Precocial development in truly marine birds is exhibited by only four species, all alcids. Other species in the family are semi-precocial or exhibit a developmental pattern intermediate between these two (Ricklefs 1973, Sealy 1973, Birkhead 1977). The movement of newly hatched murrelets away from the colonies permits them to use what appears to be a patchily distributed food supply at sea (Lack 1968, Sealy 1975a, 1975b, 1976). The young of most other marine birds that exploit patchily distributed food or distant food resources develop slowly in their nest sites because they are fed infrequently (Ashmole 1971).