Two subspecies are recognized:   The major differences between the two subspecies are slight size and plumage differences, where southern populations appear somewhat smaller and darker. Southern populations also begin and end their breeding season earlier than their northern counterparts. However, no molecular genetic analysis has been conducted to discern the two subspecies. The range of the fork-tailed storm petrel is estimated to cover 22,400,000 km2 with over 6,000,000 individuals occurring globally, making it the second-most widespread and abundant storm petrel. During the breeding season, the fork-tailed storm petrel form dense colonies on islands throughout northern California, Oregon, Washington, British Columbia, Alaska, and northeast Asia off the Kamchatka Peninsula. Most of their population is concentrated in Alaska, notably in the Bering Sea near the Aleutian Islands. Their southernmost colony is on Little River Rock in Humboldt County, with a population of 200 individuals. Their nesting habitat varies, ranging from bare rock to forests. They typically build their nests under rock crevices or roots, or burrow into soft ground with low-growing vegetation. As they frequent the open ocean, little is known about fork-tailed storm petrels outside of the breeding season. They have been frequently sighted off the Californian coast, making them the most northerly distributed storm petrel during the winter. Sparse sightings also indicate that they forage as far south as Hawaii. Fork-tailed storm petrels are generally silent and only call when entering their breeding colony at night. Their most common call is a 3- to 5-note, raspy "ana-ana-ana", heard from both sexes. This is especially intense during courtship, where their calls are accompanied by elaborate aerial displays. A higher-pitched, single note call is typically emitted by males, and it is used to locate females in noisy colonies and to maintain the pair bond. Like other storm petrels, the fork-tailed storm petrel mainly feeds on crustaceans and fish near the surface of the ocean, including amphipods, myctophids, shallow-water fish (such as greenling and sablefish), copepods, decapods, and squid. They are also extremely opportunistic and can be seen scavenging on fatty tissue of dead marine mammals and also trailing behind fishing boats. Fork-tailed storm petrels have a well-developed olfactory system and heavily rely on odour to scout for food, so often they are the first birds to arrive at a pungent food source.