A 32-year study of Razorbills Alca torda on the Isle of May, southeast Scotland, showed that the mean value of k, used to convert counts of individual birds to breeding pairs, averaged 0.795 ± 0.019 SE and increased significantly over the period. This value was much higher than the 0.67 used to convert counts of birds during censuses of colonies for Britain and Ireland in both 1985-88 and 1998-2002. Assuming that the average values recorded at the Isle of May at the time (0.767 and 0.851, respectively) were representative of other colonies, the total British and Irish populations would have been 14% and 27%, respectively higher than the published totals. Given plans for another nationwide survey in the next few years it would be useful to obtain k values from additional colonies to provide better assessment of changes in breeding numbers.

Razorbills Alca torda breed on sea cliffs and among boulders in the boreal and sub-arctic parts of the North Atlantic and in the Baltic. Although some pairs breed on cliff ledges where they are relatively easy to see, many breed in crevices or under boulders and are very difficult to observe. In Britain and Ireland the recommended counting unit for Razorbills is the presence of an individual bird in the colony, irrespective of whether it is actually breeding (Walsh et al. 1995). Sometimes, such as when comparing population estimates of different species or in population modelling, there is a need to convert counts of birds to pairs. In both the 1985-88 Seabird Colony Register and 1998-2002 Seabird 2000 surveys this was done by multiplying the counts of birds by 0.67, a conversion factor k based on studies made on Skokholm, Wales, in the early 1970s and the Isle of May, southeast Scotland, in the 1980s (Lloyd 1973, 1976; Harris 1989). A recent analysis of the value of k used to convert counts of Common Guillemots Uria aalge to pairs at two North Sea colonies documented considerable change over the last 15-30 years and showed that the use of published values can give erroneous estimates of breeding populations (Harris et al. 2015). Here we report a series of k values collected during a long-term study of Razorbills breeding at a single colony and, given the current interest in making another count of seabirds in Britain and Ireland, stress the need to obtain estimates of k values at other colonies.

We thank staff from the Institute of Terrestrial Ecology (1981-1994) and Scottish Natural Heritage (1995-2014) who made the annual whole-island counts on the Isle of May NNR. Jean-François Rail and Greg Robertson supplied unpublished data from Canadian surveys.

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