Arctic skua (Stercorarius parasiticus)

Figure 1. Trend in UK abundance index (solid line) of Arctic skua 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

National census data show the UK Arctic skua breeding population increased greatly between 1969-70 (1,000 AOT) and 1985-88 (3,400 AOT), although some of this increase was due to greater coverage during the SCR census. However, Seabird 2000 only recorded 2,100 AOT, 37% fewer than the SCR census. Figure 1 shows that the SMP annual trend was relatively stable until the early 1990s but has declined steadily ever since; the Arctic skua has probably declined more than any other seabird in the UK during the period 1986 to 2019, with the lowest index value occurring in 2017, at 83% below the 1986 baseline. The current breeding seabird census, Seabirds Count, will provide a more comprehensive assessment of the scale of the decline.

A study in 2015 estimated that the Artic skua population in Orkney, Shetland and Handa combined (which represents 88% of the UK population) was approximately 200 AOT, a decline of 81% since 1992 and 71% since Seabird 2000¹. Arctic skuas in Shetland had already declined by 42% between the SCR and Seabird 2000, and a small number of well monitored colonies suggest a further large fall in numbers; Noss, Foula, Fair Isle and Mousa, held 49 AOT in 2019, 76% fewer than were recorded during Seabird 2000 (206 AOT). In the west of Scotland, only 20 AOT were recorded on Handa in 2019, a decline of 53% since a peak of 42 AOT in 2001.

In Scotland, Arctic and great skuas breed sympatrically, usually beside large colonies of cliff-nesting seabirds². One of the main factors contributing towards this national decline is likely to be the continuous decrease in their annual productivity (no. chicks fledged per pair) and that of their host species from which they kleptoparasitise food (Arctic tern, kittiwake, common guillemot and puffin). Recent observed alterations in the marine food web in the north-east Atlantic, strongly influenced by fisheries management and climate change, are driving the decline of lesser sandeels Ammodytes marinus, a main food source of Arctic skuas and their host species^3,4,5. Another factor contributing to their decline is likely to be competition for nesting territories and predation by great skuas which have increased markedly^2,6,7,8,9,10.

Productivity

Figure 2. Trend in UK productivity (no. of chicks fledged per pair) of Arctic skua 1986–2019. Based on SMP data; view the methods of analysis.

Arctic skuas periodically experience years of very poor productivity (chicks fledged per pair), which often coincide with periods of presumed low sandeel abundance²; such periods have become more frequent since the late 1990s, with several years of very low productivity since 2003.

Annual data from the six most frequently monitored Arctic skua colonies in Shetland (92 data points between 2003–2019 from Fair Isle, Fetlar, Foula, Hermaness, Mousa and Noss) illustrate just how bad breeding seasons have become. Complete failure was recorded 47 times, with productivity below 0.20 chicks fledged per pair on a further 16 occasions; only on six occasions did productivity climb above 0.80. Arctic skuas rely on stealing fish caught by other seabirds, especially Arctic terns, black-legged kittiwakes, common guillemots and Atlantic puffins; declines in the abundance and chick provisioning of some of these host species has reduced feeding opportunities for Arctic skuas¹. Recent sandeel scarcity around Shetland is attributed to low recruitment in most years since the mid-1980s, linked to hydro-climatic changes affecting hatching dates, survival and transport of larvae from major spawning areas north and west of Orkney^11,12. More significantly, environmental conditions for sandeels appear to be worsening, with sea temperature increases and oceanographic changes affecting their physiology, food supply, phenology and survival, leading to trophic mismatch and less food for seabirds^13,14. Predation of Arctic skua chicks, and sometimes displacement or killing of adults during territory disputes by great skuas (also ultimately linked to a scarcity of alternative fish prey for great skuas), is also known to be reducing productivity in some regions^3,9,11.

Figure 3. Trend in abundance index (solid line) of Arctic skua in Scotland, 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.

National census data show the Scottish Arctic skua population increased between 1969-70 (1,000 AOT) and 1985-88 (3,400 AOT), although some of this increase was due to greater coverage during the SCR census. However, Seabird 2000 only recorded 2,100 AOT, 37% fewer than the previous census. Figure 3 shows that the SMP annual trend was stable until the early 1990s but has declined steadily since; the Arctic skua has probably declined more than any other seabird in the UK during the period from 1986 to 2019, with the lowest population index occurring in 2017, at 83% below the 1986 baseline. The current breeding seabird census, Seabirds Count, will provide a more comprehensive assessment of the scale of the decline.

A survey of the Orkney Arctic skua population in 2010 found just 380 AOT compared to 720 AOT in 2000, so numbers there alone have declined by 47% during that decade (and by 64% since 1992 when 1,043 AOT were recorded). In addition, A study in 2015 estimated that the Artic skua population in Orkney, Shetland and Handa combined (which represents 88% of the UK population) was approximately 200 AOT, a decline of 81% since 1992 and 71% since Seabird 2000¹. However, to fully ascertain the scale of the current decline, extensive survey work is still needed in Shetland where over half of the UK (and Scottish) population bred in the past. Arctic skuas in Shetland had already declined by 42% between the SCR and Seabird 2000, and a small number of well-monitored colonies suggest a further large fall in numbers; Noss, Foula, Fair Isle and Mousa, held 49 AOT in 2019, 76% fewer than were recorded by Seabird 2000 (206 AOT). In the west of Scotland, only 20 AOT were recorded on Handa in 2019, a decline of 53% since a peak of 42 AOT in 2001.

In Scotland, Arctic and great skuas breed sympatrically, usually beside large colonies of cliff-nesting seabirds². One of the main factors contributing towards the national decline is likely to be the continuous decrease in their annual productivity (no. chicks fledged per pair) and that of their host species from which they kleptoparasitise food (Arctic tern, kittiwake, common guillemot and puffin). Recent observed alterations in the marine food web in the north-east Atlantic, strongly influenced by fisheries management and climate change are driving the decline of lesser sandeels Ammodytes marinus a main food source of Arctic skuas and their host species^3,7,8,9,10. Another factor contributing to their decline is likely to be competition for nesting territories and predation by great skuas which have increased markedly^2,9,11,12,13.

Productivity

Figure 4. Trend in productivity (no. of chicks fledged per pair) of Arctic skua in Scotland, 1986–2019. Based on SMP data; view the methods of analysis.

Arctic skuas periodically experience years of very poor productivity (no. of chicks fledged per pair), which often coincide with periods of presumed low sandeel abundance²; such periods have become more frequent since the late 1990s, with several years of very low success since 2003. A recent study found that annual productivity declined from a five-year mean of 0.91 between 1992 and 1996, to 0.29 between 2011 and 2015.

Annual data from the six most frequently monitored Arctic skua colonies in Shetland (85 data points between 2003–2019 from Fair Isle, Fetlar, Foula, Hermaness, Mousa and Noss) illustrate just how bad breeding seasons have become. Complete failure was recorded 47 times, with productivity below 0.20 chicks fledged per pair on a further 16 occasions; only on six occasions did productivity climb above 0.80. Arctic skuas rely on stealing fish caught by other seabirds, especially Arctic terns, black-legged kittiwakes, common guillemot and Atlantic puffins; declines in the abundance and chick provisioning of some of these host species has reduced feeding opportunities for Arctic skuas^1,2. Recent sandeel scarcity around Shetland is attributed to low recruitment in most years since the mid-1980s, linked to hydro-climatic changes affecting hatching dates, survival and transport of larvae from major spawning areas north and west of Orkney^13,14. More significantly, environmental conditions for sandeels appear to be worsening, with sea temperature increases and oceanographic changes affecting their physiology, food supply, phenology and survival, leading to trophic mismatch and less food for seabirds^6,7,8. Predation of Arctic skua chicks and sometimes displacement or killing of adults during territory disputes by great skuas (also ultimately linked to a scarcity of alternative fish prey for great skuas) is also known to be reducing productivity in some regions^3,9,11,12.