This smallest of storm-petrels is about the size of a sparrow. This all-dark bird is almost bat-like in appearance as it flies low over the ocean with deep, rapid wingbeats. Often seen from the shore in Mexico, Least Storm-Petrels may occur closer to land than other storm-petrels. Hurricanes may drive these tiny birds inland; in 1976 Hurricane Kathleen deposited hundreds of them on California's Salton Sea.
Least Storm-Petrels weigh only three-quarters of an ounce; they are 5.7 inches long, with a 15 inch wingspan. These tiny seabirds are dark brown overall, with black wingtips and short, wedge-shaped tails, long slender wings, and a dark bill with the typical "tubenose" structure on top. In flight, these storm-petrels show a light brown diagonal bar on their wings. Sexes are similar.
Least Storm-Petrels breed only on islands off the coast of western Baja California, Mexico, and on islands in the northern Gulf of California. Following breeding, many birds wander north to California's warm coastal waters; by the middle of the fall, they have started moving south in large numbers along the coast of Central America. A few birds reach the coast of South America, spending the winter months as far south as Ecuador and Peru.
These birds, like other storm-petrel species, are true birds of the sea, coming ashore only to breed. During most of the year, they are generally found in the warm ocean waters over the continental shelf. They are more likely to move north along the California coast in years when the water temperature is higher. When they do come to land to breed, Least Storm-Petrels nest on rocky islands. This species nests in rock piles or cliff crevices, unlike many storm-petrels, which nest in burrows in the ground.
The diet of Least Storm-Petrels is largely unknown, but this species is known to forage primarily by picking very small marine life items—such as tiny crustaceans and zooplankton—off the surface of the ocean. These birds feed while fluttering just above, rather than sitting on the water.
Like others of their kind, Least Storm-Petrels are colonial nesters. During their nocturnal colony visits, whirring calls can be heard from inside the rock-crevice nests. Around July, the female lays a single white egg directly on bare rock, where it is probably incubated by both sexes. Similarly, both parents are probably responsible for feeding the young bird.
Following breeding, from August through September, these birds usually disperse irregularly north to the warm waters off the California coast, but by the middle of the fall, they begin moving south along the coast of Central America. A few birds reach the coast of South America, spending the winter months as far south as Ecuador and Peru.
BirdLife International (2007) Species Factsheet: Halocyptena microsoma. Downloaded from http://www.birdlife.org on 26/8/2007
Kaufman, Kenn. Lives of North American Birds. Houghton Mifflin Company, New York. 1996.
Sibley, David A. The Sibley Guide to Birds. Alfred A. Knopf, New York. 2000.
