Petrels and Shearwaters
Petrels and Shearwaters
Petrels and shearwaters are wide-ranging oceanic birds with a characteristic tubenose and other specialized features that equip them well for a life spent mostly at sea. Found throughout the world, these long-lived colonial nesting seabirds include some 76 species in four families, all in the family Procellariidae of the order Procelliformes. These seabirds show a great range in body size, from the giant petrel with a 6 ft (2 m) wing-span to the robin-sized diving petrel. However, they are all fairly uniform in color, either all dark, or dark and light. The sexes are externally alike. While several species are globally endangered and a number of others quite rare, a majority are numerous and thriving.
Distribution
Most petrels and shearwaters nest on isolated islands in the southern oceans, some as far south as Antarctica. Several species, however, breed in Hawaii, the northwestern United States (including Alaska), Maine, and Canada. These birds frequently range far from their birthplace, covering thousands of miles in an endless search for food. The greater shearwater, for example, nests on the Tristan da Cunha Islands in the South Atlantic Ocean but may be found in the North Atlantic from Florida to Newfoundland during the northern summer. Other species may spend months circumnavigating the Pacific.
Life history
Breeding in these seabirds occurs when an individual reaches sexual maturity, usually between three and seven years of age. After an elaborate courtship and mating, a single white egg is laid, usually in a burrow or crevice, although some of the larger members of this group lay eggs on bare ground. Adult birds show long term fidelity to their mates, and to their nest site.
Incubation time varies from about 40 days in the smaller petrels to about 55 days or more in the larger species. Both sexes incubate and care for the chick, which is fed a regurgitated mixture of rich stomach oil and fishy remains. Chicks can store large deposits of fat between feedings, which may be at intervals from several days to a week.
Chicks take their first flight when they reach 46-100 days old. The young birds spend their first year at sea before returning to land to socialize and investigate future nesting sites. Petrels and shear-waters can live a relatively long life, reaching into the upper twenties.
Adaptations
Birds that spend most of their life flying over vast, windy stretches of ocean must have a variety of ways for dealing with the stresses and demands of such an existence, and petrels and shearwaters are remarkable in their adaptations. Most species have long narrow wings designed for gliding and soaring, while some of the smaller diving petrels have short stubby wings that work well in the underwater pursuit of fish. To watch these graceful birds “shearing” the wavetops with their stiff-winged, seemingly effortless flight, is to witness a true natural wonder.
Characteristic tubular nostrils located on top of the bill serve as a means of expelling saline solution from their large salt glands, located internally near the eye sockets. The salt glands allow these birds to drink seawater without any harmful effects, since their kidneys cannot produce concentrated urine. The horny structure of the exterior nostrils protects the internal nasal passageway from the irritating salt spray, and also serves as an opening to their very efficient olfac-tory organs. Petrels and shearwaters have an excellent sense of smell, which they use to find food, burrows, and other birds of their species.
These seabirds have oily, waterproof feathers and a dense undercoat of insulating down. Their webbed feet help them swim, and are also used, especially by the storm petrels, to patter upon the ocean surface in search of floating bits of food.
The strong bill has a food-grabbing hook on the end, and the typical dark, or dark-and-light plumage helps them blend into a monochromatic landscape.
The petrels and shearwaters have a characteristic musky odor arising from their stomach oils, which are used as a food for the young, as a defensive weapon (squirted when needed), and as additional waterproofing for their feathers.
Food
Some shearwaters and petrels dive to catch fish while other species feed on the surface of the ocean where they pick up crustaceans, macroplankton, fish, squid, and even garbage from ships. Giant petrels eat the young and eggs of other birds, and one may find a variety of procellariids feasting on the remains of a dead whale or seal.
Conservation
The introduction of non-native mammals, such as rats, pigs, dogs, and cats onto islands used by breeding seabirds has led to the large scale decimation of entire colonies. The Gala´pagos petrel (Pterodroma phaeopygia), for example, is now restricted to small remnant colonies on several islands in the Gala´pagos Islands of Ecuador. It is considered critically endangered due to dramatic population declines over the past 80 years, although conservation efforts have somewhat stemmed the rate of decline. Another problem faced by some species is entanglement in fishing nets or snagging in long-lines set by fishing boats. These fishing technologies are killing huge numbers of some species of petrels and shearwaters, and are causing their populations to decrease in some regions.
Resources
BOOKS
del Hoyo, J., A. Elliott, and J. Sargatal. Handbook of the Birds of the World. Vol. 1, Ostriches to Ducks. Barcelona: Lynx Edicions, 1992.
Harrison, Peter. Seabirds: An Identification Guide. Boston: Houghton Mifflin Co., 1983.
Sibley, David Allen. The Sibley Guide to Birds. New York: Knopf, 2000.
Warham, John. The Behavior, Population Biology and Physiology of the Petrels. San Diego, CA: Academic Press, 1996.
KEY TERMS
Down— Short, springy, and spread-out under-feathers on birds, which act as insulation by trapping dead air.
Macroplankton— Larger, visible members of the free-swimming and floating organisms found in the surface waters of oceans such as shrimp, jellyfish, and copepods.
PERIODICALS
Braasch, Gary. “Antarctic Mystery: Why Are Southern Giant Petrels Thriving on One Peninsula but Declining Almost Everywhere Else?” International Wildlife 31 (March-April 2001): 52–57.
Peter Salmansohn