Arctic Skua Stercorarius parasiticus

Status;  International importance;  Population estimates;  Distribution;  Annual abundance/ productivity; Phenology/diet/survival

 

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Description

Arctic Skua vignette

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:

EC Birds Directive - migratory species

Red listed in Birds of Conservation Concern 4 (2015 update)

UK BAP - priority species

(further information on Conservation Designations for UK Taxa)

Amberin  Birds of Conservation Concern in Ireland 2014-2019 (2014 update)


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.

 


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.

 


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 page (PDF, 1.1 mb).

 

An interactive map is available on the NBN Gateway, where you can filter to display only the Seabird 2000 data.  For more recent, but less comprehensive, coverage view the distribution on the NBN with all available contributing datasets.

 

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

 


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% CLs 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 productivity are only shown if analysis of breeding success data 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. Furthermore, for 11 species where the quality of monitoring data available was considered high, population viability analysis was undertaken at the UK level and the results of this are also reported.   

 



 

Breeding abundance

 

UK Arctic skua abundance trend

Figure 1: Trend in UK abundance index (solid line) of Arctic skua 1986-2015 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

National census data show the population of Arctic skuas 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 preceding census. Figure 1shows the 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 in the period from 1986 to 2015, with the lowest population index in 2013 estimated to be 82% lower than in 1986. A survey of the Orkney population in 2010 found just 380 AOT compared to 720 AOT in 2000, so numbers there alone have declined by 47% during the last decade (and have declined by 64% since 1992 when 1,043 AOT were recorded). However, to fully ascertain the scale of the current decline, extensive survey work is needed in Shetland where over half of the UK (and Scottish) population has 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 hint at a further large fall in numbers;  (Foula, Fair Isle, Fetlar, Mousa, Noss, Black Park and Lumbister held  76 AOT between them in 2015, 73% fewer than was recorded by Seabird 2000 (280 AOT)). In the west of Scotland, only 16 AOT were recorded on Handa (Sutherland) in 2015 numbers there have declined fairly steadily since 2001 when a peak of 42 AOT was recorded. Contributory factors to the national decline include competition for nesting territories with great skuas1 (which have increased markedly), and reductions in sandeel stocks, particularly around Shetland2. Analysis of data from the Orkney population attributes the decline there to a scarcity of food coupled with predation by great skuas (also ultimately linked to a scarcity of alternative fish prey for great skuas)3.

Productivity

 

UK Arctic skua productivity trend

Figure 2: Trend in UK productivity (no. of chicks fledged per pair) of Arctic skua 1986-2015. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Arctic skuas periodically experience years of very poor productivity, 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, although 2015 was the most productive breeding seasons since 1999. Annual data from the six most frequently monitored Arctic skua colonies in Shetland (73 data points between 2003-2015 from Fair Isle, Fetlar, Foula, Hermaness, Mousa and Noss) illustrate just how bad breeding seasons have become. Complete failure was recorded 38 times, with breeding success below 0.20 chicks fledged per pair on a further 12 occasions; only on eight occasions did breeding success climb above 0.80. Arctic skuas rely on stealing fish caught by other seabirds, especially black-legged kittiwakes, Arctic terns and Atlantic puffins; declines in the abundance and chick provisioning of these host species has reduced feeding opportunities for Arctic skuas1. Predation of Arctic skua chicks by great skuas (also ultimately linked to a scarcity of alternative fish prey for great skuas) is also known to be reducing breeding success in some regions3.

 

Analysis of the SMP dataset found mean breeding success of Arctic skua was 0.52 and declined at a rate of 0.02 chicks per nest per year between 1986 and 20084. This equated to a decline in breeding success of 41% over the time period. The quality of the dataset meant a change in breeding success greater than 25% over 25 years could be detected with confidence but a change of 10% could not. Using available life history information (population size, clutch size, age at first breeding and survival rates of different age classes) to develop population viability analysis, it is predicted that if this rate of breeding success were maintained, Arctic skua populations would experience a decline of greater than 50% over 25 years. A decline of 25% over 25 years is likely unless breeding success is increased to at least 1.30, or survival increases..

 

 

Breeding abundance

 

Scotland Arctic skua abundance trend

Figure 1: Trend in abundance index (solid line) of Arctic skua in Scotland, 1986-2015 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

National census data show the population of Arctic skuas 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 preceding census. Figure 1 shows the 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 in the period from 1986 to 2014, with the population index in 2014 estimated to be 81% lower than in 1986. A survey of the Orkney population in 2010 found just 380 AOT compared to 720 AOT in 2000, so numbers there alone have declined by 47% during the last decade (and have declined by 64% since 1992 when 1,043 AOT were recorded). However, to fully ascertain the scale of the current decline, extensive survey work is needed in Shetland where over half of the UK (and Scottish) population has 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 hint at a further large fall in numbers (  Foula, Fair Isle, Mousa and Noss held 64 AOT between them in 2015, 73% fewer than was recorded by Seabird 2000 (280 AOT)). In the west of Scotland, only 16 AOT were recorded on Handa in 2015; numbers there have declined fairly steadily since 2001 when a peak of 42 AOT was recorded. Contributory factors to the national decline include competition for nesting territories with great skuas1 (which have increased markedly), and reductions in sandeel stocks, particularly around Shetland2. Analysis of data from the Orkney population attributes the decline there to a scarcity of food coupled with predation by great skuas (also ultimately linked to a scarcity of alternative fish prey for great skuas)3.

Productivity

 

Scotland Arctic skua productivity trend

Figure 2: Trend in productivity (no. of chicks fledged per pair) of Arctic skua in Scotland, 1986-2015. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Arctic skuas periodically experience years of very poor productivity, 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, although 2015 was the most productive breeding seasons since 1999. Annual data from the six most frequently monitored Arctic skua colonies in Shetland (72 data points between 2003-2015 from Fair Isle, Fetlar, Foula, Hermaness, Mousa and Noss) illustrate just how bad breeding seasons have become. Complete failure was recorded 38 times, with breeding success below 0.20 chicks fledged per pair on a further 12 occasions; only on eight occasions did breeding success climb above 0.80. Arctic skuas rely on stealing fish caught by other seabirds, especially black-legged kittiwakes, Arctic terns and Atlantic puffins; declines in the abundance and chick provisioning of these host species has reduced feeding opportunities for Arctic skuas1. Predation of Arctic skua chicks by great skuas (also ultimately linked to a scarcity of alternative fish prey for great skuas) is also known to be reducing breeding success in some regions3.

 

Analysis of the SMP dataset found mean breeding success of Arctic skua was 0.52 and declined at a rate of 0.02 chicks per nest per year between 1986 and 20084. This equated to a decline in breeding success of 41% over the time period. The quality of the dataset meant a change in breeding success greater than 25% over 25 years could be detected with confidence but a change of 10% could not. Using available life history information (population size, clutch size, age at first breeding and survival rates of different age classes) to develop population viability analysis, it is predicted that if this rate of breeding success were maintained, Arctic skua populations would experience a decline of greater than 50% over 25 years. A decline of 25% over 25 years is likely unless breeding success is increased to at least 1.30, or survival increases.

 

 

Arctic skua does not breed in England.

 

 

Arctic skua does not breed in Wales.

 

 

Arctic skua does not breed in Northern Ireland.

 

 

Arctic skua does not breed in the Republic of Ireland.

 

 

Arctic skua does not breed in Ireland.

 

 

Arctic skua does not breed on the Isle of Man.

 

 

Arctic skua does not breed on the Channel Islands.

 

 


UK Phenology, diet, survival rates

 

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

 


References

1 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:160-172. Poyser, London.

2 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.

3 Meek, E.R., Bolton, M., Fox, D. and Remp, J. 2011. Breeding skuas in Orkney: a 2010 census indicates density-dependent population change driven by both food supply and predation. Seabird. 24, 1-10.

4 Cook, A.S.C.P. and Robinson, R.A. 2010. How representative is the current monitoring of breeding success in the UK? BTO Research Report No. 573, BTO, Thetford.


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; Centre for Ecology and Hydrology; Natural Resources Wales; Department of Environment, Food and Agriculture (Isle of Man); Department of Environment, Heritage and Local Government (Republic of Ireland); States of Guernsey Government; JNCC; Manx Birdlife; Manx National Heritage; The National Trust; National Trust for Scotland; Natural England; Northern Ireland Environment Agency; The Royal Society for the Protection of Birds; Scottish Natural Heritage; Seabird Group; Shetland Oil Terminal Environmental Advisory Group; Scottish Wildlife Trust.  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.