The Ancient Murrelet is the most abundant and widespread member of the genus Synthliboramphus, the only seabirds in which the young are reared entirely at sea. It is found around the northern Pacific rim, from China to British Columbia, being most numerous in the eastern part of its range. Among auks, it is unusual not only for its chick departure strategy but also for its very diverse vocal repertoire, associated with its nocturnal colony visits, and for its extended incubation shifts, from 2–6 days.
Physically, the Ancient Murrelet is a fairly generalized auk, with a bill specialized neither for fish nor plankton, a relatively low wing-loading, and a functional morphology intermediate between specialized wing-propelled divers such as murres and the less specialized auklets. In keeping with these physical traits, Ancient Murrelets feed on a mixed diet of small fishes and large zooplankton, especially euphausiid crustacea. They forage chiefly in offshore waters to the edge of the continental shelf, but they may also occur in large numbers within a few kilometers of land, where tidal upwellings bring food close to the surface.
Ancient Murrelets normally breed in burrows dug in soft soil, in cavities under tree roots, or in shallow holes under grass tussocks; less often in rock crevices. They lay two eggs of equal size, and hatching success is high. Chicks are not fed in the burrow and depart for sea after 1–3 days. About 1.5 chicks leave the colony for every pair attempting to breed. This relatively high reproductive output (for an auk) is linked to a relatively low adult annual survival rate of about 77%. The unique breeding strategy of the Synthliboramphus murrelets and its associated traits (egg size, chick development, parent-chick recognition behavior) form a complex of adaptations that challenge the notion that seabirds must necessarily rear their young at the nest.