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Great skua (Stercorarius skua)

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:

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International importance

UK Population

Biogeographic Population

% World Population

9,600 AOT*

n/a

60.0

*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)

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.

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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 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.

 

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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 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.

 

United Kingdom

Breeding abundance

Census results show an increase, from 3,079 pairs in 1969-70 to 7,645 pairs in 1985-88 and 9,634 pairs in 1998-2002, which suggests that although the population was increasing, the growth rate was slowing down. The annual sample of great skua colonies is insufficient to produce reliable trend information because only few major colonies are surveyed frequently, or in the same year, so the trend since the last census is largely unknown. However, numbers are decreasing in some areas. In Shetland, four colonies (Hermaness, Noss, Mousa and Fair Isle) held 1,476 AOT in 2018, 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 520 AOT in 2018. Though, since 2014, the colony shows strong fluctuation with numbers increasing one year and decreasing the next (range 188-520 AOT). In the west of Scotland, four colonies (Priest Island, Canna, Fladda and Lunga and Sgeir a' Chaisteil) held 19 AOT in 2013 and 21 AOT in 2018. However, at the west coast colony of Handa, numbers had been fluctuating; falling from 266 to 135 AOT between 2009 and 2013, and increasing to 256 AOT in 2015 and 283 AOT in 2018. A complete survey of the Orkney population in 2010 found only 1,710 AOT, almost 23% less than what were recorded during Seabird 2000. A study of abundance data in Scotland from 1992 to 2015 indicated that great skuas increased at most sites, with some very large increases at smaller colonies. However, declines at the two largest colonies (Foula and Hoy) resulted in little overall change in AOTs across all colonies combined.

The above data present a complicated picture with no clear trend; surveys at the remaining UK colonies, especially in other parts of Shetland and the Western Isles, are required to accurately assess the current population status of the great skua.

Great skua diet varies geographically; in the Northern Isles, where about 94% of the UK breeding population occurs, great skua is a scavenger largely feeding on fisheries discards and sandeels1,2,3,4. Increasingly, since the 1980s, it has become a major predator of seabirds, particularly in smaller colonies, such as those in the Western Isles (where approximately 6% of the population occur)5,6. The population increase during the 1970s is likely to have been supported by high availability of discards from fishing boats, although a reduction in discards associated with decreased stocks of cod, haddock and whiting in the 1980s7 and a reduction in sandeel stocks8 led to reduced productivity and adult survival, resulting in reduced rates of population increase. It appears that density dependent competition for food and/or breeding territories are likely to limit further population growth. Indeed, analysis of the decline in Orkney in 2010 indicates the driver of population change was competition for food at a local (i.e. colony) level9. In addition, the great skua is a cold-adapted species which suffers heat stress during warm weather5. 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

great-skua-uk-prod.jpg

Figure 2. Trend in UK productivity (no. of chicks fledged per pair) of great skua 1986–2018. Based on SMP data; view the methods of analysis.

 

Periods of low productivity occurred during the late 1980s, the early 2000s and from 2011 to 2014. In the large Shetland colonies, this was probably due to decreased sandeel availability8 and low levels of discards7. These declines in productivity may have been mitigated to an extent by 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 fulmar10,11,12,13,14,15,16.

Unlike Arctic skua, great skua colonies rarely suffer complete breeding failure. Annual monitoring data from six Arctic skua colonies in Shetland (Fair Isle, Fetlar, Foula, Hermaness, Mousa and Noss) between 2003 and 2018 recorded 43 cases of complete colony failure out of a possible 85 colony-years, not one instance of complete failure for the great skua occurred over the same period. Great skuas do not breed until they are at least 5 years old, so reductions in productivity and subsequent impacts on recruitment will only be manifested in breeding numbers many years afterwards. Therefore, we should expect further decreases in their population in the coming decade.

 

Scotland

Breeding abundance

Census results show an increase, from 3,079 pairs in 1969-70 to 7,645 pairs in 1985-88 and 9,634 pairs in 1998-2002, which suggests that although the population was increasing, the growth rate was slowing down. The annual sample of great skua colonies is insufficient to produce reliable trend information because only few major colonies are surveyed frequently, or in the same year, so the trend since the last census is largely unknown. However, numbers are decreasing in some areas. In Shetland, four colonies (Hermaness, Noss, Mousa and Fair Isle) held 1,476 AOT in 2018, 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 520 AOT in 2018. However, since 2014, the colony has shown strong fluctuation with numbers increasing one year and decreasing the next (range 188–520 AOT). In the west of Scotland, four colonies (Priest Island, Canna, Fladda and Lunga and Sgeir a' Chaisteil) held 19 AOT in 2013 and 21 AOT in 2018. However, at the west coast colony of Handa, numbers had been fluctuating; falling from 266 to 135 AOT between 2009 and 2013, and increasing to 256 AOT in 2015 and 283 AOT in 2018. A complete survey of the Orkney population in 2010 found only 1,710 AOT, almost 23% fewer than what were recorded during Seabird 20009. A study of abundance data in Scotland from 1992 to 2015 indicated that great skuas increased at most sites, with some very large increases at smaller colonies. However, declines at the two largest colonies (Foula and Hoy) resulted in little overall change in AOTs across all colonies combined17.

The above data present a complicated picture with no clear trend; surveys across the remaining UK distribution of the species, especially in other parts of Shetland and the Western Isles, are required to accurately assess the current population status of the great skua.

Great skua diet varies geographically; in the Northern Isles, where about 94% of the UK breeding population occurs, great skua is a scavenger largely feeding on sandeels Ammodytes spp during the 1970s and on fisheries discards from the 1980s to the present day1,2,3,4. Increasingly, since the 1980s, it has become a major predator of seabirds, particularly in smaller colonies, such as those in the Western Isles (where approximately 6% of the population occur)5,6. The population increase during the 1970s is likely to have been supported by high availability of discards from fishing boats, although a reduction in discards associated with decreased stocks of cod, haddock and whiting in the 1980s7 and a reduction in sandeel stocks8 led to reduced productivity and adult survival, resulting in reduced rates of population increase. It appears that density dependent competition for food and/or breeding territories are likely to limit further population growth. Indeed, analysis of the decline in Orkney in 2010 indicates the driver of population change was competition for food at a local (i.e. colony) level9. In addition, the great skua is a cold-adapted species which suffers heat stress during warm weather5. 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

great-skua-scotland-prod.jpg

Figure 2. Trend in productivity (no. of chicks fledged per pair) of great skua in Scotland, 1986–2018. Based on SMP data; view the methods of analysis.

 

Periods of low productivity occurred during the late 1980s, the early 2000s and from 2011 to 2014. In the large Shetland colonies, this was probably due to decreased sandeel availability8 and low levels of discards7. These declines in productivity may have been mitigated to an extent by 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 14,15,16,17,18,19,20.

Unlike Arctic skua, great skua colonies rarely suffer complete breeding failure. Annual monitoring data from six Arctic skua colonies in Shetland (Fair Isle, Fetlar, Foula, Hermaness, Mousa and Noss) between 2003 and 2018 recorded 43 cases of complete colony failure out of a possible 85 colony-years, not one instance of complete failure for the great skua occurred over the same period. Great skuas do not breed until they are at least 5 years old, so reductions in productivity and subsequent impacts on recruitment will only be manifested in breeding numbers many years afterwards. Therefore, we should expect further decreases in populations in the coming decade.

 

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, 2014, 2015 and 201621,22.

 

Productivity

The productivity of the recent colonists 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 have only fledged chicks in 2014, 2015, 2016. On average, productivity measures 1.67 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. Mayo23. Numbers have increased slowly since then. During the recent Republic of Ireland Seabird Census the population of great skua was recorded to be at least 13 pairs, an increase of at least 1,200% since Seabird 200023.

 

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 13 pairs23.

 

Productivity

Data on the productivity of great skuas in Ireland are scarce, although the recent colonists in Northern Ireland have been closely monitored, where productivity measures an average of 1.67 chicks fledged per pair per year. The productivity of great skuas nesting in the Republic of Ireland, where as many as 15 pairs 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.

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

2 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.

3 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.

4 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.

5 Furness, R.W. 1987. The Skuas. T. and A.D. Poyser, Calton.

6 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.

7 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.

8 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.

9 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.

10 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.

11 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.

12 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.

13 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.

14 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.

15 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).

16 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.

17 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.

18 Ratcliffe, N., Furness, R.W. and Hamer, K.C. 1998. The interactive effects of age and food supply on the breeding ecology of great skuas. Journal of Animal Ecology, 67, 853–862.

19 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.

20 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.

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 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.

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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; UK 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 Department of Agriculture, Environment and Rural Affairs; 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 Great 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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SMP Report 1986–2018

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