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).
The great skua, or bonxie, is famous for its aggressive defence of territory against human intruders. The species has a very restricted breeding range – confined to the northeast Atlantic, the World population is only around 16,000 apparently occupied territories (AOTs), of which 60% are in Scotland, concentrated in Shetland and Orkney. However, its population has been increasing since 1900, and it has progressively extended its breeding range both northeast into the Barents Sea, and south into the islands of west Scotland. Closely related species breed in the Antarctic and sub-Antarctic, and show strong adaptations to cold conditions and a predatory life-style. In Scotland, great skuas nest on coastal moorland, often in loose groups of scattered nests, but with some colonies numbering thousands of pairs. When nesting at low density in small colonies, most birds in the colony feed by killing seabirds. However, when nesting in large colonies, the majority feed on fish, including fishery discards, and only a small proportion specialise in killing seabirds. Ringing has shown that great skuas from Shetland have emigrated to form colonies in many other areas as far away as north Russia, but the majority of chicks return to their natal colony to try to establish a breeding territory.
Conservation status
Great skua is currently identified as a conservation priority in the following:
- Amber listed in Birds of Conservation Concern 4 (2015 update)
- further information on Conservation Designations for UK Taxa
- Amber listed in Birds of Conservation Concern in Ireland 2020–2026
- EC Birds Directive – migratory species
International importance
|
UK Population |
% Biogeographic Population |
% World Population |
|
9,600 AOT* |
57.6% |
60.0 |
*AOT = Apparently Occupied Territories
The UK population figure (rounded to the nearest hundred) and the World populations were 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. The source for the Biogeographic population is AEWA CSR7.
UK population estimates and change 1969–2002 (census data)
Coverage of great skua breeding areas in the Seabird 2000 survey was good in most regions, although small areas of possible breeding habitat in parts of west and central mainland Shetland and Caithness were not surveyed. Coverage of nesting areas during the SCR Census (1985-88) was complete although surveys in Orkney were conducted in 1982 and counts subsequently adjusted using observed trends to estimate population size. Counts from all other areas used in the SCR Census were conducted during 1985-88. Operation Seafarer (1969-70) did not attempt to find all inland nesting skuas so will have underestimated numbers by a small amount.
Great skuas are relatively easy to census as throughout the breeding season, and especially during incubation and early chick-rearing (from early May to late June), they show very high territory attendance. Pairs that have lost eggs or young chicks almost invariably remain on territory and those that fail early (when most clutches are lost) will lay a replacement clutch. During Seabird 2000 not all colonies were counted in the same year, but this should not have affected population size estimates, as great skuas show high fidelity to breeding sites.
|
|
Operation Seafarer (1969-70) |
Seabird Colony Register (1985-88) |
Seabird 2000 (1998–2002) |
|
UK Population estimate (AOT*) |
3,079 |
7,645 |
9,634 |
|
% change since previous census |
n/a |
+148 |
+26 |
*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 Great skua found in different regions, and a map showing the location and size of colonies, is provided in the Seabird 2000 great skua results page.
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% 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
Great skua mainly breeds in Scotland with low numbers in Northern Ireland and the Republic of Ireland. In Scotland, the larger colonies are all located on islands which are monitored less frequently; therefore, a generated trend would not be valid on an annual basis.
Productivity
Great skua mainly breeds in Scotland with low numbers in Northern Ireland and the Republic of Ireland; thus, productivity trends are only provided under the UK and Scotland section.
United Kingdom
Breeding abundance
The majority of great skuas breed in the Northern Isles of Scotland, where about 94% of the UK breeding population occurs. Census results show an increase, from 3,079 AOT in 1969-70 to 7,645 AOT in 1985-88, although the 1969-70 census did not count all inland nesting skuas, so will have underestimated numbers. By the time of the Seabird 2000 census (1998–2002), the population had increased by a further 26% to 9,634 AOT.
The annual sample of great skua colonies is insufficient to produce reliable trend information because few major colonies are surveyed frequently, or in the same year, so the trend since the last census is largely unknown. However, numbers have been decreasing in some areas. In Shetland, three colonies (Hermaness, Noss and Fair Isle) held 1,476 AOT in 2019, a decrease of 49% since 2007 (the last common year all were surveyed). On Fair Isle, great skuas have been steadily increasing, from 84 AOT in 1986 to 490 AOT in 20191. However, since 2014, the colony has fluctuated with numbers increasing one year and decreasing the next (range 188-520 AOT). In the west of Scotland, two of the largest colonies (Handa2 and Hirta3) held 466 AOT in 2018-19 compared to 405 AOT during Seabird 2000, a 13% increase.
The above data present a complicated picture with no clear trend. The results from the Seabirds Count census (2015–2021) will, however, allow the current status of the UK great skua population to be assessed.
Great skua diet varies geographically; in the Northern Isles, they are scavengers mainly feeding on fisheries discards and sandeels4,5,6,7. Increasingly, since the 1980s, great skuas have become a major predator of seabirds, particularly in smaller colonies such as those in the Western Isles (where approximately 6% of the population occurs)8,9. The population increase during the 1970s is likely to have been supported by the increased availability of discards from fishing boats, although a reduction in discards associated with decreased stocks of cod, haddock and whiting in the 1980s10 and a reduction in sandeel stocks11 led to reduced productivity and adult survival, resulting in lower rates of population increase. It is likely that density dependent competition for food and/or breeding territories will limit further population growth. Indeed, analysis in 2010 of the decline in Orkney indicates that the main driver of population change was competition for food at a local (i.e. colony) level12.
From 2015 to 2019, the obligation that all catches of regulated commercial species had to be landed and counted against quota was phased in across the majority of EU fisheries. Following departure from the EU, the UK gained competency to manage fishing activities in its waters. This is regulated by the UK Fisheries Act 2020 which includes a ‘Discard prevention charging scheme’, although this has not yet been implemented. Reductions in discards are likely to have impacted seabird species that feed on them, including great skua13. In addition, great skua is a cold-adapted species which suffers heat stress during warm weather8. This currently limits their southerly extent and it is likely that climate change will further reduce available breeding habitat and their distribution in the UK.
Productivity
Figure 1. Trend in UK productivity (no. of chicks fledged per pair) of great skua 1986–2019. Based on SMP data; view the methods of analysis.
Periods of low productivity occurred during the late 1980s, early 2000s and from 2011 to 2014. In the large Shetland colonies, this was probably due to decreased sandeel availability11 and low levels of discards10. Unlike Arctic skua, great skua colonies rarely suffer complete breeding failure. This may be due to the ability of great skuas to switch to alternative food sources, often becoming cannibalistic and also preying on the chicks and adults of other seabird species including kittiwake, auks and fulmar14,15,16,17,18,19,20. Annual monitoring data from six Arctic skua colonies in Shetland (Fair Isle, Fetlar, Foula, Hermaness, Mousa and Noss) between 2003 and 2019 recorded 47 cases of complete colony failure out of a possible 92 colony-years, not one instance of complete failure for great skua occurred over the same period.
Scotland
Breeding abundance
Approximately 94% of the UK great skua breeding population is located in the Northern Isles of Scotland. Census results show an increase, from 3,079 AOT in 1969-70 to 7,645 AOT in 1985-88, although the 1969-70 census did not count all inland nesting skuas, so will have underestimated numbers. By the time of the Seabird 2000 census (1998–2002), the population had increased by a further 26% to 9,634 AOT.
The annual sample of great skua colonies is insufficient to produce reliable trend information because few major colonies are surveyed frequently, or in the same year, so the trend since the last census is largely unknown. However, numbers have been decreasing in some areas. In Shetland, three colonies (Hermaness, Noss and Fair Isle) held 1,476 AOT in 2019, a decrease of 49% since 2007 (the last common year all were surveyed). On Fair Isle, great skuas have been steadily increasing, from 84 AOT in 1986 to 490 AOT in 20191. However, since 2014, the colony has fluctuated with numbers increasing one year and decreasing the next (range 188–520 AOT). In the west of Scotland, two of the largest colonies (Handa2 and Hirta3) held 466 AOT in 2018-19 compared to 405 AOT during Seabird 2000, a 13% increase.
The above data present a complicated picture with no clear trend. The results from the Seabirds Count census (2015–2021) will, however, allow the current status of Scotland’s great skua population to be assessed.
Great skua diet varies geographically; in the Northern Isles, they are scavengers mainly feeding on fisheries discards and sandeels4,5,6,7. Increasingly, since the 1980s, great skuas have become a major predator of seabirds, particularly in smaller colonies such as those in the Western Isles (where approximately 6% of the population occurs)8,9. The population increase during the 1970s is likely to have been supported by the increased availability of discards from fishing boats, although a reduction in discards associated with decreased stocks of cod, haddock and whiting in the 1980s10 and a reduction in sandeel stocks11 led to reduced productivity and adult survival, resulting in lower rates of population increase. It is likely that density dependent competition for food and/or breeding territories will limit further population growth. Indeed, analysis in 2010 of the decline in Orkney indicates that the main driver of population change was competition for food at a local (i.e. colony) level12.
From 2015 to 2019, the obligation that all catches of regulated commercial species had to be landed and counted against quota was phased in across the majority of EU fisheries. Following departure from the EU, the UK gained competency to manage fishing activities in its waters. This is regulated by the UK Fisheries Act 2020 which includes a ‘Discard prevention charging scheme’, although this has not yet been implemented. Reductions in discards are likely to have impacted seabird species that feed on them, including great skua13. In addition, great skua is a cold-adapted species which suffers heat stress during warm weather8. This currently limits their southerly extent and it is likely that climate change will further reduce available breeding habitat and their distribution in the Scotland.
Productivity
Figure 2. Trend in productivity (no. of chicks fledged per pair) of great skua in Scotland, 1986–2019. Based on SMP data; view the methods of analysis.
Periods of low productivity occurred during the late 1980s, early 2000s and from 2011 to 2014. In the large Shetland colonies, this was probably due to decreased sandeel availability11 and low levels of discards10. Unlike Arctic skua, great skua colonies rarely suffer complete breeding failure. This may be due to the ability of great skuas to switch to alternative food sources, often becoming cannibalistic and also preying on the chicks and adults of other seabird species including kittiwake, auks and fulmar14,15,16,17,18,19,20. Annual monitoring data from six Arctic skua colonies in Shetland (Fair Isle, Fetlar, Foula, Hermaness, Mousa and Noss) between 2003 and 2019 recorded 47 cases of complete colony failure out of a possible 92 colony-years, not one instance of complete failure for great skua occurred over the same period.
England
Great skua does not breed in England.
Wales
Great skua does not breed in Wales.
Northern Ireland
Breeding Abundance
In Northern Ireland, great skuas are a recent colonist, first nesting on Rathlin Island in 2010. A single pair bred there successfully in 2011, 201421, 2015, 201622 and 201923.
Productivity
The productivity of the recent great skua colonists on Rathlin Island has been closely monitored. In 2011, a single pair nested, laying two eggs, one of which hatched and the chick subsequently fledged. This was the first successful breeding attempt by this species in Northern Ireland. Since then, breeding attempts have been made by this single pair in most years, but they only fledged chicks in 2014, 201521, 201622 and 201923. Average productivity was 0.77 chicks fledged per pair per year.
Republic of Ireland
Breeding abundance
In the Republic of Ireland, great skuas first bred in the late 1990s in Co. Mayo24 and numbers have increased slowly since then. During the recent Republic of Ireland Seabird Census, the great skua population was recorded as a minimum of 13 AOT25.
Productivity
No systematic data on the productivity of great skuas in the Republic of Ireland have been submitted to the SMP.
All Ireland
Breeding abundance
Great skuas are a recent colonist in Ireland. First nesting in the late 1990s in the Republic of Ireland and colonising Northern Ireland in 2010. It is thought there are now in excess of 14 AOT25.
Productivity
Data on great skua productivity in Ireland are scarce, although the recent colonists in Northern Ireland have been closely monitored, where productivity is an average of 0.77 chicks fledged per pair per year. The productivity of great skuas nesting in the Republic of Ireland, where as many as 13 AOT are thought to be present, is unknown or unreported.
Isle of Man
Great skua does not breed on the Isle of Man.
Channel Islands
Great 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 Parnaby, D., Cope, R., Dodd, C., Hellicar, M. and Gale, T. 2019. Fair Isle Seabird Studies 2019. Unpublished report. Joint Nature Conservation Committee, Peterborough.
2 Dorman, L. and Smith, C. 2018. Handa Island Skua Project Annual Report 2018. Unpublished report, Scottish Wildlife Trust, Edinburgh.
3 Lawrence, S. 2020. Seabird and Marine Ranger Annual Report St Kilda 2019. National Trust of Scotland, Edinburgh.
4 Furness, R. 1977. Effects of Great Skuas on Arctic Skuas in Shetland. British Birds, 70, 96–107.
5 Votier, S.C., Bearhop, S., Fyfe, R., and Furness, R.W. 2008. Temporal and spatial variation in the diet of a marine top predator – links with commercial fisheries. Marine Ecology Progress Series, 367, 223–232.
6 Votier, S.C., Bearhop, S., MacCormick, A., Ratcliffe, N and Furness, R.W. 2003. Assessing the diet of great skuas Catharacta skua using five different techniques. Polar Biology, 26, 20–26.
7 Bearhop, S., Thompson, D.R., Phillips, R.A., Waldron, S., Hamer, K. C., Gray, C.M., Votier, S.C., Ross, B.P. and Furness, R.W. 2001. Annual variation in Great Skua Diets: The importance of Commercial fisheries and predation on seabirds revealed by combining dietary analyses. The Condor, 103, 802–809.
8 Furness, R.W. 1987. The Skuas. T. and A.D. Poyser, Calton.
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) S. parasiticus on the west coast of Scotland. Bird Study, 55, 257–266.
10 Reeves, S.A. and Furness, R.W. 2002. Net loss–seabirds gain? Implications of fisheries management for seabirds scavenging discards in the northern North Sea. Unpublished RSPB Report, Sandy, UK.
11 Furness, R.W. 2002. Management implications of interactions between fisheries and sandeel dependent seabirds and seals in the North Sea. ICES Journal of Marine Science, 59, 261–269.
12 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.
13 Bicknell, A.W. J., Oro, D., Camphuysen, J.C. and Votier, S.C. 2013. Potential consequences of discard reform for seabird communities. Journal of Applied Ecology, 50, 649–658.
14 Hamer, K.C., Furness, R.W. and Caldow, R.W.G. 1991. The effects of changes in food availability on the breeding ecology of great skuas Catharacta skua in Shetland. Journal of Zoology, 223, 175–188.
15 Phillips R.A., Thompson D.R. and Hamer K.C. 1999. The impact of great skua predation on seabird populations at St. Kilda: a bioenergetics model. Journal of Applied Ecology, 36(2), 218–232.
16 Votier, S.C., Bearhop, S., Ratcliffe, N., Phillips, R.A. and Furness, R.W. 2004. Predation by great skuas at a large Shetland seabird colony. Journal of Applied Ecology, 41, 1117–1128.
17 Votier, S. C., Crane, J. E., Bearhop, S., de León, A., McSorley, C. A., Mínguez, E., Mitchell, P. I., Parsons, M., Phillips, R. A. and Furness, R. W. 2006. Nocturnal foraging by Great Skuas Stercorarius skua: implications for conservation of storm-petrel populations. Journal of Ornithology, 147, 405–413.
18 Miles, W.T.S. 2010. Ecology, behaviour and predator prey interactions of Great Skuas and Leach's Storm-petrels at St Kilda. Unpublished PhD thesis http://theses.gla.ac.uk/2297/. University of Glasgow, Glasgow.
19 Miles, W.T.S., Mavor, R., Riddiford, N.J., Harvey, P.V., Riddington, R., Shaw, D.N., Parnaby, D. and Reid, J.M. 2015. Decline in an Atlantic Puffin Population: Evaluation of Magnitude and Mechanisms. PLoS ONE, 10(7).
20 Church, C.E., Furness, R.W., Tyler, G., Gilbert, L. and Votier, S.C. 2018. Change in the North Sea ecosystem from the 1970s to the 2010s: great skua diets reflect changing forage fish, seabirds, and fisheries. ICES Journal of Marine Science, doi:10.1093/icesjms/fsy165.
21 Leonard, K. and Wolsey, S. 2014. The Northern Ireland Seabird Report 2013. British Trust for Ornithology, Thetford.
22 Booth Jones, K.A. and Wolsey, S. 2019. The Northern Ireland Seabird Report 2018. British Trust for Ornithology, Thetford.
23 Booth Jones, K.A. and Wolsey, S. 2020. The Northern Ireland Seabird Report 2019. British Trust for Ornithology, Thetford.
24 Davis, S.E., Nager, R.G. and Furness, R.W. 2005. Food availability affects adult survival as well as breeding success of Parasitic Jaegers. Ecology, 86, 1047–1056.
25 Cummins, S., Lauder, C., Lauder, A. and Tierney, T. D. 2019. The Status of Ireland’s Breeding Seabirds: Birds Directive Article 12 Reporting 2013 – 2018. Irish Wildlife Manuals, No. 114. National Parks and Wildlife Service, Department of Culture, Heritage and the Gaeltacht, Ireland.
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 >>
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