Herring gull (Larus argentatus)

Figure 1. Trends in abundance index of natural-nesting herring gulls 1986–2019 for the UK (red), Scotland (blue), England (black) 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 herring gull abundance trends since monitoring began in 1986 (Figure 1). In contrast to the UK and Scotland trend – which have stayed below the baseline since monitoring began in 1986 – the England trend had its highest index values between 1994 and 2000 and Wales between 1999 and 2006, in 2011 and between 2013 and 2014. Although, since 2000, all trends have declined and have shown less fluctuation than previously. More detailed information can be found in the individual sections below. The number of colonies monitored frequently are only sufficient to allow meaningful abundance trends to be produced for the UK, Scotland, England and Wales.

Productivity

Figure 2. Trends in productivity index of natural-nesting herring gulls 1989–2019 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.

A large proportion of the nests monitored for productivity are from colonies in Scotland and as such, the UK and Scottish trends are closely matched. No Scottish productivity data have been submitted to the SMP since 2014. Natural-nesting herring gulls breed in all regions covered in this report; however, a valid productivity trend could only be generated for the UK, Scotland and Wales.

Figure 3. Trend in abundance index (solid line) of natural-nesting herring 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.

There are uncertainties as to how representative the SMP sample is of the entire UK herring gull breeding population (i.e. natural and urban-nesting herring gulls combined), as only a relatively low proportion of the sample comes from the urban nesting population (6% average per-annum over the sampling range). We, consequently, have low confidence in the ability of SMP sample data to predict trends for the entire UK herring gull population and do not, therefore, include a combined urban and natural nesting population trend in this report (Table 1). Census data should, therefore, be solely used for this purpose.

UK census data, where each census is a complete count of all known colonies at that time, show that coastal nesting herring gulls decreased by 48% from 1969-70 (285,900 AON) to 1985-88 (149,200 AON), with a continuing decline, albeit at a slower rate (-13%), between the 1985-88 and 1998–2002 censuses (130,230 AON).

Annual SMP data (Figure 3) indicate that after the Seabird Colony Register Census period, natural nesting abundance decreased until 1992, after which it increased for a time but then fell to its lowest ever value in 2015, at 61% below the 1986 baseline. Since then, it has been fluctuating and in 2019 was 56% below the baseline.

At the time of the Seabird 2000 census, approximately 14% of the UK herring gull population nested on buildings, compared to natural habitats; a proportion and total number that increased from 1976¹ (when just 63 pairs nested on roofs) to 1993-95² (10,900) and 1998–2002 (20,000)³. The abundant food supply in urban areas provided by street litter and insecure refuse bags/bins, combined with abundant safe nesting sites, has probably facilitated this increase². The current size of the UK urban nesting herring gull population is not known.

The decrease in the coastal natural-nesting herring gull population may be indicative of a decline in the entire UK breeding population, although this will not be known until the current Seabirds Count census (2015–2021), which includes natural and urban-nesting gulls, has been completed. The reasons for the apparent decline in the natural nesting population are not well understood. Botulism is thought to have been a major factor in the decline between the first two censuses and possibly thereafter; refuse tips may have been the source of the Clostridium botulinum bacterium that caused the disease, which also is widespread in wetland sediments³. A decrease in the availability of food scavenged from refuse tips (associated with changes in refuse management in recent years)³; and reductions in the availability of discards from fisheries^4,5 may have also contributed to a decrease in the natural nesting herring gull population. Ground predators have also had an effect at some colonies⁶.

Productivity

Figure 4. Trend in UK productivity (no. of chicks fledged per pair) of natural-nesting herring gull 1986–2019. 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.

Although there were fluctuations in UK natural-nesting herring gull productivity between 1989 and 1999, there does appear to be an underlying declining trend during this period. Between 2000 and 2010, productivity increased and was relatively stable, although never high, with an average of 0.60 chicks fledged per pair annually. In 2015, productivity declined to its lowest level since sampling began (0.30), although by 2019 had increased to 0.57 chicks fledged per pair (Figure 4).

Figure 5. Trend in abundance index (solid line) of natural-nesting herring gulls in Scotland, 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.

The number of herring gulls nesting in coastal areas of Scotland declined severely between 1969-70 and 1998–2002. National census data show a 42% decline in the population between Operation Seafarer and the Seabird Colony Register, with a further 23% decline by Seabird 2000. Due to urban colonies being surveyed infrequently, no trend for the entire herring gull population in Scotland (i.e. urban and natural nesting combined) can be generated from data submitted to the SMP.

In Scotland, the SMP index (Figure 5) shows a steady decline in natural nesting herring gull abundance to 56% below the 1986 baseline in 2009. Since then, the abundance index has fluctuated, although has remained well below the baseline. In 2019, the index had fallen to 60% below the baseline, the third lowest value recorded since monitoring began.

At the last census, Scotland held the second largest proportion of urban roof-nesting gulls within the UK (33%). Numbers nesting on buildings in towns and cities increased from 1976¹ (55 pairs) to 1993–1995² (3,568 pairs) and to 1998–2002 (5,843 pairs)³. The current number of urban nesting gulls in Scotland is unknown but is likely to have increased in some areas. Very few urban areas have been surveyed since the last census, so no meaningful summary can be provided to indicate if herring gulls are increasing or decreasing in these areas.

Productivity

Figure 6. Trend in productivity (no. of chicks fledged per pair) of natural-nesting herring gulls in Scotland, 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.

Despite fluctuations in the Scottish natural-nesting herring gull productivity trend index between 1989 and 2000, there may have been an overall decline during that period. Between 2000 and 2010, productivity increased and was relatively stable, although never high, with an average of 0.60 chicks fledged per pair annually. In 2015, productivity fell to 0.45 chicks fledged per pair (Figure 6). Insufficient productivity data for natural-nesting herring gulls have been submitted to the SMP between 2016 to 2019 to produce a trend.

Most information on natural-nesting herring gull productivity in Scotland comes from a study of the effects of American mink Neovison vison control on the breeding success of gulls and terns on west coast islands. This introduced mammal can have a significant depressive effect on breeding success, although usually not to the same extent as that seen for common gull. Herring gull productivity data collected from this study area between 1996 and 2013 found colonies with successful mink control fledged an average of 0.91 chicks per pair per year, compared to 0.61 at colonies with no, or unsuccessful, mink control; so, on average, mink lowered breeding success by 33% (range 0–71%). In 2014, success at these two groups of colonies was 0.99 and 0.58 chicks fledged per pair, respectively – a reduction of 41% due to the effects of American mink⁷. Trapping, specifically targeting colonies where the American mink are most active, has helped to raise the number of young fledged over at least the last decade and thus may be one of the causes of the upward trend in productivity visible in Figure 4 since the late 1990s. Predation of herring gull chicks by other gulls has also been responsible for reduced productivity on the Isle of May⁷.

Figure 7. Trend in abundance index (solid line) of natural-nesting herring gull in England, 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

In common with Scotland and Wales, the natural nesting coastal herring gull population in England declined greatly between the censuses of 1969-70 and 1985-88. By Seabird 2000 (1998–2002), although the population had increased, numbers were still some 18,000 pairs lower than recorded during the 1969-70 census.

The SMP abundance trend (Figure 7) indicates that the natural-nesting herring gull population in England declined until the early 90s but then increased, to a maximum of 26% above the 1986 baseline in 1994. The herring gull population maintained positive index values until 2000, then plummeted to 42% below the baseline in 2001. Index values continued to decline and reached their lowest value, of 79% below the 1986 baseline, in 2016 and 2019.

At the last census, England held the largest proportion of urban roof-nesting gulls within the UK (60%). The number of roof-nesters has increased greatly, from 1,960 pairs in 1976¹ to 6,383 pairs in 1993-95 and to 12,284 pairs by Seabird 2000³. However, the current size of the urban population is unknown, though recent increases have been documented in towns in south-west and north-west England⁸. The fourth seabird census (Seabirds Count, 2015-21) will provide a comprehensive picture of trends in the UK herring gull breeding population.

Productivity

Relatively few data are available on the productivity of natural-nesting herring gulls at English colonies hence a valid trend could not be generated.

Figure 8. Trend in abundance index (solid line) of natural-nesting herring gulls in Wales, 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.

Coastal herring gull numbers fell by 77% in Wales between Operation Seafarer and the Seabird Colony Register and, although a subsequent increase of 26% was recorded by Seabird 2000, numbers were still 34,600 AON lower than in 1969-70. The SMP annual sample indicates that natural-nesters decreased in the late 1980s but began increasing in the early 1990’s until 2000. In 2018, a suite of 125 colonies were counted which held 5,945 AON compared to 6,769 during Seabird 2000, a decline of 12%. This sample represents just over 50% of the natural-nesting population counted during Seabird 2000 and, therefore, may be indicative of the current state of the Welsh population.

Urban roof-nesting herring gulls in Wales increased from 772 AON in 1993-95¹ to 1,826 AON in Seabird 2000³. In 2011, a survey of urban gulls in Cardiff recorded 640 AON. This represents a remarkable increase, as previous estimates indicated 425 AON in 1975, four pairs in 1993 and none during Seabird 2000 (1998–2002).

Productivity

Figure 9. Trend in productivity (no. of chicks fledged per pair) of natural-nesting herring gulls in Wales, 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 of natural-nesting herring gulls at colonies in Wales declined between 1996 and 2008 but has increased since then (Figure 9).

On Skomer, one of the SMP key sites, average productivity for all years monitored between 1962 and 2019 was 0.67 chicks fledged per pair. Productivity in 2019 was 0.28, well below the overall island average and the lowest since 2002⁹.

Figure 10. Number of herring gull AON on the Calf of Man, 1986–2019.

Between Operation Seafarer and the Seabird Colony Register there was a decline of 9% in the number of herring gulls nesting on the Isle of Man. A larger decline then occurred so that, by Seabird 2000, numbers had fallen a further 21% to 7,126 AON. In 2017, a census of the Isle of Man recorded 1,033 herring gull AON, representing a decline of 86% since Seabird 2000¹². The decline was greatest at coastal sites (down 85% from 1999 to 2017) compared to urban sites (where a 31% decline was observed). The Calf of Man is the only colony that has been surveyed regularly since Seabird 2000 and reflects the declines seen in the Isle of Man herring gull population (Figure 10). In 2019, 463 AON were recorded on the Calf of Man.

Productivity

No systematic data on the productivity of herring gulls on the Isle of Man have been submitted to the SMP.

Figure 11. Estimated adult survival rate of herring gull on Skomer, 1986–2018.