SITE USE AND FIDELITY IN THE COMMON GUILLEMOT URIA-AALGE

The site fidelity of 470 colour-ringed Common Guillemots Uria aalge wi th at least 1 year of breeding experience was followed on the Isle of May from 1982 to 1993, On average, 85.7% of birds recorded breeding at a site in any year were present at the same site in the following sea son, Of those which did not retain their site, 35% had moved to anothe r site, 25% were nonbreeders and 40% were not seen, Site- fidelity of birds which returned to the colony the next year was 91%, There were n o significant age or sex effects, but there were significant (and unex plained) year and area effects. Most birds which changed sites moved l ess than 2 m, Some birds which obtained a new site improved their nest ing success, but many others became nonbreeders; it is speculated that the former chose to move site, whilst the latter had been evicted.