Black-headed gull (Chroicocephalus ridibundus)

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

The UK and England black-headed gull abundance trends appear to be following a similar pattern, with index values declining from the late 1980s to the beginning of the 2000s. Since then, the trends have been increasing (Figure 1). While the UK index for much of the monitoring period closely mirrored the English trend, a gap between the two has become evident in recent years, with England lower than the UK. More detailed information can be found in the individual country sections below. Black-headed gulls breed in all areas covered by this report apart from the Channel Islands, however, in most of these, a valid abundance trend cannot be generated due to insufficient data being submitted to the SMP.

Productivity

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

There has been considerable regional variation in black-headed gull productivity trends since monitoring began in 1986 (Figure 2). A large proportion of the nests monitored for productivity are based in England and as such, the UK and English trends are similar.

Data from other regions are sparse; thus, no meaningful trends can be provided for these.

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

National census data show that the number of coastal nesting black-headed gulls in the United Kingdom was relatively stable between 1969-70 and 1998–2002. However, there are differences within the census data for the constituent countries of the UK (see individual country accounts). Data submitted to the SMP show an increase in the abundance index to the early 1990s but a decline thereafter until 2003. After this, the trend gradually increased until 2018 where it was over 35% above the baseline, although in 2019 it had decreased to 21% above (Figure 3).

Productivity

Figure 4. Trend in UK productivity (no. of chicks fledged per pair) of black-headed gull 1987–2019. Based on SMP data; view the methods of analysis.

Over the monitoring period, black-headed gull productivity has fluctuated markedly and is likely to have been affected by American mink Neovison vison predation^1,2 (and, to a lesser extent, by rats Rattus sp.), as well as changes in food supply and periods of inclement weather during breeding seasons. This fluctuating productivity trend is common to black-headed gull colonies throughout the UK. In 2019, an average of 0.48 chicks were fledged per pair (Figure 4).

Figure 5. Abundance of black-headed gull at Loch of Strathbeg, Sands of Forvie and Insh Marshes, 1986–2019.

The number of black-headed gulls nesting in coastal areas of Scotland declined severely between 1969-70 and 1998–2002. National census data show that almost 50% of the population had disappeared between Operation Seafarer and the Seabird Colony Register, with a further decline of 28% by the time of Seabird 2000.

Due to colonies being surveyed infrequently, no annual trend can be generated from data submitted to the SMP. However, counts at three of the largest and most frequently monitored colonies (Loch of Strathbeg, Sands of Forvie and Insh Marshes) show how numbers have changed at each site over the reporting period (Figure 5). The Sands of Forvie colony had almost disappeared by the mid-1990s but, by 2018, had recovered and held over 2,100 AON. In 2019, it decreased slightly to 1,963 AON. However, Loch of Strathbeg and Insh Marshes have declined considerably over the reporting period, now holding a fraction of the numbers they once did. Data from 51 coastal colonies counted in 2019 totalled 2,336 AON (including Sands of Forvie and Loch of Strathbeg), an increase of 14% compared to the 2,033 AON recorded at the same colonies during Seabird 2000. However, drawing conclusions about the fortunes of black-headed gulls in Scotland is difficult without extensive coverage of inland colonies which held far greater numbers compared to coastal colonies during the last census. The current census, Seabirds Count (2015–2021), will provide an update on the status of black-headed gulls in Scotland.

Productivity

Figure 6. Trend in productivity (no. of chicks fledged per pair) of black-headed gull in Scotland, 1987–2019. Based on SMP data; view the methods of analysis.

Productivity of black-headed gulls in Scotland has fluctuated considerably over the monitoring period. There is no clear trend, although birds appear to have been more successful overall during the latter half of the recording period. However, productivity was only 0.28 chicks fledged per pair in 2018 and 0.39 in 2019 (Figure 6).

Productivity is affected by mammal predation, especially by American mink Neovison vison at west coast colonies³. Comparisons of productivity at colonies where American mink were controlled against those with no control, or where control was unsuccessful, found that on average, between 1997 and 2011, American mink lowered success from 0.79 to 0.32 chicks fledged per pair – an estimated 59% reduction. However, from 2012 to 2014, success at colonies where American mink were controlled (0.06, 0.44 and 0.00 in each year, respectively) was actually lower than at colonies with no, or unsuccessful, control (0.44, 0.65 and 0.69 in each year, respectively), suggesting other factors (e.g. predation by large gulls, predation by otters Lutra lutra, or due to inclement weather) were impacting on productivity.

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

Census results for coastal nesting black-headed gulls in England show an increase of 14% in the number of nests recorded between 1969-70 and 1985-88. Results from Seabird 2000 showed little change took place between 1985-88 and 1998–2002. The abundance trend of black-headed gulls from SMP data indicates increases may have continued up until the early 1990s (Figure 7) before declining. Abundance was then relatively stable until 2006 when the index reached its lowest point since monitoring began. However, abundance has increased since then and, between 2010 and 2015, reached values equal to, or above, those recorded in the late 1980s and early 1990s. In 2018, the index was 53% above the 1986 baseline, one of the highest values ever recorded. In 2019, it had declined to 31%.

The breeding population at inland colonies (about 100 were known at the time of Seabird 2000) may have increased too, although an insufficient number are routinely surveyed to deliver a robust trend. Two inland colonies, Belmont Reservoir and Old Moor, that were surveyed in 2018 held a total of 13,555 AON, a very large increase since Seabird 2000, when they held 70 AON and 200 AON, respectively. Based on the latest UK population estimate of black-headed gull (138,000 pairs), Belmont Reservoir holds c. 8%⁴ of the UK breeding population and is probably the largest inland black-headed gull colony in the UK.

Productivity

Figure 8. Trend in productivity (no. of chicks fledged per pair) of black-headed gull in England, 1990–2019. Based on SMP data; view the methods of analysis.

The productivity of black-headed gulls breeding in England has fluctuates markedly during the monitoring period. Particularly low productivity levels were recorded prior to 1996, often due to high tides washing out nests. Between 1996 and 2001, colonies fledged an average of over one chick per pair per year but this was followed by a reduced mean productivity of 0.55 chicks per year per pair between 2002 and 2009. In 2012, failure at several colonies was caused by flooding or predation leading to another poor season (0.24 chicks fledged per pair). Since then, productivity has rapidly increased and in 2019 was 0.63 chicks fledged per pair (Figure 8).

Figure 9. Numbers of black-headed gull AON at Strangford Lough, 1986–2019.

Seabird 2000 recorded just over 4,000 pairs of black-headed gulls nesting around the Northern Ireland coastline, 12% fewer than recorded by the SCR but almost three times that found by Operation Seafarer. However, the national population must have been far higher between the SCR and Seabird 2000 censuses, as numbers at Strangford Lough alone (Figure 9), climbed above 7,000 AON at the colony’s peak in the mid-1990s, before declining prior to Seabird 2000. Numbers at Strangford Loch then increased again but, from 2008 to 2012 declined considerably (from 4351 to 1015 AON respectively). Since 2013 numbers have stabilised to between 1,200 and 1,300 AON each year. The colonies at Carlingford Lough, Copeland Island and Rathlin need to be counted to better understand the status of the coastal-nesting population of black-headed gulls in Northern Ireland.

Few inland colonies are regularly monitored, although those at Lower Lough Erne and Portmore Lough held 1,517 AON in 2018 compared to 1,033 AON during Seabird 2000. On Lough Neagh, which held 30,000 breeding AON of black-headed gulls on 12 islands in the 1980s, counts of the main breeding islands estimated 11,595 individuals in 2016, 8,120 in 2017 and 8,906 in 2018⁵. In addition, black-headed gulls have abandoned breeding on Shallow Flat and Coney Island Flat, and have decreased in number on Padian Island, Owen Roe and Scaddy Island⁶. No Northern Ireland abundance data were submitted to the SMP for 2019.

Productivity

Data submitted to the SMP on black-headed gull productivity in Northern Ireland are sparse. At Portmore Lough, 124 AONs produced 159 chicks (1.28 chicks fledged per pair), almost identical to 2017. In 2019, average productivity was 1.08 chicks fledged per pair.