The Razorbill is a bird of the temperate North Atlantic and
adjacent parts of the Arctic Ocean. They breed on both sides of the
Atlantic and in the east they breed as far south as Brittany
(France), north to Svalbard and east to the White Sea in north-west
Russia. Razorbills breeding in Britain and Ireland winter along the
Atlantic coast of Europe from southwest Norway to Iberia and North
Africa, and into the western Mediterranean. Immature Razorbills
move significantly further away from their natal colonies than do
adults and generally further south, though occasionally they stray
west as far as Greenland and the Azores (Merne 2002).
Razorbills breed mainly on small ledges or in cracks of rocky
cliffs and in associated screes, and on boulder-fields. Rarely,
colonies have been found up to 300m inland (Harris & Wanless
1997). Razorbills are usually associated with colonies of other
seabirds, and small numbers scattered among large concentrations of
Common Guillemots and Black-legged Kittiwakes can easily be
overlooked. Razorbill 'nest' sites are usually hidden from view,
but the presence of a colony is clearly indicated by the attendance
of off-duty birds standing close by. Since it is not usually
possible to count occupied sites, Razorbills are difficult to
census. Hence, prior to Operation Seafarer (1969-70), very little
was known about its numbers and population trends in Britain and
Ireland. Furthermore, interpreting differences between counts of
Razorbills conducted during Operation Seafarer (1969-70) and the
SCR Census (1985-88) is difficult, since most counts in Operation
Seafarer were expressed as pairs, while the SCR Census counted the
number of birds attending the colonies (on land). Despite
methodological differences between the two censuses, during
1969-1988, there appeared to be an increase in the total number of
Razorbills breeding in Britain and Ireland. For example, in
Shetland, where individual birds were counted in both censuses,
numbers of Razorbills doubled between 1969 and 1988. In Ireland
during the same period, substantial increases in Razorbill numbers
on the east coast were offset by declines in the south and west.
These declines were coincident with large numbers of Razorbills
recorded drowned in fishing nets, especially in mono-filament
salmon drift-nets that were set close to colonies on the south and
west coasts of Ireland in the 1970s. During this period large
numbers of Razorbills were also drowned in sardine nets while
wintering in areas off the mouth of the Tagus River in
Portugal.
Census Methods
The count unit specified for Razorbills during Seabird 2000
was 'individuals on suitable breeding ledges', which excluded birds
on rocks below the high water mark and on the sea adjacent to the
colony. Counts of individual birds included off-duty adults,
non-breeders and immatures as well as - where nest-sites were
visible - brooding and incubating birds.
At a few sites it was possible to actually find and count
apparently occupied sites or nests (AOS or AON) of Razorbills.
However, in order to compare counts between sites and between
years, all counts of AOSs and AONs were divided by 0.67 to estimate
the equivalent count of individuals (after Lloyd et al.
1991). This conversion factor was based on studies on the Isle of
May and Skokholm, where the ratio of AONs or AOSs to birds present
ranged from 0.59 to 0.77 (Harris 1989) and 0.30 to 0.55,
respectively (Lloyd 1975). In Seabird 2000, only 3% of the
population estimate (in terms of birds) of Britain and Ireland was
converted from counts of AOSs and AONs.
To allow comparison between Seabird 2000 and the two previous
censuses, counts of AOSs and AONs from Operation Seafarer and the
SCR Census were converted to counts of birds using the same
adjustment used for Seabird 2000 counts (see above). In the SCR
Census, converted counts comprised only 5% of the total estimate of
Razorbills in Britain and Ireland. In contrast to both the SCR
Census and Seabird 2000, most counts of Razorbills in Operation
Seafarer were expressed as pairs (i.e. AOSs or AONs) and comprised
78% of the total population estimate for Britain and Ireland in
1969-70. While the organisers of Operation Seafarer recognised the
difficulties involved in surveying breeding Razorbills, it is
unclear how surveyors determined the number of pairs present in
each colony (Cramp et al. 1974). Therefore comparisons of
Operation Seafarer counts of pairs of Razorbill with subsequent
counts of individuals should be treated with caution.
In Britain and Ireland, egg-laying usually begins in late
April or early May, with a peak in mid-May. The single chick
hatches from the last week in May and leaves, with its male parent,
about 3 weeks later. Most chicks have left the colonies by late
July, with stragglers sometimes remaining to mid-August. Seabird
2000 prescribed counting Razorbills between 1 and 21 June, to
coincide with the late incubation and main nestling period, and
during 08:00 to 16:00 hrs (BST) to coincide with the period of most
consistent attendance by birds at the colony from day-to-day. This
period does not necessarily coincide with the maximum numbers of
birds attending the cliffs during a season, but instead provides
the most comparable measure of attendance when using one-off counts
during censuses such as Seabird 2000.
In Seabird 2000, 43% of counts were conducted at the
prescribed period and time, and a further 21% was on the prescribed
period, but either not at the correct time or at a time that was
not noted by the surveyors. 36% of counts were conducted outside
prescribed date and time, but the majority of these were within one
week either side of the prescribed count period. During the SCR
Census, surveyors were instructed to count Razorbills within the
same dates and times as prescribed during Seabird 2000, but the
actual time was not recorded by surveyors. However, 37% of counts
were conducted on the correct dates, but 51% was conducted outside
this period and no date was recorded by surveyors for the remaining
12%. The same times and dates were not prescribed during Operation
Seafarer and some counts were carried out late in the season (in
July or even early August), when many successful adults would have
left with their chicks and when failed breeders would have
deserted. As the breeding season progresses the number of immatures
in attendance tends to increase, which also makes counts after the
prescribed period unreliable.
During Seabird 2000 and the SCR Census, surveyors were
instructed to avoid counting Razorbills during heavy rain or fog,
and in winds higher than Beaufort scale force 4, which all affect
the level of attendance by Razorbills at a colony, as well as the
ability of the observer to count them (no guidance in this respect
was given for Operation Seafarer). Any one of these conditions
constituted 'poor' weather, whereas all other weather conditions
were described as 'good'. At least 73% of Seabird 2000 counts were
conducted during 'good' weather and only 5% in 'poor' weather
(weather conditions were not recorded for 22% of counts). Weather
conditions were not recorded by surveyors during the SCR
Census.
Survey coverage
Coverage in Britain and Ireland during Seabird 2000 was almost
complete, though some Irish colonies were not surveyed, at Inishark
(Galway) and Aughris Head (Sligo); these colonies, however, were
relatively small during the SCR Census (22, 133 and 19 birds,
respectively).