Population trends of Kittiwake Rissa tridactyla, Black Guillemont Cepphus grylle and Common Guillemot Uria aalge in Shetland, 1978-98

Kittiwake Rissa tridactyla numbers in Shetland, monitored by periodic nest counts at all colonies, declined from 54 600 pairs in 1981 to 23 000 pairs in 1998, probably due to low food availability and increased predation by Great Skuas Catharacta skua. Pre-breeding counts of Black Guillemots Cepphus grylle along selected stretches of coastline showed variable trends. In Yell Sound, an increase of 155% between 1983 and 1998 probably represented recovery after mortality from oil pollution in 1979. Elsewhere, some decreases were associated with localised oil pollution. Common Guillemot Uria aalge numbers at four colonies increased in the late 1970s and early 1980s, but then declined up to 1990; thereafter, numbers increased at all colonies. At the largest colony, Guillemot numbers had returned to their previous peak by the mid 1990s, but at the three smaller colonies they remained at c. 50% of early 1980s levels. Large-scale change in food availability in the North Sea is thought to have caused increased winter mortality during the early 1980s, whereas reduced abundance of sandeels may have contributed to reduced colony attendance of non-breeding and off-duty birds and therefore apparently low population levels from 1989-91.