Seabird 2000
Great Black-backed Gull
Larus marinus
The Great Black-backed Gull has an extensive breeding range
across the north Atlantic and adjacent seas, and historically,
Britain and Ireland have hosted most of the world population after
Iceland and Norway. They breed mainly in the Outer and Inner
Hebrides and the Northern Isles of Scotland. These regions offer
extensive areas of the preferred breeding habitat of well-vegetated
rocky coastline with stacks and cliffs. The 20th century saw
widespread expansion of the Great Black-backed Gull breeding range
and numbers on both sides of the Atlantic. In Britain at least,
population increase was remarkable given that a period of decline
rendered the species virtually extinct as a breeder towards the end
of the previous century. InBritain and Ireland as a whole, there
was little change in the size of the coastal breeding population of
the Great Black-backed Gull between 1969-70 and 1985-88. Great
Black-backs nest almost exclusively in coastal habitats, but will
occasionally nest inland[1] at freshwater sites as well as on the
roofs of buildings.
Census Methods
During Seabird 2000, the recommended census unit for Great
Black-backed Gulls was the apparently occupied nest (AON). Counts
of AONs were conducted from either within the colony or from a
suitable vantage point. At colonies that could not be accessed
(e.g. on stacks or on islands) and where there was no suitable
vantage point, flush counts of individuals attending the colony
were made. Counts of individuals were divided by two to provide a
rough approximation of the number of AONs present. This is the
least accurate method for censusing breeding gulls, as such counts
will include an unknown percentage of non-breeders and attendance
at the colony by both breeders in a pair is highly variable during
the day and throughout the breeding season. The accuracy of counts
of individuals is difficult to assess. During Seabird 2000, 18% of
the population estimate for Britain and Ireland was obtained from
counts of individuals, compared to 29% during the SCR Census
(1985-88).
Survey Coverage
No major gaps in survey coverage are known for
Seabird 2000 – most of the larger colonies (i.e. of several to
several hundreds of pairs) are well established. Great Black-backs
often nest at low densities in mixed-species colonies, usually with
Lesser Black-backed Gulls, but their visually conspicuous plumage
and large size coupled with a distinctively deep voice, probably
reduces the chances of such pairs being overlooked. However,
solitary nests or pairs, especially those in remote areas, might
easily have been missed. Seabird 2000 represents the first attempt
to census all inland breeding colonies of gulls. However, only 22
pairs of Great Black-backs were found to be breeding inland during
Seabird 2000.
Data presentation
Counts of breeding gulls within each admin area during Seabird
2000 and during previous censuses are presented differently from
other species to take into account the variation in coverage of
inland and coastal colonies during Seabird 2000 and the previous
two censuses. Since no inland colonies were surveyed during
Operation Seafarer (1969-70), Table 1 compares counts from all
three censuses from coastal colonies only, but also gives the total
number present in both inland and coastal colonies during Seabird
2000. Table 3 gives counts of breeding Great Black-backs within
each admin area during Seabird 2000 and during two previous surveys
of roof-nesting gulls in Britain and Ireland, conducted in 1976
(Monaghan & Coulson 1977) and in 1993-95 (Raven & Coulson
1977).
Census Methods Data Processing and
Analysis References Seabird 2000
[1] Coastal colonies were regarded as those being within 5km
of the high water mark; inland colonies were further from the sea
than this. All colonies in Orkney, Shetland and the Western Isles
were regarded as coastal.
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