The Common Guillemot is one of the most abundant seabirds in
the temperate and colder parts of the northern hemisphere with very
large populations in the Atlantic and the Pacific Oceans and the
adjacent areas of the Arctic Ocean. In the northeast Atlantic its
range extends from Portugal in the south to Spitzbergen in the
north and includes the Baltic. Two subspecies, not easily separable
in the field, breed in our area, the dark-mantled nominate race
aalge occurs in most of Europe including Scotland and
possibly northern England, and the smaller, much browner mantled
albionis occurs in England, Wales, Ireland, Helgoland,
France and Iberia. A bridled morph, with a striking white eye-ring
and spectacle occurs in the Atlantic but not in the Pacific. The
frequency of this morph increases with latitude from less than 1%
in southern Britain and southern Ireland to 20-25% in Shetland.
Common Guillemots breed at most places around the coasts of
Britain and Ireland where there is suitable cliff habitat.
Guillemots are extremely gregarious, colonial breeding is the norm
and colonies can contain many tens of thousands of individuals.
Breeding areas are situated where the birds are safe from mammalian
predators such as foxes, feral cats, and mink. This means that on
the mainland, they are confined to sheer cliffs or in among
boulders at the bases of cliffs where access is difficult even from
the sea. On islands, cliffs and the tops of large stacks are
preferred but where such habitat is absent guillemots breed among
rocks or even on flat open ground. No nest is built, the single
relatively large egg being incubated on the bare rock, guano or
soil on a wide variety of breeding sites including large flat,
broad ledges where birds are crowded together at average densities
of about 20 pairs/m2, narrow ledges, isolated sites that
are little more than toeholds, grassy banks, on top of, or under,
boulders and elsewhere, even under bushes. Breeding success is
highest where birds breed at high density or where sites are well
protected from predators.
Numbers of Common Guillemots breeding in Britain and Ireland
have increased substantially during the last 30 years, and the
increase continued up to Seabird 2000, albeit at a reduced
rate.
Survey Coverage
Colonies are usually very conspicuous and are therefore easily
found. The locations of most are well known so coverage for this
species is likely to have been extremely high. With the exception
of Aughris Head (Sligo) that had 1012 birds in 1985-88, only the
very smallest and most isolated, for instance Rockall 300 km west
of St Kilda where a single pair bred in 1992 (Belaoussoff 1993),
and those that have just been established will have not been
visited during Seabird 2000.
Census Methods
A large colony of cliff-nesting seabirds is an awe-inspiring
and noisy place. In Britain and Ireland, Common Guillemots often
breed in association with Black-legged Kittiwakes, Razorbills and
Northern Fulmars. The size and scale of some colonies can be
intimidating to the counter, but most nesting areas can be viewed
from comfortable and safe cliff-top vantage points. Few colonies
can be counted from below the cliff since birds breeding towards
the back of ledges are often hidden from view. The normal counting
method is to divide a cliff-face into manageable sections using
prominent cracks and other physical features, make a diagram of the
area and then count each section systematically. Birds on tidal
rocks and those on the sea are excluded. Counting from the sea is
difficult and often very uncomfortable for those not used to small
boats; as a result counts of large colonies from the sea can be
subject to large error and should only be made in good conditions
and when there is no reasonable alternative counting method.
Colonies where birds breed out of sight in caves or under boulders
pose particular problems and estimates usually have to be based on
brief visits to the colony to count eggs and chicks.
The census unit for the Common Guillemot is the individual
bird. Some previous estimates were based on only counts of birds
thought to be incubating eggs or brooding chicks (Gibson 1950) but
this technique is unreliable for large-scale censuses. It is
impossible to know for some accounts made prior to Operation
Seafarer whether apparently incubating/brooding birds or all
individuals were counted, which reduces their usefulness. When
reporting the result of any count, the units involved must be
unambiguously stated.
Counts of individuals comprise birds with eggs or chicks,
their mates, failed breeders and immature birds. Although an egg or
chick is rarely left unattended, since there is serious risk of
either being predated, there is considerable daily, annual and
seasonal variation in attendance by other birds. Conveniently, it
is well established that numbers are relatively stable in the
middle part of the day during late incubation and the main
chick-rearing periods, even in the Arctic in continuous daylight.
In some colonies, but not all, the numbers of birds present are
depressed during or immediately after heavy rain or strong winds,
so counts are normally not made in bad weather. The timing of
breeding is fairly consistent among colonies in Britain and
Ireland, therefore during Seabird 2000 observers were asked to make
counts between 0800 and 1600 hr BST between 1 and 21 June, on days
when the wind was not stronger than Beaufort Force 4 and not during
heavy or continuous rain. About 43% of the Seabird 2000 counts were
conducted during the prescribed conditions, a further 25% was
counted within the recommended dates but either outwith the
recommended times (3%) or gave no time (14%). Most of the remainder
came from counts made, often for understandable logistical reasons,
in late June or early July. Such late counts will have
substantially under-estimated colony-size since by then many of the
successful males will have taken their chicks to sea, leaving the
females to attend the nest-site. In addition, the numbers of
non-breeders visiting the ledges will also have declined
considerably. This problem was particularly acute in Caithness
where 75% of the total Seabird 2000 count came from parts of the
coast counted late in the season. Numbers there will have been
substantially higher than the count of over a quarter of a million
individuals suggested. During the SCR Census (1985-88) the
proportion of counts within the optimum dates was 42%, though date
was not recorded in 11%. The time of counts was not routinely
recorded during the SCR Census. There was therefore, a considerable
imprecision in the calculation of rates of change between the two
censuses. However, the general findings are backed up by systematic
standardized counts made annually at 15-20 colonies dispersed
around Britain.
All counts relating to Seabird 2000 refer to individuals
counted at colonies. Counts of birds can, if required, be converted
into an approximate estimate of the number of pairs by multiplying
by a correction factor 0.67 to allow for the presence of mates and
non-breeders. This factor that has been shown to be generally
applicable in Britain but there will inevitably be differences
between years and between colonies depending on local feeding
conditions and whether the population is increasing or declining.
British and Irish totals have been converted to pairs for
comparison with counts from other countries that are almost always
expressed as pairs or breeding adults.