The Black Guillemot or 'tystie' is a circumpolar species,
concentrated around the North Atlantic, Barents Sea, Baltic and
smaller numbers around the Chukchi Sea in northern Alaska and
north-eastern Siberia. Approximately half of the Black Guillemots
breeding in Britain and Ireland do so around the Northern Isles,
with the remainder confined mainly to the coasts and islands of
north and west Scotland, Ireland and the Isle of Man. Their
distribution is within the core range is determined by the
availability of suitable nest cavities that are safe from land
predators such as rats, mink, stoats and otters. Between 1969-70
and 1985-91, there was an expansion in the range of Black
Guillemots, in particular the colonisation of new sites around the
Irish Sea, including man-made structures (e.g. harbour walls,
jetties, piers), and in to north east Scotland.
The Black Guillemots is one of the more problematic seabird
species to survey. It tends to breed away from the large seabird
cliff colonies and prefers small rocky islands and low-lying,
indented stretches of rocky coast. Black Guillemot nests are hidden
in rock crevices and under boulders, which makes them extremely
difficult to census during the breeding season. During Operation
Seafarer (1969-70), counts were conducted along with other
cliff-nesting seabirds during June. At this time of year, Black
Guillemots are either hidden away in nest chambers brooding young
or are away feeding. Most Black Guillemots feed close inshore, so
may be visible to surveyors, but may be easily missed especially if
they are feeding or breeding away from colonies of other breeding
seabirds. Operation Seafarer therefore underestimated the British
and Irish population by an unknown amount. Between 1982 and 1991,
as part of the SCR Census, a survey of the number of adult Black
Guillemots was conducted prior to the breeding season (late March –
early/mid May), during the first three hours of daylight (06.00 –
09.00 BST) when they congregate close inshore for courtship and
mating. Such counts have been found to be the most repeatable and
accurate way of assessing the size of Black Guillemot populations.
Unfortunately, during the SCR census, counts of Black Guillemots in
the Republic of Ireland were conducted in June and not during the
pre-breeding period.
During Seabird 2000 a pre-breeding survey of Black Guillemots
was conducted throughout Britain and Ireland. This was the first
opportunity in many areas to examine changes in the population of
Black Guillemots since 1982-91. In The Republic of Ireland, this
was the first national pre-breeding survey.
Census Methods
The methods used during Seabird 2000 were identical to those
used during the SCR Census, when pre-breeding surveys of Black
Guillemots were conducted in Britain and Northern Ireland during
1982-1991 (see Lloyd et al. 1991. NB. The results of
pre-breeding surveys conducted in 1991in parts of Argyll & Bute
and in Kyle and Carrick, Stewartry and Wigtown were not included in
Lloyd et al but are included in the results of the SCR
Census summarised in Table 1). The count unit used was the number
of adult Black Guillemots visible on land or on the sea within 300m
of the shore. Adults seen more than 300m offshore were assumed to
be feeding rather than associated with a pre-breeding courtship
group. At some colonies it is actually possible to find nest
chambers and these were counted as apparently occupied
sites (AOS). Counts of AOS were converted to individuals by
multiplying by two.
Surveyors were instructed to count Black Guillemots between 26
March and 15 May at 06.00-09.00 (BST), in winds no stronger than
Beaufort force 4 and in calm sea conditions. Overall, 86% of counts
were conducted on the prescribed date and at the prescribed time,
9% on prescribed date but time was not recorded, 3% on the
prescribed dates but later than 0900 (BST) and just 2% were
conducted outside the prescribed dates.
Survey Coverage
In Britain most counts (72%) were conducted from boats,
whereas in Ireland 67% of counts were conducted from land. The use
of fast inflatable boats allowed stretches of coast of 20-30km to
be covered in a single morning, in contrast to only 5km on foot.
Small inflatable boats are advantageous in that they can be used
very close to the shore, entering geos and coves where Black
Guillemots are often difficult to see, especially when they are
perched on rocks. Later in the survey period, Black Guillemots are
more likely to be on land and may be difficult to see from cliff
tops above and often require active flushing to be visible to
observers. In Scotland, boats enabled small teams of surveyors to
cover large stretches of coastline in single years. In 2000, the
whole of the north west mainland coast of Scotland was surveyed
from Cape Wrath (Sutherland) south to Oban (Argyll and Bute),
including the Isles of Skye and Mull, in addition to eastern
Caithness and half of the sites on the Isle of Lewis (Western
Isles). The following year, just three survey teams covered the
remainder of the Argyll and Bute coastline (Oban to Campbell town,
Kintyre, including Islay and Jura), the north coast of mainland
Scotland, the southern Western Isles and Wigtown. Table 1 lists the
year in which surveys in each county were conducted during both
Seabird 2000 and the SCR census.
Data presentation
Site-by-site comparisons of counts from the SCR Census and
Seabird 2000 were only possible in Britain and Northern Ireland. In
the Republic of Ireland most counts of Black Guillemots during the
SCR Census were conducted in June and were not comparable with
Seabird 2000 pre-breeding counts. The Black Guillemot is the only
species in this book for which comprehensive site-by-site
comparisons with the SCR Census have been possible The main reason
for this is that the spatial scales at which counts were conducted
during the SCR Census and Seabird 2000 were much more compatible in
the case of Black Guillemots than any other species. During most of
the SCR census, the position of each distinct group of pre-breeding
birds was recorded with a single OS grid reference. During Seabird
2000, surveys of all coastal nesting species including Black
Guillemots were conducted along discrete sections of coastline
called 'sub-sites', 1km or more in length. It was therefore
straightforward to compare counts of Seabird 2000 sub-sites with
those of the contiguous SCR count points.
The intervening period between SCR and Seabird 2000 counts
varied considerably between different sites e.g. 15-19 years on
Shetland, 10-11 years in Argyll. Therefore the change in numbers at
each individual site was expressed as percentage change per annum
(see Figure 2).