Great Skua Stercorarius skua
Status; International importance;
Population
estimates; Distribution; Annual abundance/
productivity; Phenology/diet/survival
Description
The following was 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, 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 birds.
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 3 (2009 update)
(further information on Conservation Designations for UK Taxa)
Amber listed in
Birds of Conservation Concern in Ireland 2 (2007
update)
International importance
*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, however, 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. However, the proportion of great skuas
nesting 'inland' is small.
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
(which is 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 cause problems, as great skuas that
have nested in one colony have never been recorded moving to breed
elsewhere in a subsequent year.
| |
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 where colonies are found and how large they are 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
Breeding abundance
Figure 1: Trend in UK
abundance index of great skua 1986-2010.
The annual sample of great skua colonies is
insufficient to produce reliable trend information. The graph shows
an interpolated and extrapolated trend using population estimates
obtained from censuses in 1985-88 and 1998-2002. The extrapolated
population index increased by 48% between 1986 and 2010; in the
longer term, census results show an increase, from 3,100 pairs in
1969-70 to 7,700 pairs in 1985-88, suggesting that the population
growth rate has decreased since 1969. However, a complete survey of
the Orcadian population in 2010 found only 1,710 AOT, almost
23% fewer than was recorded during Seabird 2000. Surveys of the
rest of the UK (Scottish) population, especially that of
Shetland, are required to ascertain whether this
decline is more widespread.
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. 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
Figure 2: Trend in UK
productivity (no. of chicks fledged per pair) of great
skua 1986-2010. Based on SMP data; view
the methods
of analysis (PDF 158 kb).
Periods of low productivity occurred
during the late 1980s and in the early 2000s in the large Shetland
colonies, probably due to decreased availability of
sandeels4 and low
levels of discards3.
Great skuas do not breed until they are 5-12 years old, so
reductions in productivity will only be manifested in breeding
numbers many years afterwards. Therefore, we can expect further
decreases in the rate of population increase in the coming
decade. Productivity has been moderately high during
2006-2010.
Within the British Isles this species
breeds only in Scotland. Thus all data and
text displayed for the United Kingdom is
fully relevant for Scotland but is not repeated
here.
This species does not breed in England.
This species does not breed in Wales.
This species does not breed in Northern
Ireland.
This species does not breed in the Republic of
Ireland.
This species does not breed in Ireland.
This species does not breed on the Isle of
Man.
This species 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. & A.D.
Poyser, Calton.
3 Reeves, S.A. and R.W. Furness. 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 J. Mar. Sci. 59,
261-269.
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; Countryside Council for Wales; Department of
Agriculture, Fisheries and Forestry (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.