Results from seabird monitoring programmes are increasingly being used to assess the state of the marine environment, but data are time consuming to collect. We used monitoring data from a colony of Common Guillemots Uria aalge in the North Sea to determine whether temporal changes in breeding success over a 34- year period were reflected in six consistently monitored plots and if individual plots showed significant differences in breeding success. Annual mean breeding success showed a four-fold difference over the study period varying from 0.261 to 0.848 young fledging per pair laying. Although the annual pattern of change was broadly reflected in all six monitoring plots there were significant differences in annual breeding success and changes in breeding success over time among the plots. Monitoring any single plot thus gave an approximate indication of the overall patterns of change over time, including periods when breeding success was reduced. However, significant among plot differences indicated that at least at this colony, robustness of colony level estimates of breeding success and changes over time were improved by monitoring multiple plots.

The Seabird Monitoring Programme (SMP) was established in 1986 with the aim of ensuring that sample data on breeding numbers and breeding success of the 25 species of seabird that regularly breed in Britain and Ireland are collected to enable their regional and national conservation status to be assessed. The SMP is led and coordinated by the UK Joint Nature Conservation Committee (JNCC) in partnership with other organizations (http://jncc.defra.gov.uk). Monitoring the status of the Common Guillemot (henceforth Guillemot) Uria aalge is a high priority since the species is widespread around the coasts of Britain and Ireland and is known to be at risk from multiple anthropogenic threats such as oiling, drowning in fishing gear, accumulation of toxic chemicals and heavy metals, as well as extreme storm events resulting in wrecks (Bourne 1976; Mitchell et al. 2004). Considerable effort has been invested in developing appropriate protocols for monitoring the numbers of this species in terms of the timing and numbers of counts and the number of sample plots needed to allow changes over time to be assessed statistically (Birkhead & Nettleship 1980; Anker-Nilssen et al. 1996; Sims et al. 2006, 2007). However, no comparable evaluation has been carried out on protocols for monitoring breeding success.

The current recommendations for monitoring the breeding success of the Guillemot given in the The Seabird Monitoring Handbook for Britain and Ireland suggest following five or more plots of about 50 breeding pairs selected randomly or otherwise dispersed through the colony to include a range of different breeding habitats, with checks made every 1–2 days (Walsh et al. 1995). To our knowledge, no assessment of whether this level of replication of the number of study plots is adequate to provide a robust estimate of changes of breeding success at a colony level has been made. From a practical point of view, such an assessment is needed because monitoring breeding success of Guillemots is time consuming. For instance, Birkhead & Nettleship (1980) considered that it took 3 hours per day to follow a sample plot of 80 pairs. Since in Britain and Ireland breeding at any colony spans about three months it is not surprising that few studies have monitored more than 2–3 plots and some rely on estimates from a single plot.

The Isle of May, Firth of Forth, southeast Scotland is one of the four key UK SMP sites (the others being Fair Isle, Shetland, Skomer, southwest Wales, and Handa/Canna, northwest Scotland), where detailed monitoring of seabird numbers and breeding success are undertaken under contract to JNCC. On the Isle of May, breeding success of Guillemots has been monitored by checks of six plots several times a day throughout the entire breeding season since 1984. Results of this work have been used extensively to investigate the effects of changing conditions in the North Sea and to construct integrated population models of this Guillemot population (Kokko et al. 2004; Ashbrook et al. 2010; Newell et al. 2015; Lahoz- Monfort et al. 2017). Here we use individual monitoring plot data to assess whether overall temporal patterns of change were similar and whether there was evidence of significant differences in mean breeding success among the plots.

We thank the many people who have helped collect data on the Isle of May. Scottish Natural Heritage and its predecessors allowed us to work on the Isle of May National Nature Reserve and fieldwork was funded by the Natural Environment Research Council and Joint Nature Conservation Committee’s integrated Seabird Monitoring Programme. Liz Humphreys and an anonymous reviewer improved the manuscript with their comments.

Anker-Nilssen, T., Erikstad, K. E. & Lorentsen, S.-H. 1996. Aims and effort in seabird monitoring: an assessment based on Norwegian data. Wildlife Biology 2: 17−26. [Crossref]

Ashbrook, K., Wanless, S., Harris, M. P. & Hamer, K. C. 2010. Impacts of poor food availability on positive density dependence in a highly colonial seabird. Proceedings of the Royal Society B: Biological Sciences 277: 2355−2360. [Crossref]

Birkhead, T. R. 1977. The effect of habitat and density on breeding success in the common guillemot Uria aalge. Journal of Animal Ecology 46: 751−764. [Crossref]

Birkhead, T. R. & Nettleship, D. N. 1980. Census methods for murres Uria species - a unified approach. Canadian Wildlife Service Occasional Paper 43: 1−25.

Bourne, W. R. P. 1976. Seabirds and pollution. In: Johnson, R. (ed.), Marine Pollution: 403−502. Academic Press, London.

Gaston, A. J. & Nettleship, D. N. 1981. The Thick-billed Murres of Prince Leopold Island. Canadian Wildlife Service Monograph No. 6. Canadian Wildlife Service, Ottawa.

Harris, M. P. & Wanless, S. 1988. The breeding biology of guillemots Uria aalge on the Isle of May over a six-year period. Ibis 130: 172−192. [Crossref]

Harris, M. P., Wanless, S., Barton, T. R. & Elston, D. A. 1997. Nest site characteristics, duration of use and breeding success in the Guillemot Uria aalge. Ibis 139: 468−476. [Crossref]

Kokko, H., Harris, M. P. & Wanless, S. 2004. Competition for breeding sites and sitedependent population regulation in a highly colonial seabird, the common guillemot Uria aalge. Journal of Animal Ecology 73: 367−376. [Crossref]

Lahoz-Monfort, J. J., Harris, M. P., Wanless, S., Freeman, S. N. & Morgan, B. J. T. 2017. Bringing it all together: multi-species integrated population modelling of a breeding community. Journal of Agricultural, Biological and Environmental Statistics 22: 140−160. [Crossref]

Mitchell, P. I., Newton, S. F., Ratcliffe, N. & Dunn, T. E. 2004. Seabird populations of Britain and Ireland. T. & A.D. Poyser, London.

Murphy, E. C. & Schauer, J. H. 1996. Synchrony in egg-laying and reproductive success of neighboring common murres, Uria aalge. Behavioral Ecology and Sociobiology 39: 245−258. [Crossref]

Newell, M., Wanless, S., Harris, M. P. & Daunt, F. 2015. Effects of an extreme weather event on seabird breeding success at a North Sea colony. Marine Ecology Progress Series 532: 257−268. [Crossref]

Reynolds, T. J., King, R., Harwood, J., Frederiksen, M., Harris, M. P. & Wanless, S. 2009. Integrated data analysis in the presence of emigration and mark loss. Journal of Agricultural, Biological and Environmental Statistics 14: 411−431. [Crossref]

Sims, M., Elston, D. A., Harris, M. P. & Wanless, S. 2007. Incorporating variance uncertainty into a power analysis of monitoring designs. Journal of Agricultural, Biological and Environmental Statistics 12: 236−249. [Crossref]

Sims, M., Wanless, S., Harris, M. P., Mitchell, P. I. & Elston, D. A. 2006. Evaluating the power of monitoring plot designs for detecting long-term trends in the numbers of common guillemots. Journal of Applied Ecology 43: 537−546. [Crossref]

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: a compilation of methods for survey and monitoring of breeding seabirds. Joint Nature Conservation Committee, Peterborough.