Great Skua Stercorarius skua

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

 

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Description

Great 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).

 

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:

EC Birds Directive - migratory species

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 2014-2019 (2014 update)


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.

 


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 (PDF, 1.2 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

 

The annual sample of great skua colonies is insufficient to produce reliable trend information. 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. Few major colonies are surveyed frequently, or in the same year, so the trend since the last census is largely unknown. However, numbers are increasing in some areas. In Shetland, four colonies (Hermaness, Noss, Mousa and Fair Isle) held 1,747 AOT in 2013, an increase of 27% since 2007 (the last common year all were surveyed). On Fair Isle in 2015, only 188 AOT were recorded compared to 424 the previous year, a decline of 55.6%. In the west of Scotland, six colonies (Priest Island (Ross and Cromarty), Canna (Lochaber), Mingulay, Berneray (both western Isles), Fladda and Linga (both Argyll and Bute)) held 97 AOT in 2013 and 136 AOT in 2014. In 2015, only Priest Island (5 AOT), Canna (8 AOT) and Minulay (101 AOT) were counted; however, the total of 114 AOT also represents a large increase since all were surveyed in 2009 (61 AOT). However, at another west coast colony, Handa (Sutherland), numbers had been fluctuating, falling from 266 to 135 AOT between 2009 and 2013 and increasing to 256 AOT in 2015. A complete survey of the Orkney population in 2010 found only 1,710 AOT, almost 23% fewer than was recorded during Seabird 2000. Some redistribution of the Orkney population had also occurred with movement away from the stronghold on Hoy and the establishment of new colonies and increases in smaller ones.

 

In Northern Ireland, great skuas are a recent colonist, first nesting in 2010, with the first successful breeding in 2011. One pair was present in 2012, but did not breed successfully, with one or two pairs present in 2013 although there was no evidence of breeding. A single pair nested in 2014 and fledged two chicks. One pair was present in 2015 and fledged one chick.

 

The above data present a complicated picture with no clear trend; surveys across the remaining UK distribution of the species, especially 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, they feed largely on fisheries discards and sandeels1. In smaller colonies, such as those in the Western Isles (where the remaining 6% occur) great skuas tend to specialise in eating seabirds2. The population increase during the 1970s is likely to have been supported by high availability of discards from fishing boats, but reductions in discards associated with decreased stocks of cod, haddock and whiting in the 1980s3 - and a reduction in sandeel stocks4 - led to reduced productivity and adult survival, resulting in reduced rates of population increase. It appears that density dependent competition for food or breeding territory is 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) level5. In addition, the great skua is a cold-adapted species which suffers heat stress during warm weather2. This currently limits its southerly extent and it is likely that climate change will further reduce available breeding range in the UK.

Productivity

 

UK Great skua productivity trend

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

 

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 sandeel4 availability and low levels of discards3. However, in periods of severe food shortage, great skuas will become cannibalistic and prey on the chicks in neighbouring great skua territories. For example, although not quantified, high levels of intra-specific depredation have been reported on Fair Isle in the last 10 years. 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 2015 recorded 38 cases of complete colony failure out of a possible 72 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.

 

 

Breeding abundance

 

The annual sample of great skua colonies is insufficient to produce reliable trend information. 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. Few major colonies are surveyed frequently, or in the same year, so the trend since the last census is largely unknown. However, numbers are increasing in some areas. In Shetland, four colonies (Hermaness, Noss, Mousa and Fair Isle) held 1,747 AOT in 2013, an increase of 27% since 2007 (the last common year all were surveyed). Numbers on Fair Isle decreased in 2015; only 188 AOT were recorded compared to 424 AOT in 2014, a decline of 55.6%. Elsewhere in Shetland, a decline of 28% was recorded on Foula between 2000 and 2007 from 2,293 to 1,657 AOT. In the west of Scotland, six colonies (Priest Island, Canna, Mingulay, Berneray, Fladda and Lunga) held 97 AOT in 2013 and 136 AOT in 2014; this also represents a large increase since all were surveyed in 2009 (61 AOT). However, at another west coast colony, Handa, numbers had fallen from 266 to 135 AOT between 2009 and 2013. A complete survey of the Orkney population in 2010 found only 1,710 AOT, almost 23% fewer than what was recorded during Seabird 2000. Some redistribution of the Orkney population had also occurred with movement away from the stronghold on Hoy and the establishment of new colonies and increases in smaller ones.

 

The above data present a complicated picture with no clear trend; surveys across the remaining Scottish distribution of the species, especially 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, they feed largely on fisheries discards and sandeels1. In smaller colonies, such as those in the Western Isles (where the remaining 6% occur) great skuas tend to specialise in eating seabirds2. The population increase during the 1970s is likely to have been supported by high availability of discards from fishing boats, but reductions in discards associated with decreased stocks of cod, haddock and whiting in the 1980s3 - and a reduction in sandeel stocks4 - led to reduced productivity and adult survival, resulting in reduced rates of population increase. It appears that density dependent competition for food or breeding territory is 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) level5. In addition, the great skua is a cold-adapted species which suffers heat stress during warm weather2. This currently limits its southerly extent and it is likely that climate change will further reduce available breeding range in the UK.

 

Productivity

 

Scotland Great skua productivity trend

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

 

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 availability of sandeels4 and low levels of discards3. However, in periods of severe food shortage, great skuas will become cannibalistic and prey on the chicks in neighbouring great skua territories. For example, although not quantified, high levels of intra-specific depredation have been reported on Fair Isle in recent years.

 

 

Great skua does not breed in England.

 

 

Great skua does not breed in Wales.

 

 

Breeding abundance

 

In Northern Ireland, great skuas are a recent colonist. A single pair nested in 2010 and 2011, with the first successful breeding in 2011. One pair was present in 2012 but did not breed successfully. In 2013, two adults were present throughout the summer and were later joined by two more birds, although there was no evidence of breeding and the behaviour of the birds suggested there was no nest6. A single pair bred successfully in 2014 and 20157.

 

Productivity

 

The breeding success 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, successful breeding attempts have been made by this single pair in 2014 and 20156,7. On average, productivity measures 0.80 chicks fledged per pair per year with no statistical significant variation over time.

 

 

Breeding abundance

 

In the Republic of Ireland, the first breeding occurred in the late 1990s in Co. Mayo. Numbers have increased slowly since then and, although there has not been a complete systematic survey, there are now thought to be approximately 15 pairs in the north-west of the country7.

 

Productivity

 

No systematic data on the productivity of great skuas in the Republic of Ireland have been submitted to the SMP.

 

 

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 in excess of 15 pairs all though no complete systematic survey has been carried out. Overall the population is healthy and increasing. However, great skuas have been shown to be serious predators of Leach’s storm-petrels on St Kilda (Western Isles, Scotland)8, consuming as many as 21,000 individuals per year9, so there may be potential cause for concern in relation to storm-petrel populations on islands off the west coast of Ireland10.

 

Productivity

 

Data on the breeding success of great skuas in Ireland are scarce. The recent colonists in Northern Ireland have been closely monitored, where productivity measures an average of 0.80 chicks fledged per pair per year with no statistical significant variation over time. The breeding success of great skuas nesting in the Republic of Ireland, where as many as 15 pairs are thought to be present, is unknown or unreported.

 

 

Great skua does not breed on the Isle of Man.

 

 

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 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.

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

3 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.

4 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.

5 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.

6 Leonard, K. and Wolsey, S. (eds.). 2015. Northern Ireland Seabird Report 2014. British Trust for Ornithology, Thetford.

7 Leonard, K. and Wolsey, S. (eds.). 2016. Northern Ireland Seabird Report 2015. British Trust for Ornithology, Thetford.

8 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.

9 Miles, W.T.S. 2010. Ecology, behaviour and predatorprey 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.

10 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. J. Ornithol. 147: 405–413.

 


 

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