Xantus’s murrelet is thought to be a monogamous bird which arrives at its island breeding colony around mid-February, about three weeks before it lays its eggs. The female lays two eggs, eight days apart, into a crevice in a cliff, at the back of a small cave, under a boulder, or occasionally, on bare ground beneath thick vegetation (2). Following the laying of the second egg, the male and female will take it in turns to incubate the eggs for the next 34 days. During this time, the eggs are highly vulnerable to predation by deer mice (Peromyscus maniculatus) (2). The chicks are very well-developed on hatching, with limbs that are already 98 percent of the size of adults’ (5), and are taken to sea, just one to two days later, by their parents (2). Here, unique to Synthliboramphus, the remainder of the chick’s development takes place (6).
In April or May, Xantus’s murrelet will move north and west, away from the breeding islands, travelling rather slowly to their wintering areas where they will remain until late November or early December (2). Xantus’s murrelet feeds mainly on larval fish, as well as other small prey such as sandeels and crustaceans (2). Diving down to maximum depths of 21 metres and remaining underwater for up to 28 seconds, Xantus’s murrelet uses its wings to propel itself through the water to pursue and capture its prey (7). During the breeding season, northern anchovies (Engraulis mordax) comprise a significant part of this murrelet’s diet, and thus breeding effort has been noted to be lower when anchovy abundance is low (2).