Mitchell, P.I., Newton, S.F., Ratcliffe, N. & Dunn, T.E.
Summarises the findings of Seabird 2000, a national initative to census all the bredding seabirds in Britain and Ireland which incorporated the work of over 400 individuals counting seabird numbers at coastal and inland sites over 15 years.
Introduction
Seabird Populations of Britain and Ireland summarises
the results of
Seabird 2000 – a census of all seabirds breeding
in Britain and Ireland during 1998-2002. Seabird 2000 was
launched on 12 April 1999 by Elliot Morley MP and fieldwork was
completed in 2002. Over 1000 surveyors took part and censused 3,200
colonies along 40,000 km of coastline and at 900 inland
sites. The project is a partnership between JNCC, the
Government's conservation agencies, RSPB, the Seabird Group,
Shetland Oil Terminal Environmental Advisory Group (SOTEAG),
BirdWatch Ireland and National Parks and Wildlife Service (Dept. of
Environment, Heritage and Local Government - Republic of
Ireland).
Seabird 2000 followed on from two previous censuses:
Operation Seafarer in 1969-70 and The Seabird Colony
Register in 1985-88, thus allowing population trends over
15-30 years to be assessed. In contrast to the previous two
censuses, Seabird 2000 surveyed inland colonies of seabirds,
namelyBlack-headed Gulls, Common Gulls, Lesser Black-backed Gulls,
Great Cormorants and Common Terns.
In the book, species experts address the main factors causing
change in the population of each species, highlighting any major
threats and topics requiring further study.
Seabird Populations of Britain and Ireland is a major
contribution to the implementation of the Birds Directive (EC
Council Directive on the Conservation of Wild Birds – 79/409/EEC)
in the UK and the Republic of Ireland, with over 40% of Special
Protection Areas (SPAs) listed because of seabird interest. The
book provides an accurate baseline on which to compare future
monitoring. For the first time, accurate baseline estimates
were obtained for populations of nocturnal species (i.e. petrels
and shearwaters) breeding on the remotest of the British
Isles.
International context
Britain and Ireland's seabird populations are an important
component of global biodiversity and the British Isles are one of
the richest areas in the world for seabirds. Just under 8 million
seabirds from 25 species breed in Britain and Ireland, including
90% of the world's Manx Shearwaters, 68% of Northern Gannets and
60% of Great Skuas. This compares strongly with other major
seabird centres: Caribbean (1.3 million breeding seabirds from 22
species), Falkland Islands (over 4 million, 22 species), Barents
Sea (13 million, 25 species), New Zealand (20 million, 55 species)
and Alaska (20 million, 24 species).
That Britain and Ireland host such important seabird
populations is due to many reasons, including: 1) an extensive
coastline offering a wide spectrum of nesting habitats ranging from
high cliffs and offshore islands to dunes and shingle beaches; 2) a
rich and diverse marine environment served by deep water and
surface currents, which encourage high primary productivity and
associated growth of zooplankton and fish, both major food sources
for seabirds; 3) a ready food supply for seabirds at sea in the
form of fisheries waste and discards; 4) minimal human exploitation
of seabirds and their eggs.
Current statust & trends
Numbers of seabirds breeding in Britain and Ireland have risen
steadily over the last 30 years from around 5 million in 1969-70,
to over 6 million in 1985-88, to almost 8 Million in
1998-2002.
25 species of seabird currently breed in Britain and Ireland,
the coastal populations of 13 species have increased in size by
more than 10%, three have decreased by more than 10% and five have
changed by less than 10% (trends for four species were impossible
to assess due to incomplete previous surveys).
The most abundant seabird in Britain and Ireland is the
Common Guillemot, of which thereare 1.6 million,
more than double the number in 1969-70. There are around 1.2
million Atlantic Puffins breeding in Britain and
Ireland, and some colonies on islands along the North Sea coast of
Britain have increased substantially in size over the last 15
years. Numbers of Northern Fulmars breeding in
Britain and Ireland increased throughout most of the
20th century, and there are now 1.1 million breeding
birds, 74% more than in 1969-70. However, the total population of
Northern Fulmars in Britain and Ireland has changed little since
1985-88 and while numbers continue to increase in some areas,
numbers within the densest areas (i.e. in northern Scotland) have
declined slightly. Black-legged Kittiwakes remain
one of the most abundant seabirds in the British Isles, totalling
800,000 breeding birds, but this represents a decline of 23% since
1985-88. This decline is due mainly to successive years of poor
breeding success related to low availability of their main food
sandeels in the North Sea, particularly around Shetland where the
number of breeding Kittiwakes has declined by 69% in the last 15
years.
A major achievement of Seabird 2000 was to survey the
nocturnal European and Leach's Storm-petrels that
nest under ground on the remotest of the British Isles, which due
to inherent logistical problems had never been accurately counted
before. The nests of both species are hidden away in earth burrows
or in rock crevices, under boulders or in stone walls and are
therefore extremely difficult to find. Seabird 2000 used a
technique called 'tape playback', which relies on the fact that
petrels hidden away in burrows will call back to taped recordings
of their calls. By counting the number of responses to the taped
calls, surveyors could estimate the number of breeding pairs within
each colony, without actually having to see any birds. A total of
180 remote and mostly uninhabited islands around the British Isles
were visited by Seabird 2000 surveyors.
European Storm-petrels were found breeding on
125 islands, in colonies varying in size from 1-2 pairs to several
thousand pairs, the largest being 27,000 pairs on an island in Co.
Kerry. The total population in Britain and Ireland was estimated at
125,000 pairs, which represents 3-11% of the world population,
although uncertainty about how many exist in their other main
strongholds in Iceland and the Faeroe Islands makes it difficult to
assess the global importance of the British and Irish
populations.
Leach's Storm-petrels were confined to
colonies on several islands situated along the Atlantic Frontier of
Scotland and on one island in Co. Mayo. All colonies were situated
close to the edge of the continental shelf, adjacent to deep
oceanic water where Leach's Storm-petrels congregate to feed on
plankton brought to the ocean's surface by deep water
upwellings. Around 48,000 pairs of Leach's Storm-petrels were
found breeding in the British Isles, of which 94% were on just four
small islands in the St Kilda archipelago situated 80km west of the
Outer Hebrides. The population in Britain and Ireland
represents less than 1% of the world population of Leach's
Storm-petrels, which are more numerous in the western Atlantic and
in the Pacific, but their relative scarcity in Europe means that
they receive special protection under the Birds Directive.
But on St Kilda they appear to be under serious threat from
predation by Great Skuas.
The first comprehensive survey of Manx
Shearwaters was conducted in Britain and Ireland during
Seabird 2000. Population estimates were derived from tape playback
surveys and counts of apparently occupied burrows. A total of 40
colonies were surveyed and up to 14 more were thought to be present
in Scotland and Ireland but the numbers involved were not thought
to be substantial. Of the 332,000 pairs breeding in Britain, 82%
were on Rum in NW Scotland and on three adjacent islands off SW
Wales. In Ireland, the majority of breeding Manx Shearwaters were
in colonies on islands in Co. Kerry, the largest colony of 10,000
pairs was on Inshtooskert.
Only 14 of the 21 Northern Gannet colonies in
Britain and Ireland were surveyed during Seabird 2000. It was
decide to transfer resources to other work (e.g. storm-petrel
surveys) since all gannet colonies were surveyed in 1994/95 and a
repeat survey is planned for 2004. Colony-specific trends were used
to estimate the current size of those colonies not surveyed during
Seabird 2000. The total population was estimated at 259,311
pairs.
There has been a 15% increase in the number of Great
Cormorants nesting in Britain and Ireland since 1985-88,
mostly due to an influx of birds from mainland Europe to inland
waters in England. In 1985, there was just one inland colony in
England numbering 151 pairs, but by 2000, this colony had increase
to 370 pairs and there was a total of over 1,300 pairs breeding at
29 inland sites in England. Most of these inland nesters are of the
European race sinsensis, which characteristically nest in
trees; but DNA analysis revealed that some inland nesters are of
the British race carbo, which more usually nest on small
coastal islands and stacks. In contrast to the expanding
inland population, the endemic coastal populations have shown
substantial declines in parts of north and east Scotland and have
remained stable in Ireland. Cormorants inevitably come into
conflict with anglers and unfortunately illegal killing cannot be
ruled out as a cause of some of the declines.
Numbers of Arctic Skuas breeding in Scotland
have declined by 37% since 1985-88, while at the same time, numbers
of Great Skuas have increased by 26% and a pair
bred in Ireland for the first time. These contrasting trends are
not unconnected: Great Skuas have had a direct negative impact on
Arctic Skua numbers by depredating their chicks and by taking over
their territories, often killing adult Arctic Skuas in the process.
Arctic Skuas feed their chicks on sandeels that they steal from
host-species such as Black-legged Kittiwakes and Atlantic Puffins
and in Shetland, a reduction in sandeel stocks around Shetland has
resulted in low productivity of Arctic Skuas and their hosts.
Declines in the size of their host populations as a result of poor
productivity and partly due to predation from Great Skua, have
further reduced the food available to Arctic Skuas. Great
Skuas feed predominantly on fish discarded from trawlers, but as
the quantity of fish discarded declined in recent years due to
diminishing fish stocks and to measures taken to conserve them,
Great Skuas are turning to alternative prey such as other seabird
species.
Great Skuas are thought to compete with Great
Black-backed Gulls, but there is no evidence to suggest
that the increase in Great Skua numbers in Northern Scotland is
having any widespread detrimental effect on Great Black-back
numbers. Indeed, the total number of Great Black-backed gulls
breeding in Britain has changed little over the last 30 years;
although numbers in Ireland have decreased by 28% since 1985-88.
There has been little study of this species in the British Isles so
that factors underlying any changes are poorly understood.
During the 1980s and 1990s the number of gulls nesting on
rooftops in British towns and cities increased sharply. In
1998-2002, 20,000 pairs of Herring Gulls nested on
rooftops in Britain, more than double the number during the last
survey of urban gulls in 1994. There were also 11,000 pairs of
roof-nesting Lesser Black-backed Gulls in
1998-2002, which is four times the number in 1994. Rooftops provide
safe, predator-free nest sites that are often situated right next
to an abundant food source provided by domestic and commercial
waste. However, the number of Herring and Lesser Black-backed Gulls
nesting on rooftops is a small proportion of the total population
that nest mainly on coastal cliff tops, offshore islands and
inland, by lakes and on moorland. At these natural sites Herring
Gulls are faring much worse than their urban con-specifics – the
total population of Herring Gulls breeding in Britain has declined
by 50% since 1969-70. Over the last 30 years there has been a
general decline in commercial fishing around the British Isles and,
combined with changes in fishing practices (i.e. reduced mesh sizes
resulting in fewer discards and retention of offal for conversion
to fish meal), has led to a reduction in food available to Herring
Gulls and other seabirds that scavenge behind trawlers.
In Ireland the decline in Herring Gull
numbers has been even more dramatic and has occurred mainly in the
last 15 years when numbers throughout Ireland declined by 90%.
Avian Botulism is thought to be major cause of this decline.
In contrast, numbers of Lesser Black-backed
Gulls breeding in Britain and Ireland have increased over
the last 30 years by 77%. 22% of Lesser Black-backs breed at inland
sites, the majority, of which, nest on a Lancashire moor, where
numbers have increased from about 8,000 pairs in 1985-88 to around
19,000 pairs in 1998-2002.
Common Gulls in the British Isles nest
predominantly in Scotland (97%) and inland (57% of pairs breeding
in Britain and Ireland during Seabird 2000), but previous censuses
have been confined to coastal colonies. Numbers of coastal-nesting
Common Gulls have increased by 65% and 39% since 1969-70 and
1985-88 respectively. However, the distribution of Common gulls
nesting throughout Britain and Ireland appears to have constricted
over the last 30 years, with the number of OS 10-km squares
occupied by breeding Common Gulls having declined by 44% and 32%
since the Atlas surveys in 1968-72 and 1988-91 respectively.
Formerly the world's largest colony of Common Gulls, a colony in
the Grampian Hills that held 24,500 pairs in 1989 became extinct in
the late 1990s, but a nearby colony of almost 18,000 pairs changed
little in size during the same period.
Black-headed Gulls also nest inland (44% of
pairs breeding in Britain and Ireland during Seabird 2000), but are
much more widely distributed than Common Gulls. Since 1985-88,
Black-headed Gulls have shown contrasting fortunes on different
sides of the Irish Sea, with a 6% increase in numbers
breeding in Britain, but a 70% decline in Ireland. Declines in
numbers breeding inland in England and Wales were offset by an
increase in the total number breeding in Scotland. But
throughout Britain, their breeding range appears to have
constricted markedly over the last the 30 years and the number of
OS 10-km squares occupied by breeding Black-headed Gulls has
declined by 60% and 50% since the Atlas surveys in 1968-72 and
1988-91 respectively. In Ireland the decline in numbers has been
greatest inland and the breeding distribution has constricted by
95% and 77% since the Atlas surveys in 1968-72 and 1988-91
respectively. The decline in Ireland has been attributed to
predation by American Mink.
During the last 50 years Mediterranean Gulls
have been expanding their breeding range from their stronghold in
eastern Europe and around the Black Sea, and in 1968 a pair bred in
southern England. Up until the late 1980s only a handful of
breeding attempts were made by Mediterranean Gulls in Britain, but
from then on, numbers breeding each year began to rise sharply and
by 1999-2002, there were 108 pairs in Britain, mostly confined to
southeast England and a further 5 pairs in Ireland.
Since 1985-88 the numbers of three of the five tern species
breeding in Britain and Ireland have declined: Sandwich
Tern (-11%), Arctic Tern (-29%) and
Little Tern (-25%). The causes of the declines of
both Arctic and Little Tern populations are thought to be due to
successive years of poor productivity resulting from different
reasons: to a reduced availability of sandeels around Shetland -
the main breeding stronghold of Arctic Terns; and to the loss of
Little Tern chicks and eggs to predators such foxes and Kestrels.
The causes of the decline in numbers Sandwich Terns are less clear,
though persecution on their wintering grounds in northwest Africa
may have had an impact. This is also thought to have contributed to
a two-thirds reduction in the number of Roseate
Terns breeding in Britain and Ireland since 1969-70. Now
the rarest breeding tern in the British Isles at just 790 pairs,
Roseate Terns have increased in numbers slightly over the last 15
years, probably due to the efforts of conservationists on a tiny
island near Dublin, where nest boxes have been provided on
specially built terraces, which have attracted birds from colonies
across the Irish Sea in Britain, where the Roseate Tern is now the
rarest breeding seabird at just 52 pairs. Common
Tern numbers in Britain and Ireland have remained stable
over the last 30 years but localised impacts from mammalian
predators (e.g. Mink, foxes) and from competition with large gulls
(i.e. Larus spp.) have resulted in regional variation in
population trends. Common Terns have a greater tendency than other
tern species to nest inland. The number of inland OS 10-km squares
occupied by breeding Common Terns in Britain has declined by 51%
since the New Atlas survey in 1988-91.
Black Guillemots were surveyed by counting
pre-breeding aggregations of adults close inshore during the first
hours of daylight (05.00 – 09.00 BST) during April and early May,
prior to laying. The UK total of 39,000 adults was very similar to
that from a previous pre-breeding survey of Black Guillemots
conducted in 1982-91. Numbers of Black Guillemots in the
Northern Isles, the core of the UK range, had increased by 14%
despite considerable numbers of adults being killed in Shetland
during the Braer oil spill in 1993. Elsewhere, population
trends varied locally, with colonies on offshore islands, on the
whole, having increased in size, whereas those on inshore islands
and along parts of NW mainland Scotland had reduced in size or
disappeared. The concurrent spread of American Mink into NW
Scotland is a possible cause of these declines, since their
presence would reduce the amount of safe nesting habitat available
to Black Guillemots. The Seabird 2000 pre-breeding survey was the
first such survey to be conducted in the Republic of Ireland and
recorded a total of 3,367 adults.
Causes of change
Seabirds are long-lived, delay breeding until they are several
years old, display high rates of annual survival of adults but low
rates of post-fledging survival. Breeding population size is
most affected by factors that influence adult survival rather than
breeding success and post-fledgling survival, unless such effects
are sustained over several years. Factors causing reduced
adult survival include senescence, disease, reduced food
availability, predation, hunting/culling and stochastic events such
as oil spills and severe storms.
Seabird populations in Britain and Ireland have increased in
size over the last century as a direct result of increased
protection from hunting and persecution at British and Irish
colonies and non-breeding areas overseas.
During the last 15 years two large spills occurred close to
the British coast, resulting from oil tanker accidents: the
Braer (Shetland, January 1993) and Sea Empress
(Pembrokeshire, February 1996). Thousands of seabirds died but the
impact of both spills was far less than if they had occurred during
the breeding season, since most species tend to disperse away form
the colonies during the non-breeding season. However, two spills
recently occurred in the Bay of Biscay within the non-breeding
areas of auks from colonies in Britain and Ireland - the Erika
(Brittany, France, December 1999) and the Prestige (Galicia, Spain,
November 2002). Both spills killed large numbers of auks, but the
majority were immature birds, so that the impact on breeding
populations was much less than if the same number of adults were
killed. However, the true impact of this mortality on the breeding
colonies in Britain and Ireland will only be evident several years
later when these young birds recruit into the breeding population.
The very fact that large numbers of seabirds aggregate together to
breed makes them very vulnerable to even localised spills should
they occur during the breeding season.
Food availability tends to have a much more dramatic effect on
breeding success than it does on adult survival. Between 1985 and
1990, the sandeel stock around Shetland collapsed, leading to
successive years of breeding failures of Arctic Terns, Arctic
Skuas, Great Skuas, Black-legged Kittiwakes and Atlantic Puffins.
Since then, sandeel availability and breeding success has
fluctuated. As a result of successive years of poor breeding
success and subsequently low recruitment, the breeding populations
of Arctic Skuas and Black–legged Kittiwakes in Shetland have
declined by 42% and 69% respectively since 1985-88, and numbers of
Arctic Terns breeding throughout Orkney and Shetland (considered to
be one large metapopulation) declined by 41% between 1980 and 2000.
Arctic Terns (and all other tern species) and Black-legged
kittiwakes feed on fish just below the surface, whereas other
piscivorous species can reach food much deeper below the surface by
pursuit diving (auks, European Shags and Great Cormorants) or by
plunge-diving (Northern Gannets) and so tend to have a greater
ability to obtain enough food to raise chicks even when fish stocks
are low. However, diving species are by no means immune to the
effects of food shortages. For instance, on the Isle of May the
breeding success of European Shags has been
positively correlated with the size of the local sandeel
stock and in years of poor sandeel availability up to 60% of
the breeding population of shags on the island have deferred
breeding. While the British populations of sub-surface feeders have
tended to fair better than surface feeders over the last 15-30
years, the European Shag population Britain and Ireland has
declined by 25% since 1985-88.
The extent to which fishing has affected the availability of
small prey fish like sandeels and in turn, their reliant seabird
populations is poorly understood. The collapse of the sandeel stock
around Shetland in the mid-1980s was believed to be a result of
changes in oceanography rather than any effect of fishing. Most of
the sandeel fishing in the North Sea occurs beyond the foraging
range of seabirds in UK colonies, apart from around Shetland and
over the Wee Bankie, within range of the large seabird colony on
the Isle of May. A precautionary ban was imposed on the Shetland
fishery in 1990-95 and subsequent catches have been limited to low
levels; and a ban has been imposed in the eastern North Sea since
2000, with reopening dependant on Kittiwake breeding success, which
has shown some signs of improvement since the ban, whilst that of
Common Guillemots and Razorbills in the same area has declined.
Clearly further work is required to decipher the complicated
relationships between fisheries and seabird populations in order to
determine the true impact of future changes in fisheries
management.
Commercial fisheries do not always compete with seabirds over
fish, and indeed fishing trawlers are an important source of food
for some species. As a result of depleted stocks of sandeels around
Shetland, Great Skuas became more reliant on scavenging discards
from fishing trawlers and their aggressive nature meant that they
were able to out-compete other species. Great Black-backed Gulls,
Herring Gulls, Northern Fulmars and Northern Gannets also rely on
food provided by the fishing industry. However, over the last 30
years there has been a general decline in commercial fishing around
the British Isles and, combined with changes in fishing practices
(i.e. reduced mesh sizes resulting in fewer discards and retention
of offal for conversion to fish meal), has led to a reduction in
food available to seabirds, which has been suggested as the cause
of a concurrent decline in numbers of Herring Gulls breeding in
Britain. The amount of food provided by fishing is set to decline
even further in the future following the recent collapse of white
fish stocks in the North Sea and is likely to impact on other
species that rely on scavenging.
Island refuges of many seabird species are under
threat from introduced mammalian predators such as rats, cats and
mink. Seabirds nest colonially in dense aggregations that represent
abundant food sources for such predators. They avoid predation by
nesting in sites such as cliffs, offshore islands, spits or remote
beaches where predators are scarce or absent. The number of
predator-free nest sites can limit population size and the
colonization or introduction of predators to such sites has
resulted in the extirpation of some seabird species. For instance,
rat predation has been associated with population declines and
extirpation of Manx Shearwaters on Canna and Calf of Man, and of
Puffins on Ailsa Craig and Puffin Island, Gwynedd. Furthermore, the
distribution of European and Leach's Storm Petrels is confined to
islands without rats. Rat eradication projects have been
implemented on some islands has resulted in the recolonisation by
some seabird species.
American Mink were introduced to Britain and
Ireland when they escaped from fur farms and now occupy Lewis,
Harris and mainland Scotland as far north as the Great Glen and
beyond. Mink are adept swimmers and can easily cross several
kilometers of sea to access inshore islands and can decimate the
resident colonies of terns, gulls and Black Guillemots. Mink
predation has resulted in the decline in population sizes of some
species in west Scotland and on inland lochs in Ireland. Mink
control has been implemented at key colonies along the west coast
of Scotland and these are effective in increasing seabird
productivity and preventing extirpation. A five-year project to
eradicate mink from the Uists and reduce numbers in Harris is
currently being implemented in the Western Isles.
Climate change is likely to impact on seabird populations. The
breeding success of some seabird populations in the UK has already
been linked to climatic fluctuations in the North Atlantic, such as
the North Atlantic Oscillation (NAO). Projected consequences of
global warming in UK waters, such as sea level rises, increased
storminess and rises in sea/air temperatures are likely to have a
direct impact on seabird populations. For instance, rising sea
levels may reduce the amount of breeding habitat available for
shoreline nesting species such as terns; winter storms can cause
large-scale 'wrecks' of seabirds and summer storms can wash whole
colonies from cliffs.