NEST-SITE CHARACTERISTICS, DURATION OF USE AND BREEDING SUCCESS IN THE GUILLEMOT URIA-AALGE

The fate of all eggs laid by Guillemots Uria aalge in seven study area s on the Isle of May was recorded in 12 consecutive seasons, The avera ge success at each site tvas related to its physical characteristics, bird density, position on the cliff and availability of suitable habit at for ticks, The numbers of neighbours and walls, type of site, slope where the egg was incubated and distance from the top of the cliff al l had a significant effect on both hatching and breeding success, Pote ntial tick habitat had no significant effect, Much variation in hatchi ng and breeding success remained unexplained. Sites used when the colo ny was less than half its current size and most frequently in the peri od 1984-1995 were the most successful. Since an average Guillemot pair remains together at a site for only 3-4 years, the consistently high success of many sites over a period of 10-12 years suggests that the m ost: used sites were occupied by a succession of high-quality birds.