Breeding colonies form in spring; each pair has a single brood consisting of one egg each year (1). The egg is laid in a crevice or hole, or a nest of pebbles (1). The young leave the breeding colony while still unfledged, at around 18 days after hatching and are looked after by the parents for some time (7). In winter, the diet is known to consist mainly of fish such as herring, whiting and sand eel, although crustaceans and worms are also eaten (4). Patterns of movement are complex, but it seems that birds in their first year travel further distances than adults (4).