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Puffinus gravisgreat shearwater

By Maryanne Spady

Geographic Range

Greater shearwaters are a marine species distributed throughout the region of the Atlantic ocean. Puffinus gravis breed in the far south Atlantic, mainly on the islands of the Tristan da Cunha group, the Falkland Islands, and Gough Island. P. gravis then migrates to the North Atlantic during the winter, reaching noth-east Canada and sometimes reaching as far north as Greenland. Migration to the breeding grounds involves a flight east, past Britain and Iberia, than turning south to reach their Southern Hemisphere breeding grounds.

(del Hoyo, Elliot, and Sargatal 1992; Farrand 1985; Gooders 1978)

Habitat

P. gravis is a marine, pelagic bird species, frequenting cool offshore and pelagic waters, and breeding on sloping ground, mainly in grassland or woodland areas (del Hoyo, Elliot, and Sargatal 1992; Gooders 1978)

Physical Description

Puffinus gravis is a large shearwater species, 43-51cm in length, with a wingspan of 100-118cm. Colouring is unique with a combination of pale underparts with a poorly defined dark patch on the belly. White bands of plumage occur across the uppertail-coverts and also across the hindneck, emphasizing the Greater Shearwaters strongly capped appearance. Upperparts are dark gray-brown to black with light feather edgings to give a scaled appearance. Flight feathers have black upper- and undersurfaces. The tubenosed bill is long, thin, and black. The legs are pink to grey with webbed feet, and eyes are brown. Chicks have bluish-grey down and juveniles are similar to adults but greyer with paler fringes to feathers to give less scaled appearance. There is no sexual dimorphism in terms of size or colour between males and females.

(Campbell 1974; del Hoyo, Elliot, and Sargatal 1992; Farrand 1985; Gooders 1978)

  • Range mass
    715 to 950 g
    25.20 to 33.48 oz

Reproduction

The breeding season commences in October and often lasts until December. Breeding takes place in the Southern Hemisphere and is restricted to the far south oceanic islands: Tristan da Cunha group, Falkland Island, and Gough Island. Puffinus gravis is a colonial nester, nesting in burrows or crevices among boulders along hilly island shores. The female produces a single, white, oval, and slightly pointed egg. The egg is incubated for 53-57 days, and an altricial offspring is then hatched. Both male and female care for young. Offspring become independent of their parents at about 105 days. (del Hoyo, Elliot, and Sargatal 1992; Gooders 1978)

Lifespan/Longevity

Behavior

Greater Shearwaters are a social species, flying, hunting, and migrating together in flocks, and can often be seen as flocks following ships for food. They are expert flyers with the majority of their life spent in flight. These birds take to the air by flapping and then soar using air currents. The name 'shearwater' comes from the behaviour of soaring just above the surface of the ocean, closely following the contour of the waves with their wings outstretched, shearing the water surface with the pointed tips of the wings.

Puffinus gravis is also agile underwater, capable of rotating 180 degrees, using wings for maneuvering. These birds, despite their graceful appearance in the air and under water, are clumsy and awkward on land.

Puffinus gravis is a transequatorial migrant, breeding in the Southern Hemisphere and wintering in the Northern Hemisphere.

Individuals exhibit a harsh raucous call when feeding, much like a gull, and they also exhibit a vocal croak upon returning to a nest at night. Calls are sexually dimorphic, the females with shorter calls, and with a lower frequency inspiratory phrase than the frequency of the first harmonic of the longer expiratory phrase. This is reversed in males. Individuals can discriminate between female and male calls which is potentially useful in mate selection. Greater Shearwaters are sufficiently large to be relatively secure from predation by Great Skuas (Catharacta antarctica), a major predator in this area. As a result of this, Greater Shearwaters have no nocturnal habits as they can display during the day.

(Brooke 1988; Brown, Bourne, and Wahl 1978; del Hoyo, Elliot, and Sargatal 1992; Gooders 1978)

Communication and Perception

Food Habits

Greater Shearwaters feed in groups where aggressive intraspecific feeding competition is evident through lashing with bills and wings. They eat mostly fish and squid, with occasional feeding on crustaceans, fish entrails, and other refuse discarded by fishing vessels. They hunt by plunge-diving from heights of 6-10m or taking prey from surface seizing or pursuit diving.

Surface-seizing consists of the 'walking on water' that is usually associated with Storm Petrels. Without entirely folding its wings, a Greater Shearwater lands on the water surface with its feet, balances with its wings, and "walks" forward over the water as it picks up food from near the surface.

Plunge-diving involves striking the water surface from heights of 6-10 m with belly and feet and then instantly lowering the head under the water surface to lead into a smooth submersion. Occasionally, a Greater Shearwater might briefly halt 0.5 m above the water surface, spread its feet, then plunge headfirst underneath the water. After the dive, the bird bursts out of the water and almost directly into flight.

(Brown, Bourne, and Wahle 1978; del Hoyo, Elliot, and Sargatal 1992; Gooders 1978)

Economic Importance for Humans: Positive

In the past, Greater Shearwaters had been used to provide food and bait for seamen, but this practice has long since been discarded and aside from the occasional contact with fishermen, and Tristan islanders, P. gravis has mostly no contact with humans.

(del Hoyo, Elliot, and Sargatal 1992; Cassidy 1990)

Conservation Status

Greater Shearwaters are not globally threatened. They are abundant with enormous total populations of a minimum 5 million breeding pairs on Tristan da Cunha, 600,000 to 3 million pairs on Gough Island, and small numbers on Falkland Islands. Their breeding range is restricted with only 4 sites known. Where the problem lies is with exploitation by Tristan islanders. Each year, a few thousand adults, and 50,000 chicks are taken. This could lead to a population collapse unless a quota system is established to allow for rational exploitation. More research is required in this area, as well as in the areas of the impact of harvesting on Greater Shearwaters, and the importance of other causes of mortality and population dynamics, in order to determine the maximum sustainable levels of exploitation. Another issue to be addressed is that of the occasional Greater Shearwater that is snared by fishermen's baited hooks.

Very little research has been done, but is required to ensure the maintenance of healthy populations of Puffinus gravis worldwide.

(del Hoyo, Elliot, and Sargatal 1992)

Contributors

Maryanne Spady (author), University of Alberta, Cindy Paszkowski (editor), University of Alberta.

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

visual

uses sight to communicate

References

Brooke, M. 1988. Sexual Dimorphism in the Voice of Greater Shearwaters. Condor, 100(2): 319-323.

Brown, R., W. Bourne, T. Wahl. 1978. Diving by Shearwaters. Condor, 80: 123-125.

Campbell, B. 1974. The Dictionary of Birds in Colour. London: Michael Joseph Ltd.

Cassidy, J. 1990. Book of North American Birds. Montreal: Reader's Digest Association, Inc..

Farrand, Jr., J. 1985. The Audubon Society Master Guide to Birding. New York: Alfred A. Knopf, Inc..

Gooders, J. 1978. The Orbis Encyclopedia of Birds of Britain and Europe-Birds of Ocean and Estuary. London: Orbis Publishing.

del Hoyo, J., A. Elliot, J. Sargatal. 1992. Handbook of the Birds of the World Volume 1. Barcelona: Lynx Edicions.

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