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Arctic skua (Stercorarius parasiticus)

The following has been adapted from original text by Robert W. Furness and Norman Ratcliffe in Seabird Populations of Britain and Ireland (with permission from A&C Black, London).

 

In Britain, the Arctic skua is confined to breeding in north and west Scotland, at the southern extremity of its circumpolar, high latitude breeding range. In Scotland, most nest in moorland colonies close to aggregations of auks (common guillemots, razorbills and Atlantic puffins), black-legged kittiwakes and Arctic terns from which they obtain food by piracy. In a few places, such as the extensive moors of Caithness, the species can be found further inland in rather scattered breeding territories, where feeding on berries, insects and small birds may be more important. Unlike the larger great skua, Arctic skuas do not normally scavenge behind fishing boats or feed as members in multi-species flocks of seabirds on surface shoals of fish, as their smaller size means they cannot compete in such situations. Although numbers nesting in Scotland increased in the 1970s and 1980s, most of their breeding sites have been established for many decades or centuries with few new colonies formed, resulting in a remarkably static breeding range.

Conservation status

Arctic skua is currently identified as a conservation priority in the following:

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International importance

UK Population

Biogeographic Population

% World Population

2,100 AOT*

8.4 (NE Atlantic)

1.0

*AOT = Apparently Occupied Territories

The UK population figure (rounded to the nearest hundred) was derived from data in Mitchell, P.I., Newton, S.F., Ratcliffe, N. and Dunn, T.E. (eds.) 2004. Seabird Populations of Britain and Ireland. Poyser, London. This was also the source of figures for the Biogeographic and World populations.

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UK population estimates and change 1969–2002 (census data)

Once they have been recruited into a colony, Arctic skuas usually breed every year returning to the same territory year after year. However, in seasons when food supply is particularly poor, birds may fail to lay. This appears to have been the case during surveys of Arctic skua in Shetland in 2000 and 2001 which may have resulted in an underestimate of the number that would normally be breeding under more favourable conditions. Breeding success in Shetland was also poor in 2000 and 2001, and birds that lost eggs early on may have shown low attendance in the territory, possibly resulting in some territories being missed. Counts in Shetland in 2002 were affected by poor weather conditions in some parts, with a considerable area surveyed in fog which may also have contributed to an underestimate of numbers.

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998–2002)

UK Population estimate (AOT*)

1,039

3,388

2,136

% change since previous census   

n/a

+226

-37

* AOT = Apparently Occupied Territories

For census results for individual countries and Ireland, the Channel Islands and the Isle of Man see under relevant sections below.

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Distribution/abundance

The Seabird 2000 census provides the most comprehensive recent assessment of the distribution and abundance of breeding seabirds. Numbers of Arctic skua found in different regions, and a map showing the location and size of colonies, is provided in the Seabird 2000 Arctic skua results.

The locations sampled during the annual Seabird Monitoring Programme provide some information on distribution and are accessible via the Seabird Monitoring Programme online database.

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Annual abundance and productivity by geographical area

With reference to the regional accounts below please note the following:

Breeding abundance: graphs of abundance index with 95% confidence limits are only shown for a region where the trend produced has been deemed accurate (see methods of analysis). Where a trend was thought to be inaccurate, graphs of abundance at major colonies in a region may be shown instead, particularly if such colonies hold greater than 10% of the regional population, are monitored frequently and may thus help illustrate regional population fluctuations outwith national censuses. Occasionally, too few data have been collected regionally to produce either of these.

Productivity: graphs of estimated reproductive success (hereinafter referred to as productivity) are only shown if data analysis produced a significant result for regional and/or year effects (again see methods of analysis). If results were not significant, then a regional mean productivity value is given. However, on some occasions, too few data are available from which to provide a meaningful average.

 

Overview of all regions

Breeding abundance

Within the range covered by this report, Arctic skua only breeds in Scotland; thus, the abundance trend is provided under the UK and Scotland sections only.

 

Productivity

Within the range covered by this report, Arctic skua only breeds in Scotland; thus, the productivity trend is provided under the UK and Scotland sections only.

 

United Kingdom

Breeding Abundance

AC UK abundance.jpg

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 species3,4,5. Another factor contributing to their decline is likely to be competition for nesting territories and predation by great skuas which have increased markedly2,6,7,8,9,10.

 

Productivity

AC UK productivity.jpg

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 Orkney11,12. 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 seabirds13,14. 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 regions3,9,11.

 

Scotland

Breeding abundance

AC Scot abundance.jpg

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 species3,7,8,9,10. Another factor contributing to their decline is likely to be competition for nesting territories and predation by great skuas which have increased markedly2,9,11,12,13.

 

Productivity

AC Scot productivity.jpg

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 skuas1,2. 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 Orkney13,14. 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 seabirds6,7,8. 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 regions3,9,11,12.

 

England

Arctic skua does not breed in England.

 

Wales

Arctic skua does not breed in Wales.

 

Northern Ireland

Arctic skua does not breed in Northern Ireland.

 

Republic of Ireland

Arctic skua does not breed in the Republic of Ireland.

 

All Ireland

Arctic skua does not breed in Ireland.

 

Isle of Man

Arctic skua does not breed on the Isle of Man.

 

Channel Islands

Arctic skua does not breed on the Channel Islands.

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UK phenology, diet, survival rates

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

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References

1 Perkins, A., Ratcliffe, N., Suddaby, D., Ribbands, B., Smith, C., Ellis, P., Meek, E. and Bolton, M. 2018. Combined bottom-up and top-down pressures drive catastrophic population declines of Arctic skuas in Scotland. Journal of Animal Ecology, 87(6), 1573–1586.

2 Furness, R.W. and Ratcliffe, N. 2004. Arctic Skua Stercorarius parasiticus. In: Mitchell, P.I., Newton, S.F., Ratcliffe, N. and Dunn, T.E. eds. 2004. Seabird Populations of Britain and Ireland, pp. 160–172. Poyser, London.

3 Phillips, R.A., Caldow, R.W.G. and Furness, R.W. 1996. The influence of food availability on the breeding performance and reproductive success of Arctic Skuas. Ibis, 138, 410–419.

4 Frederiksen, M., Edwards, M., Richardson, A.J., Halliday, N.C. and Wanless, S. 2006. From plankton to top predators: bottom-up control of a marine food web across four trophic levels. Journal of Animal Ecology, 75, 1259–1268.

5 Régnier, T., Gibb, F.M. and Wright, P.J. 2017. Importance of trophic mismatch in a winter-
hatching species: evidence from lesser sandeel. Marine Ecology Progress Series, 567, 185–197.

6 Wanless, S., Harris, M.P., Newell, M.A., Speakman, J.R., and Daunt, F. 2018. Community-wide decline in the occurrence of lesser sandeels Ammodytes marinus in seabird chick diets at a North Sea colony. Marine Ecology Progress Series, 600, 193–206.

7 Furness, R. 1977. Effects of Great Skuas on Arctic Skuas in Shetland. British Birds, 70, 96–107.

8 Phillips, R.A., Furness, R.W. and Stewart, F.M. 1998. The influence of territory density on the vulnerability of Arctic skuas Stercorarius parasiticus to predation. Biological Conservation, 86, 21–31.

9 Jones, T., Smith, C., Williams, E. and Ramsay, A. 2008. Breeding performance and diet of Great Skuas Stercorarius skua and Parasitic Jaegers (Arctic Skuas) Stercorarius parasiticus on the west coast of Scotland. Bird Study, 55, 257–266.

10 Phillips, R.A., Caldow, R.W.G. and Furness, R.W. 1996. The influence of food availability on the breeding effort and reproductive success of Arctic Skuas Stercorarius parasiticus. Ibis, 138, 410–419.

11 Wright, P.J. and Bailey, M.C. 1993. Biology of Sandeels in the Vicinity of Seabird Colonies at Shetland. Fisheries Research Report, No. 15/93. Aberdeen: SOAFD Marine Laboratory.

12 Poloczanska, E.S., Cook, R.M., Ruxton, G.D. and Wright, P.J. 2004. Fishing vs. natural
recruitment variation in sandeels as a cause of seabird breeding failure at Shetland: a modelling approach. Journal of Marine Science, 61, 788–797.

13 Frederiksen, M., Furness, R.W. and Wanless, S. 2007. Regional variation in the role of bottom-up and top-down processes in controlling sandeel abundance in the North Sea. Marine Ecology Progress Series, 337, 279–286.

14 Burthe, S., Daunt, F., Butler, A., Elston, D., Frederiksen, M., Johns, D., Newell, M., Thackeray, S.J. and Wanless, S. 2012. Phenological trends and trophic mismatch across multiple levels of a North Sea pelagic food web. Marine Ecology Progress Series, 454, 119–133.

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Partners

Data have been provided to the SMP by the generous contributions of its partners, other organisations and volunteers throughout Britain and Ireland. Partners to the SMP are: BirdWatch Ireland; The British Trust for Ornithology; Department of Agriculture, Environment and Rural Affairs (Northern Ireland); Department of Environment, Food and Agriculture (Isle of Man); Department of the Environment, Climate and Communications (Republic of Ireland); JNCC; Manx Birdlife; Manx National Heritage; The National Trust; National Trust for Scotland; Natural England; Natural Resources Wales; NatureScot; The Royal Society for the Protection of Birds; Scottish Wildlife Trust; Seabird Group; Shetland Oil Terminal Environmental Advisory Group; States of Guernsey Government; UK Centre for Ecology and Hydrology. More about the SMP partners >>

 

Image of Arctic skua appears courtesy of Ian Rendall ©, is subject to international copyright law and may not be reproduced in any form whatsoever.

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Categories:

SMP Report 1986–2019

Published: .

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