The breeding distribution of Black Guillemots Cepphus grylle in Orkney and Shetland, 1982–84

Surveys of adult Black Guillemots associated with breeding habitat in the pre-breeding period recorded totals of 6883 in Orkney and 12,008 in Shetland. These populations are much larger than was previously thought, largely due to improved census methods and coverage. Although the distribution of suitable sites influenced the distribution of nests, breeding was widespread with most colonies comprising fewer than 20 birds. Nest sites accessible to ground predators were used on islands free from Brown Rats or Stoats but on infested islands Black Guillemots bred only in higher cliffs. The lowest breeding densities were in Yell Sound and it is likely that the population there was still recovering from the effects of a large oil spill.