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Ardenna gravis
Unlike many migratory bird species, the great shearwater breeds in the southern hemisphere and spends the non-breeding season in the northern hemisphere, wandering across the Atlantic Ocean. Nesting sites are limited to a handful of locations in the South Atlantic, and apart from breeding, the species almost never comes ashore.
Great Shearwater
Great Shearwater flying over the ocean
Great Shearwater getting ready to land in the sea
Great Shearwater in-flight over the blue sea
Scientific name:
Ardenna gravis
Family:
Petrels and shearwatersConservation status:
Green
Length:
43cm to 51cm
Wingspan:
100cm to 118cm
Weight:
715g to 950g
Great shearwaters are large seabirds, with rather nondescript sooty-brown and white plumage. Their underparts are white, with a darker brown belly patch, and their wing feathers are brown, fringed with paler edges, which gives a scaled appearance. The underwings are white, with a dark rear edge, and the tail is darker than the rest of the upper body with a white horseshoe-shaped patch.
Great shearwaters have a sooty brown face and crown. A white neck ring, sometimes incomplete, leads into a white throat and breast. The bill is gray and hooked, and the legs and feet are pinkish.
Females and males share the same coloring, although the sexes can usually be told apart when alongside males as they are slightly smaller.
On fledging, juvenile great shearwaters are fairly similar to adults, although as their plumage is fresh, coloring is more vibrant and less washed out than an adult bird’s worn plumage. In younger birds, the white collar may also be absent.
Great Shearwater in-flight
The second part of the great shearwater’s scientific name, ‘gravis’, means heavy or weighty, reflecting the large size of this seabird species. Females are slightly smaller and lighter in weight than males.
At sea, a nasal squawking or braying call is heard when foraging over the ocean waters. At breeding colonies, a repetitive and vigorous call can be heard by great shearwaters on the ground.
Squid, octopuses and crustaceans are the main elements of a great shearwater’s diet during the breeding season, whereas once they set off on their post-breeding expeditions, fish (mackerel and capelin) and fish offal are also important. Great shearwaters usually catch their prey by plunge-diving at least 2 m (6.6 ft) beneath the surface of the water.
Adult great shearwaters regurgitate partly digested fish and squid into the bills of their young. One particular issue for this species is the ingestion of plastic in adult birds which is then fed to chicks.
Great Shearwater resting out at sea
Great shearwaters are pelagic birds that forage in cool offshore waters, and breed in burrows dug into the earth on sloped terrain, particularly in grassland landscapes with tussocks and in woodlands. Their four island breeding colonies in the South Atlantic are characterized by inhospitable craggy cliffs and undisturbed low-level vegetation.
Great shearwaters only breed at four known sites in the South Atlantic Ocean, three in the Tristan de Cunha archipelago, and the fourth in the Falkland Islands. Once breeding is complete, birds embark on a north-western migration spending several months at sea in the North Atlantic Ocean, around the coasts of Canada, the United States, the Caribbean, and north-west Europe and West Africa.
Great shearwaters breed at three main sites in the South Atlantic: Nightingale Island, the Inaccessible Islands, and Gough Island, in Tristan da Cunha, and Kidney Island in the Falkland Islands (Islas Malvinas). Of these, there are 2 to 3 million pairs that breed on Nightingale, 2 million pairs on the Inaccessible Islands, and between 600,000 and 3 million pairs on Gough Island. Kidney Island’s population is far lower, with only between 50 and 100 pairs recorded.
Great shearwaters have a large and stable population of around 15 million individuals. However, due to their limited breeding sites and their preference for open sea habitats during the non-breeding season, sightings are rare.
Great Shearwater in-flight over the ocean
Sightings of great shearwaters are reported along the Atlantic coast of the United States during their post-breeding migrations, as they move northwards from their breeding grounds off South America. Rarely seen inland, except when blown off course during a hurricane, great shearwaters are usually only seen far out at sea, regularly spotted following fishing trawlers and feeding on discarded fish offal.
Great shearwaters are only ever seen way offshore of the British Isles, with sightings concentrated along the south-western coast of England and north and west coasts of Scotland. The best time of year for a chance sighting is between July and late September as they begin their return migration to their South Atlantic breeding grounds.
Great shearwaters are among the longest-lived bird species, and although the average lifespan is unknown, the oldest individual has been recorded at 55 years of age. Age at first breeding is also unknown, but for similar species such as sooty shearwaters, maturity isn’t reached until around 5 years of age.
Great shearwaters’ habitats are largely predator-free, meaning they can continue with nesting activities in daylight, rather than being confined to hours of darkness. As a large species, they are not pursued by great skuas, one of the region’s chief predatory avians.
Great shearwaters are protected by Canada’s Migratory Birds Convention Act of 1994, the United States Migratory Bird Treaty Act of 1918, and the UK’s Wildlife and Countryside Act, 1981. These legislations make it illegal to kill, injure, capture, or trade a great shearwater.
Great shearwaters have a stable population and are considered a species of least concern worldwide, and rated with Green status on the British Birds of Conservation Concern list.
Threats include being caught as by-catch by the fishing industry in South America and ingestion of plastics at sea. Several thousand adult birds and up to 50,000 juvenile birds are harvested each year from the Nightingale Island colony.
Great Shearwater taking-off from the water
Nest burrows are excavated by male and female great shearwaters and can reach up to 1 m (3.3 ft) in length. If no suitable sites for burrows are present, cracks and crevices between boulders and large rocks or open grassy sites may be used as an alternative.
Great shearwaters arrive back on breeding grounds from September onwards, and breeding begins with mating in October. Laying peaks in mid-November, followed by an incubation period of 53 to 57 days. Incubation is shared between both parents.
Great shearwaters lay one single plain white egg, which measures 78 mm by 49 mm (3 in by 1.9 in).
Monogamous and long-term pair bonds are formed between great shearwaters, with pairs raising one brood each season. It’s unknown whether pairs remain together through the nonbreeding season or if they go their separate ways and reunite on their breeding grounds each year.
Great Shearwater in-flight over natural habitat
Great shearwaters are colonial breeders, but do show territorial behavior around the entrance to their nest burrows, and will challenge intruders that approach too closely. Flocks of great shearwaters forage and migrate together.
Once breeding is complete, great shearwaters leave their colonies and follow the Gulf Stream north, spending April to September wandering throughout the Atlantic Ocean.
These migrations take them as far north as the coasts of Canada and Greenland, before crossing the Atlantic to Norway, Iceland, and the British Isles before they return to their breeding grounds in late September, following the coast of Africa on their return leg.
Great shearwaters travel between the South Atlantic Ocean in the southern hemisphere’s summer and the North Atlantic Ocean’s nutrient-rich waters once summer arrives in the northern hemisphere, benefiting from an abundant supply of fish all year round. Once their breeding grounds start to become too inhospitable to survive in, the movement towards the north and richer foraging grounds begin.
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