(c) Glen Tepke
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With flocks including tens of thousands of individuals, Sooty Shearwaters rank high on the social seabird scale. In windy weather these shearwaters glide effortlessly above the waves, sometimes flying in long lines or resting in dense rafts on the water. Sooty Shearwaters have made recent headlines for their spectacular long-distance migration. Recent tagging experiments have shown that birds breeding in New Zealand may travel to Japan, Alaska and California, and back—a total of 40,000 miles per year.
Sooty Shearwaters average 17 inches in length, with a wingspan of 40 inches, and weigh 1.7 pounds. In good light, this relatively large shearwater shows the uniformly dark chocolate-brown plumage which is responsible for its name. The bird's long, narrow dark wings sport distinctive silvery under-wing coverts, and the bill is relatively long and dark, compared with similar shearwaters.
Although the Sooty Shearwater is at times the most abundant bird off the California coast, it nests only in Australia, New Zealand, and southern South America. New Zealand has by far the most birds, with five million pairs in 80 breeding colonies—approximately half the world's population. Colonies on the Snares Islands south of New Zealand alone contain 2,750,000 pairs.
Sooty Shearwaters generally concentrate around upwellings over cooler waters, or where cool and warm waters meet. They may feed close to shore in areas where the water is deep. They breed on isolated southern ocean islands that have either soil or rock crevices needed for nesting burrows.
Sooty Shearwaters forage by plunging into the water from low flight, making shallow dives from the surface, or seizing prey at or just below the surface, to catch various small fish, shrimp and other crustaceans, squid, and jellyfish. These birds swim well underwater, propelled by their wings, and may feed in association with other seabirds and marine mammals, often following whales in particular to catch fish disturbed by them. They also follow fishing boats to take fish scraps thrown overboard.
The nesting season for Sooty Shearwaters in Australia and New Zealand runs from September to May. The birds nest on islands in nocturnally active colonies. Five- to nine-year-olds may begin breeding; they typically mate for life. The pair may perform dueting courtship calls. Both parents dig a burrow up to 10 feet long, or find a suitable rock crevice, and line the nest chamber at the end of the burrow with grass and leaves. Both parents incubate the single white egg for up to 56 days. Once hatched, parents spent their days feeding at sea. Returning to feed the chick at night, they typically crash land through the tree canopy and hit the ground with a resounding thump. These noisy nocturnal visitations occur with less and less frequency as the chick matures. The parents eventually cease to visit, and finally, after about 97 days in the nest, the chick departs from the nest by night, and heads off to sea.
Tracking tagged sooty shearwaters by satellite, researchers recently mapped the small bird's annual 74,000-kilometer search for food in a giant figure eight over the Pacific Ocean, traveling north from their southern Australia and New Zealand breeding colonies via Polynesia at the end of the nesting season in March to May, to their foraging grounds in Japan, Alaska and California by September, and then returning to their breeding colonies by November. Making this journey—the longest migration ever electronically recorded—in only 200 days, the birds averaged a surprising 350 kilometers daily. In some cases, a breeding pair made the entire journey together. Some non-breeders are present off the California coast during all seasons.
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