Among the most northerly breeding species of the family Hydrobatidae, the Fork-tailed Storm-Petrel is found only in the North Pacific Ocean. It is one of the most common marine birds breeding in Alaska and the second most abundant and widespread of the storm-petrels (5-10 million individuals). Although most abundant in Alaska, this small seabird nests along the North American coast from northern California to Alaska and also along the northeast coast of Asia. After the long nesting season, individuals move southward and disperse over much of the North Pacific Ocean.
Like other storm-petrels, this species is sexually monomorphic in plumage and general morphology: one of its distinguishing characteristic is its bluish-gray color, resembling the prions (Pachyptila spp.), a group of petrels from the Southern Hemisphere. Like almost all small procellariiforms, it is mainly pelagic, spending up to 8 months of the year at sea. Size of breeding colonies varies widely, some containing thousands of individuals, while others hold only a few. Strictly nocturnal, the species is monogamous and both sexes invest equally in breeding, lay a clutch of 1, and raise no more than a single chick each year. Incubation and nesting periods are protracted, chick growth is slow, reproductive maturity is delayed, and adult survival is high. Pairs generally nest in burrows or crevices in talus slopes, but they also use burrows they excavate and sometimes use side chambers of other burrowing seabirds. Their ex-treme life-history characteristics are thought to result from exploiting a highly unpredictable en-vironment.
Two subspecies are recognized, the differences between them being clinal, however. In northerly habitats (for example, the Aleutian Islands), individuals are lighter in coloration and slightly larger, and may feed closer to shore. Individuals in more southerly populations-Washington, Oregon, and California-are smaller in size and generally more gregarious. The species does not usually form large flocks, but is often seen foraging in small groups over continental shelf waters or at the shelf break. Following ships during the day, the species is often attracted by boat lights at night. Its main diet is zooplankton, nekton, and small fish, which it usually captures while hovering, pattering with wings partly spread, or dipping at the surface of the sea.
Reproductive adaptations of the Fork-tailed Storm-Petrel-such as low incubation temperature, tolerance to egg neglect, torpor in chicks, and variable interannual growth rates-represent extremes among the family Hydrobatidae. The constraints of predation, spatial and temporal availability of food, and climate are more evident in features of its breeding biology than in most other seabirds. The opportunistic nature of this petrel in its habitat use, ability to forage over broad areas, and proclivity for ingesting surface contaminants-plus the tolerance of eggs and young to extreme inattention-make it an intriguing subject for ecological, physiological, and conservation research.
Much is known about Fork-tailed Storm-Petrel breeding biology, particularly on East Amatuli Island, Alaska ( Boersma et al. 1980 ). Comparative studies in southern and western colonies are lacking. Studies examining its life at sea, navigation, and orientation and homing abilities are likely to prove rewarding. Conservation will require removal of introduced predators on nesting islands and also a better understanding of interactions with other species.