Abstract

The Common Guillemot Uria aalge population on Skomer Island, Pembrokeshire, comprised around 100,000 pairs prior to the Second World War, after which it rapidly declined to the point that by 1972, when the study described here started, it had reached about 2,000 pairs. The aim of the study was to under-stand the population biology of the Guillemot and what was causing the decline. Chronic oil pollution was considered a significant factor. From 1980, the popula-tion began to increase and has since continued to do so at 5% per annum; in 2022, around 21,000 pairs were present on the island. Estimates of adult and immature survival, based on annual resightings of colour-ringed individuals, together with measures of reproductive success, have helped to quantify factors causing the pop-ulation increase. The effects of oil pollution and climate change on survival have also been quantified. Since 1972, the median egg-laying date of Guillemots on Skomer has advanced by over two weeks. This effect is likely to be due to climate change, but so far with no apparent negative effects. An important focus of this study is the reliability and repeatability of the methods employed to monitor Guillemots and identifying the limitations of other monitoring methods.IntroductionAs an undergraduate, I was invited to join Ian Prestt’s team, who were studying Grey Herons Ardea cinerea,and spent several happy weeks in 1970 at the Monks Wood Experimental Station, Cambridgeshire. I revelled in the station’s scientific atmosphere and enjoyed talking to its researchers. Towards the end of my visit, I mentioned to John Parslow that I would be keen to assist with other projects. A few months later, he suggested that I might help with a project on Common Guillemots Uria aalge. This was another fish-eater at the top of the food chain that, like the Grey Heron, might have been suffering from pesticide poisoning. The day after my last university examination, in May 1972, my father drove me to Wales to start several weeks of Guillemot observations on Skomer Island, Pembrokeshire, following Parslow’s instructions. Only later did I recognise just how perceptive John’s approach was in rejecting some of the crude methods employed by earlier Guillemot researchers. Most previous researchers had estimated breeding success by climbing onto the breeding ledges, flushing all the adult Guillemots and counting their eggs and chicks (e.g. Tuck 1961). John Parslow’s common-sense approach was to observe Guillemots from a hide and obtain an estimate of breeding success from birds undisturbed by human activity.