The breeding biology of Black Guillemots Cepphus grylle in Shetland

The breeding biology of Black Guillemots Cepphus grylle was studied from 1982 to 1984 in Shetland, on islands lacking introduced mammalian predators. Adult survival (94%), clutch size (1.75-1.92) and breeding output (1.05-1.34 young fledged per breeding pair) were very high, indicating that conditions were relatively favourable on the islands studied. The timing of breeding varied significantly between years and was somewhat later than in other temperate parts of the range. In 1984 food was probably more readily available (possibly due to better weather conditions) in the pre-breeding period, which enabled more inexperienced birds to reach the condition threshold for breeding. It is likely that these birds tended to lay single-egg clutches in more open sites which were more susceptible to egg predation by crows. Nest-site quality and food availability were the key determinants of breeding productivity.