The use of k values to convert counts of individual Razorbills Alca torda to breeding pairs

Mike P. Harris*, Mark A. Newell & Sarah Wanless

Centre for Ecology & Hydrology, Bush Estate, Penicuik, Midlothian EH26 0QB, UK.

Full paper

Abstract

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.

Introduction

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.

Acknowledgements

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-Fran├žois Rail and Greg Robertson supplied unpublished data from Canadian surveys.

References

Ashbrook, K., Wanless, S., Harris, M. P. & Hamer, K. C. 2008. Hitting the buffers: conspecific aggression undermines benefits of colonial breeding under adverse conditions. Biology Letters 4: 630-633.

Birkhead, T. R. & Nettleship, D. N. 1980. Census methods for murres Uria species: a unified approach. Canadian Wildlife Service Occasional Paper 43.

Chapdelaine, G., Diamond, A. W., Elliot, R. D. & Robertson, G. W. 2001. Status and population trends of the Razorbill in eastern North America. Canadian Wildlife Service Occasional Paper 105.

Cook, A. S. C. P., Parsons, M., Mitchell, I. & Robinson, R. A. 2011. Reconciling policy with ecological requirements in biodiversity monitoring. Marine Ecology Progress Series 434: 267-277.

Grecian, V.D. 2004. 'The effect of physical and biological parameters on the breeding success of Razorbills (Alca torda L. 1758) on Machias Seal Island, NB in 2000 and 2001'. MSc thesis, University of New Brunswick.

Harris, M. P. 1989. Variation in the correction factor used for converting counts of individual Guillemots Uria aalge into breeding pairs. Ibis 131: 85-93.

Harris, M. P., Heubeck, M., Newell, M. A. & Wanless, S. 2015. The need for year-specific correction factors (k values) when converting counts of individual Common Guillemots Uria aalge to breeding pairs. Bird Study 62: 276-279.

Lloyd, C. S. 1973. Attendance at auk colonies during the breeding season. Skokholm Bird Observatory Report 1972: 15-23.

Lloyd, C. S. 1976. An estimate of the world breeding population of the Razorbill. British Birds 69: 298-304.

Lloyd, C., Tasker, M. L. & Partridge, K. 1991. The Status of Seabirds in Britain and Ireland. Poyser, London.

Merne, O. J. & Mitchell, P. I. 2004. Razorbill Alca torda. In: Mitchell, P. I., Newton, S. F., Ratcliffe, N. & Dunn, T. E. (eds.) Seabird populations of Britain and Ireland: 364-376. Poyser, London.

Robertson, G. J. & Elliot, R. D. 2002. Population size and trends of seabirds breeding in the Gannet Islands, Labrador. Canadian Wildlife Service Technical Report Series 393.

Walsh, P. M., Halley, D. J., Harris, M. P., del Nevo, A., Sim, I. M. W. & Tasker, M. L. 1995. Seabird monitoring handbook for Britain and Ireland. JNCC / RSPB / ITE / Seabird Group, Peterborough.