted to preserve detail in the abundance index). Based on SMP data; view the methods of analysis.

During the Seabird 2000 census, the great black-backed gull population in Wales was 47% larger than recorded during the Seabird Colony Register (SCR). However, despite the increase, the population was still well below that recorded during Operation Seafarer. In 2018, 38 colonies held 276 AON compared to 236 AON during Seabird 2000, an increase of 17%. In 2019, the abundance index was at 115% (Figure 8). On Skomer Island, one of four SMP key monitoring sites, the population of great black-backed gull has been in gradual decline since 1999. However, recent data suggests that the island’s population may be recovering from earlier setbacks between the 1960s and 80s⁴; a trend even more marked on the nearby Skokholm Island⁵. This might be due to the discontinuation of control measures combined with the abundance of prey (particularly seabirds and rabbits) on the island⁶.

Productivity

On average great black-backed gulls at colonies in Wales fledged 0.89 chicks per pair between 1986 and 2019.

Figure 9. Abundance Number of great black-backed gull AON at Strangford Lough, 1986–2019.

Abundance data submitted to the SMP for great black-backed gull in Northern Ireland are sparse; thus, no meaningful annual trend can be provided. Most great black-backed gull colonies in Northern Ireland hold only a few pairs. Great Minnis’s Island on Strangford Lough is by far the most important, holding over 40% of the population during the Seabird Colony Register census. It declined by over 50% between 1986 (86 AON) and 2000 (41 AON); and one year later held only one AON. The colony has gradually increased since then to a peak of 129 AON in 2018, although had declined to 107 AON in 2019⁷. A further 46 AON were recorded at three other colonies in 2019, suggesting that the great black-backed gull population in Northern Ireland has more than doubled since Seabird 2000. Numbers of great black-backed gulls have not been counted on the Copeland Islands since 2012⁸.

Productivity

Data submitted to the SMP on the productivity of great black-backed gulls in Northern Ireland are sparse; thus, no meaningful average productivity value can be provided.

Figure 10. Number of great black-backed gull AON on the Calf of Man, 1986–2019.

Between Operation Seafarer and the Seabird Colony Register censuses, great black-backed gulls nesting on the Isle of Man increased by 37% with little further growth by the time of Seabird 2000. A census of the Isle of Man in 2017 found that numbers of great black-backed gulls had fallen severely to 85 AON, a decline of 79% since Seabird 2000¹¹. Only one colony, the Calf of Man (Figure 11), has been surveyed regularly since Seabird 2000. In 2019, 54 AON were recorded on the Calf of Man.

Productivity

Data submitted to the SMP on the productivity of great black-backed gulls on the Isle of Man are sparse; thus, no meaningful average productivity value can be provided.

Figure 11. Frequency of occurrence of food items within a five-metre radius cross-shaped transect around 25 great black-backed gull nests on Skomer, 2017 (top chart), 2018 (middle) and 2019 (bottom).

Figure 12. The number of Manx shearwater carcasses found per great black-backed gull nest on Skomer, 1959, 1965, 1973, 1992 and 2008–2019.

Survival

No systematic data have been collected as part of the Seabird Monitoring Programme.