Monitoring the breeding success of Common Guillemots Uria aalge: the value of multiple plots

Abstract

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.