Measuring seabird productivity: inter-plot differences in the effect of site openness on the breeding success of the Common Guillemot Uria aalge

Measures of seabird breeding success are often used as bio-indicators for the state of the marine environment. For many seabirds, productivity is estimated by averaging data from a number of survey plots. This may lead to evidence of change being confounded if plots differ in the factors affecting productivity. To test for such differences, the productivity of the Common Guillemot Uria aalge was monitored at four plots on Skomer Island in 2011 and 2012. The relationship between site openness and breeding success varied between plots, and the plots themselves differed in productivity. At three plots, open sites had lower breeding success than more protected sites, but at one plot, no relationship was observed between site openness and productivity. Analysis of historical data revealed inter-plot differences in the relationships between productivity and the number of breeding birds in a plot. At only one plot was there evidence consistent with the theory that increasing numbers of breeding birds lowered productivity by forcing the use of more open sites. Inter-plot differences mean that evidence of changes in productivity may be confounded in estimates based on data from multiple plots. Specifically, increasing populations may act to reduce productivity at some plots but not at others. Simple analyses of site openness, breeding success and breeding numbers can provide important information about the factors driving changes in Guillemot productivity.