Lesser black-backed gull (Larus fuscus)

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

There has been regional variation in natural-nesting lesser black-backed gulls abundance trends between countries since monitoring began in 1986 (Figure 1). The abundance index in Wales peaked in 1993, after which it steadily declined to a similar index value in 2019 as the UK and Scotland (between 62% and 57% below the baseline). The trend in Scotland had a lower and later peak at 15% above the baseline in 1998. Since then, it has fluctuated but declined overall, reaching its lowest point in 2019 at 57% below the baseline. A valid abundance trend could not be generated for the other regions due to the small number of colonies monitored elsewhere.

Productivity

Figure 2. Trend in UK breeding productivity (no. of chicks fledged per pair) of natural-nesting lesser black-backed gulls 1989–2019 for the UK (red) and Scotland (blue). Further details can be found in the sections below. Based on SMP data; view the methods of analysis.

Considerable fluctuation has occurred in the UK and Scotland lesser black-backed gull productivity trends since monitoring began in 1986. For much of the monitoring period productivity in Scotland was higher than that of the UK, although since 2011 has often been lower. UK productivity in 2018 and 2019 was above the long-term average. No Scottish productivity data have been submitted to the SMP since 2014 and the number of colonies monitored in other regions are insufficient to allow the production of meaningful productivity trends.

Figure 3. Trend in abundance index (solid line) of natural-nesting lesser black-backed gulls in the UK, 1986–2019 with 95% confidence limits (dotted lines). This abundance trend excludes urban nesting gulls from the sample and, therefore, may not be representative of trends in the entire UK population. Based on SMP data; view the methods of analysis.

Note: ‘Natural-nesting’ is defined as on moors, cliffs, marshes, beaches and other areas of semi-natural habitat, while ‘urban-nesting’ is defined as on human-built structures.

National census data indicate lesser black-backed gulls nesting in coastal colonies increased by 29% from 1969-70 (48,000 pairs) to 1985-88 (62,000 pairs). Increases after 1969 were probably a result of increased food availability from fishery discards¹ and from landfill sites. A further 40% increase (to 87,000 pairs) occurred between the 1985-88 census and Seabird 2000 (1998–2002).

There are uncertainties as to how representative the SMP sample trend is of the lesser black-backed gull UK population since the last seabird census (1998–2002). Uncertainties relate to the relatively small number of urban nesting sites contributing to the annual SMP sample (annual average 5% of sites 1999–2014). The trend in Figure 3 is, therefore, based only on the natural-nesting component of the SMP sample and may not reflect the UK population trend i.e. that of both natural and urban nesting lesser black-backed gulls. Between 2002 and 2018, confidence intervals derived from the natural-nesting lesser black-backed gull sample have run parallel to the abundance index and have become increasingly narrow, giving confidence that the underlying data indicate a real downward trend of their population over the last 15–20 years.

If the SMP trend between the two previous censuses periods is compared to the more accurate trend based on actual census data (i.e. between the 1985-88 and 1998–2002 censuses), it is apparent that although the direction of trend is accurately reflected in the SMP sample, the magnitude of change appears to be over-estimated. The change in the coastal lesser black-backed gull population between the two censuses using census data is +40% whereas the change estimated from the SMP sample over the same period is +71%. We, therefore, have low confidence in the ability of SMP sample data to describe trends in the UK coastal lesser black-backed gull population and advise that 1969-70 and 1998–2002 census data should solely be used for this purpose.

Several major colonies surveyed since Seabird 2000 in Scotland, England and Wales appear to be in decline, while increases have been recorded in the smaller population in Northern Ireland. For example, in 2018 two of the largest colonies in Wales (Skomer and Skokholm) had declined by 46% and 56% since Seabird 2000, respectively. Similar has been observed in England where the major colony at Orford Ness declined by 98% from 5,500 AON in 2001 to 335 AON in 2013 and 97 AON in 2018. However, the neighbouring colony at Havergate Island increased from 290 AON during Seabird 2000 to 1,327 AON in 2018. The colony at Walney Island has decreased by 91% (19,487 AON during Seabird 2000 to 1,981 in 2018) and Bowland Fells by 21% (18,518 AON during Seabird 2000 to 14,627 AON in 2018). In contrast, the Ribble and Alt Estuary colony increased by 69% from 4,150 AON in 1998 to 7,022 in 2016. Ailsa Craig, the largest colony in Scotland during Seabird 2000, now only holds 146 AON in 2018, a decline of 64%. The causes of these declines may be due to a decrease in the availability of domestic refuse, reduced discards from fisheries², predation, cannibalism³ and human disturbance.

Three censuses of roof-nesting gulls were undertaken in 1976⁴, in 1993-95⁵, and during the 1998–2002 Seabird 2000 census⁶. The number of roof-nesting lesser black-backed gulls increased dramatically between these surveys – by 7.7 and 4.3 times respectively since the 1976 census. These censuses counted all inland and coastal gulls nesting on buildings from vantage points and were seen as absolute counts at the time. However, the first two censuses stated that, because of difficulties in locating all nests at colonies, population sizes were likely to be underestimates. The Seabird 2000 census suffered similar methodological issues and did not attain complete coverage so will have also underestimated the size of the roof-nesting population. The degree of under-estimation is, however, likely to be modest, particularly for Operation Seafarer, as inland/roof-nesting only became commonplace from the mid-1970s onwards⁴. The UK trend in roof-nesters since Seabird 2000 is unknown due to insufficient survey coverage, though colonies in south-west and north-west England have expanded in range (comparing Seabird 2000 with the 2007-11 Breeding Bird Atlas)⁷.

The causes of the population increase in urban areas may have been facilitated by an abundance of locally available food (e.g. from fast-food street litter and domestic/commercial rubbish bins), and safe (predator-free) nesting sites on buildings.

Productivity

Figure 4. Trend in UK productivity (no. of chicks fledged per pair) of natural-nesting lesser black-backed gull 1989–2019. This productivity trend excludes urban nesting gulls from the sample and, therefore, may not be representative of trends in the entire UK population. Based on SMP data; view the methods of analysis.

Productivity measured at UK natural-nesting colonies has fluctuated widely over the recording period and, while often low, has shown an increasing trend (Figure 4). Local productivity rates have been linked to nesting habitat^8,9, parental condition¹⁰ and availability of fishery discards¹¹. American mink Neovison vison is known to lower productivity at some colonies in Scotland but factors influencing this parameter in other parts of the UK are largely unknown or poorly reported.

Recently, there were three years with high productivity – 2010, 2012 and 2014. In 2019, productivity was 0.52, equal to the national average of chicks fledged per pair between 1989 and 2019.

Figure 5. Trend in abundance index (solid line) of natural-nesting lesser black-backed gull in Scotland, 1986–2019 with 95% confidence limits (dotted lines; drawing of upper limit restricted to preserve detail in the abundance index). This abundance trend excludes urban nesting gulls from the sample and, therefore, may not be representative of trends in the entire UK population. Based on SMP data; view the methods of analysis.

Between the Seabird Colony Register and Seabird 2000 censuses, coastal-nesting lesser black-backed gull numbers increased by 10%. Prior to this, a large increase was recorded by the SCR census, 62% higher than Operation Seafarer. Reasons for the increase are unknown but, for example, on the Isle of May (the largest colony in Scotland that is counted frequently), numbers increased from the late 1980s to the mid-1990s, with a particularly steep increase between 1992 and 1993, associated with the cessation of gull control measures undertaken in 1984-88 to reduce gull predation on other seabirds¹².

In Scotland, the SMP annual natural-nesting lesser black-backed gull abundance index shows a steep decline in 1987, then an increase between 1992 and 2000, to a maximum peak value of 19% above the baseline. Since then it has fluctuated between 3% and 48% below the 1986 baseline (Figure 5). In 2019, the index was at its lowest on record, at 57% below the baseline. It should, however, be noted that confidence limits are wide throughout the monitoring period and this index should, therefore, be used with caution.

Urban-nesting lesser black-backed gulls increased considerably in Scotland between surveys in 1993-95⁵ (1,346 AON) and Seabird 2000 (3,846 AON), although little information on their status has been available since then. At the last census, Scotland held the second largest proportion of urban-nesting lesser black-backed gulls in the UK (33%).

Productivity

Figure 6. Trend in productivity (no. of chicks fledged per pair) of natural-nesting lesser black-backed gull in Scotland, 1989–2014. This productivity trend excludes urban nesting gulls from the sample and, therefore, may not be representative of trends in the entire UK population. Based on SMP data; view the methods of analysis.

Productivity of natural-nesting lesser black-backed gulls in Scotland has been variable over the recording period, with a slight increasing trend up to 2010 (Figure 6). Predation by American mink at colonies in south-west Scotland is the main cause of low productivity in some, but not all, years. Higher productivity may have been the result of efforts to remove this destructive species from affected colonies. However, productivity from 2011 to 2014 was lower than before, probably due to a combination of factors such as predation, poor weather and poor feeding conditions. In 2014, the most recent year for which data are available, 0.26 chicks were fledged per pair, the second lowest value recorded by the SMP.

Figure 7. Number of lesser black-backed gull AON at South Walney (Cumbria), 1986–2019.

National census data show numbers of coastal-nesting lesser black-backed gulls in England changed little between Operation Seafarer and the Seabird Colony Register but had doubled by the time of Seabird 2000 to just over 44,000 AON.

The SMP sample trend for natural-nesting lesser black-backed gulls is not provided as it has extremely wide confidence intervals, although since Seabird 2000 numbers at several natural nesting sites have decreased. For example, at South Walney (Figure 7) numbers have decreased 98% since a peak of 22,200 AON in 1997. Declines have also occurred at other large colonies between Seabird 2000 and 2018; such as Bowland Fells in north-west England (-21%) and Orford Ness in south-east England (-98%). However, the colony at Havergate Island increased from 290 AON during Seabird 2000 to 1,670 AON in 2019. The causes of these declines may be linked to past control measures at Bowland Fells SPA due to extensive culling of lesser black-backed gulls (to protect breeding red grouse and wading birds) and predation of their chicks and eggs by foxes Vulpus vulpus at Orford Ness¹³. No data were submitted to the SMP from Ordford Ness and Bowland Fells in 2019.

Urban-nesters increased greatly between surveys in 1976⁴ (127 AON), 1993-95⁵ (954 AON) and Seabird 2000 (6,550 AON)⁶. However, the number of gulls nesting in some urban areas may have been underestimated during Seabird 2000, or not counted at all. Since then, little information is available, although large increases have been recorded in some cities and towns in south-west and north-west England¹⁴. At the last census, England held the largest proportion of urban roof-nesting gulls within the UK (60%).

Productivity

Few data on lesser black-backed gull productivity at English colonies have been collected and no urban gull productivity data are submitted to the SMP. On average, natural-nesting lesser black-backed gulls fledged 0.45 chicks per nest per year between 2007 and 2019.

Figure 8. Trend in abundance index (solid line) of natural-nesting lesser black-backed gulls in Wales, 1986–2019 with 95% confidence limits (dotted lines; drawing of upper limit restricted to preserve detail in the abundance index). This abundance trend excludes urban nesting gulls from the sample and, therefore, may not be representative of trends in the entire UK population. Based on SMP data; view the methods of analysis.

In Wales, the number of coastal-nesting lesser black-backed gulls increased by 74% between Operation Seafarer and the Seabird Colony Register with similar numbers recorded during Seabird 2000. Counts at two of the largest Welsh colonies, Skomer and Skokholm do, however, suggest that the natural nesting lesser black-backed gull population in Wales may have declined considerably since the last census. These colonies held a total of 12,426 AON in 2000 and only 6,499 in 2018, a decline of 48%. No data from these islands were submitted to the SMP in 2019. Decreases in adult survival rate and productivity are likely to be the main drivers of this long-term breeding population decline, although it is also possible that some birds have relocated. The reversal of factors responsible for earlier population increases, namely a decrease in the availability of domestic refuse and reduced discards from fisheries, may also be impacting the population^1,15.

In Wales, annual SMP data (Figure 8) suggest that natural nesters increased during the late 1980s and early 1990’s. Since then, the abundance index shows a sustained decline, with the lowest value being recorded in 2019, when it was 56% below the 1986 baseline.

Urban-nesters increased between surveys in 1976⁴ (198 AON), 1993-95⁵ (201 AON) and Seabird 2000 (394 AON)⁶. No nationwide census has been undertaken since, but 2,696 AON were reported from Cardiff alone in 2011, suggesting a large increase may have occurred in some urban areas.

Productivity

Figure 9. Estimated number of chicks fledged per pair of natural-nesting lesser black-backed gull on Skomer Island, 1986–2019.

The estimated number of chicks fledged per pair of natural-nesting lesser black-backed gull on Skomer Island fluctuates widely but has often been low, averaging 0.36 chicks fledged per pair between 1986 and 2019. Years with poor productivity have been linked with a reduction in food availability and quality during the chick rearing period, largely due to the reduction of discards from fisheries and chicks being fed on earthworms¹⁶. In 2010, lesser black-backed gulls on Skomer had their most successful breeding season in the period of the SMP, fledging 0.89 chicks per pair. Only on five other occasions in 28 years of monitoring has success at this colony been above 0.60.

Figure 10. Number of lesser black-backed gull AON at Strangford Lough, 1986–2019.

Lesser black-backed gulls breed at approximately 30 colonies in Northern Ireland, although few are monitored in any one year, so a representative annual trend is not available. Numbers doubled between Operation Seafarer and the Seabird Colony Register and had more than doubled again by Seabird 2000. Recent counts of both natural and urban nesting lesser black-backed gulls suggest that the Northern Ireland population has increased overall since Seabird 2000.

At Strangford Lough (Figure 10), one of the more frequently monitored colonies, natural-nesting lesser black-backed gull numbers increased from 50 AON in 2003 to a peak of 438 AON in 2015, although held only 316 AON in 2019. In 2019, Lighthouse Island (Copeland Islands) held 547 natural nesting lesser black-backed gull AON, an increase of 188% since Seabird 2000. Inland, 1,316 AON were recorded at Lower Lough Erne in 2018, compared to just over 100 AON during Seabird 2000, and a total of 1,584 individuals were recorded at colonies on Lough Neagh in 2019¹⁷ where only 559 were recorded during Seabird 2000.

The number of urban-nesting lesser black-backed gulls in Northern Ireland increased from eight AON in 1993-95⁴ to 63 AON during Seabird 20006. No urban-nesting lesser black-backed gulls were recorded during the 1976 census. The urban nesting population has increased since Seabird 2000, with at least 101 AON being recorded in Belfast city centre and harbour in 2018, and an additional 12 AON recorded in the wider Belfast area. In addition, 77 urban-nesting adults were recorded at a colony in Carrickfergus¹⁸. However, due to the complexity of the roof-scape and the limited number of vantage points, this is likely to be an underestimate.

Productivity

Data submitted to the SMP on lesser black-backed gull productivity in Northern Ireland are sparse; thus, no meaningful average productivity value can be given.

Figure 11. Number of lesser black-backed gull AON on the Calf of Man, 1986–2017.

The population of lesser black-backed gulls on the Isle of Man more than doubled between Operation Seafarer and Seabird 2000 from 54 to 114 AON, although in 2017 a census there recorded just 36 AON. The most comprehensive time series of breeding lesser black-backed gull data comes from the Calf of Man, which held c.25% of the Isle of Man population during Seabird 2000 (28 AON in 1999) and 27 AON during the 2017 census (Figure 11)²⁰. In 2019, 35 AON were recorded on the Calf of Man.

Productivity

No systematic data on the productivity of lesser black-backed gulls on the Isle of Man have been submitted to the SMP.

Figure 12. Estimated adult survival rate of lesser black-backed gull on Skomer, 1986–2018.

The single study site for estimates of adult survival rate of lesser black-backed gull provided to the SMP comes from the large population breeding on Skomer (Figure 12), where survival from 1986 to 2018 averaged 87%, although there has been considerable variation over time²². Survival of breeding adult birds between 2017 and 2018 was 93%, the highest level since 2013.

The decline in survival rate between 1994 and 2003 coincided with a rapid decrease in the number of lesser black-backed gulls breeding on Skomer, presumably caused by very low breeding success, lowering recruitment to the breeding population, although unsuccessful adults may have been deserting Skomer in favour of other breeding locations. Survival of breeding adult birds between 2016 and 2017 dropped to 79%, the lowest level since 2003. Poor adult survival may be one of the drivers behind long-term decline in breeding numbers on Skomer but it is not known how many of the ‘missing’ birds die over the winter and how many move elsewhere²³.