Scripps's (formerly Xantus's) Murrelet, along with the closely related Craveri's Murrelet (Synthliboramphus craveri), is one of the southernmost species in the Alcidae, a family that is predominantly northern in distribution. This small black and white seabird has a relatively limited breeding distribution on offshore islands from southern California to central Baja California, Mexico. Postbreeding dispersal is largely to the north, as far as is known; large numbers of birds spend the late summer and fall over the outer continental shelf off central California, and small numbers occasionally occur in similar habitats north to Washington and southern British Columbia.
Unlike other alcids, Scripps's Murrelet and other Synthliboramphus murrelets—Craveri's Murrelet, Ancient Murrelet (S. antiquus), and Japanese Murrelet (S. wumizusume)—have a highly precocial posthatching development pattern. Two eggs are laid in rock caves or crevices or under dense shrubs. Each egg weighs approximately 22 percent of an adult bird, which is among the largest relative to adult size laid by any bird. After hatching, the young spend only one or two days in the nest before leaving the island under cover of darkness, often scrambling or tumbling 100 meters or more down steep sea slopes and cliffs to follow their parents to sea. The young are then fed and raised to independence well out of sight of land.
The largest nesting colony of Scripps's Murrelet in the United States is on Santa Barbara Island in the California Channel Islands. Important colonies off the coast of Baja California are on Islas Los Coronados, Islas San Benito, and Isla Guadalupe. The species has been extirpated on some of the Baja California islands by introduced cats and other predators, and it is threatened on other islands. Although the colony at Santa Barbara Island has maintained numbers in the low thousands since the mid-1970s, it is very localized and subject to several threats, including oil spills and other pollution as well as avian and mammalian predation.
This species' former common name honored the Hungarian John Xantus, a prodigious and colorful collector and student of natural history during the mid- to late 1800s. While employed by the U.S. Coast Survey at Cabo San Lucas, Baja California, Xantus collected specimens of 130 bird species. Three of these, including Xantus's Murrelet, were new to science ( Mearns and Mearns 1992a ). The specimens Xantus collected from southern Baja California remain among a relatively few records of Scripps's/Xantus's Murrelet from that far south.