The study examined the characteristics of nesting cavities of Black Guillemots Cepphus grylle at a pier in the harbour of Bangor, Co. Down, Northern Ireland during the period 1985-2001. Although the 15 nesting cavities in the pier appear superficially to be the same, there must be differences that the bird detect as some cavities were used every year and others rarely. Whilst one cavity was never used, another was used in all 17 years of the study. A number of nest cavity characteristics were measured and used as independent variables in a multiple regression with the total number of eggs laid in the cavity as the dependent variable. An exposure index (width x height / depth of cavity) and distance to the nearest steps were the two significant independent variables indicating that Black Guillemots chose the least exposed and least disturbed cavities in the pier.