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Marine Mammals
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 Greater Shearwater

Image Credit: USGS

 Greater Shearwater
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Scientific Name: Puffinus gravis

Category: Seabirds

The greater shearwater is a seabird that is usually 45-50 cm (18 inches) long from bill to tail. It has a wingspan of 105-122cm (3 ½ feet) and weighs just under 1kg (about 2 pounds). As they fly, they glide very close to the ocean surface and their wing tips almost touch the water. This type of flying is called shearing. The birds have a dark back and light underbelly. Most sea birds have this type of coloring. It acts as a protective mechanism against predators both in the air and underwater. Their predators have a more difficult time seeing them as the shearwaters blend into the sky or the ocean. The Greater Shearwaters have a brown patch on their underside and dark patches on their shoulders. Their heads, eyes, and bills are black and they have pink legs. One of the best ways to identify them in the wild is by a white band that separates the brown rump from the tail. These sea birds often sit on the ocean surface in noisy groups of 50-100 and are often seen in the same places as sea gulls. Their cries are extremely loud (eeyah) and can be heard over great distances.

The breeding grounds are found in the Tristan da Cunha volcanic island group located in the southern hemisphere southwest of Cape Town, South Africa in the southern Atlantic Ocean. This group of six islands is the most inaccessible island group in the world. In fact, they have no airport and did not get TV access until 2001! The total human population is under 300, making this a peaceful habitat for the breeding of shearwaters without human interference. Within this island group, the shearwaters breed on Nightingale Island (uninhabited) and Gough Island which has a weather station manned by only six people. A smaller shearwater population breeds on the Falkland Islands off the coast of Argentina in South America. The breeding grounds of the Greater Shearwaters are in the southern hemisphere and after breeding, they migrate to the northern hemisphere. Most other sea birds migrate in the opposite direction, moving north to breed and then returning south to their feeding grounds in the Southern Hemisphere.

The Greater Shearwaters move north along the coasts of South America and North America. They are often seen in the Canadian provinces of Nova Scotia and Newfoundland, where the summers tend to be cool. They feed generously on krill and herring in preparation for the next leg of their migration. They leave the east coast of Canada in September to fly over the Atlantic Ocean to Great Britain and then they fly back south along the northwest coast of Africa. Here they cross the Atlantic Ocean again and fly south along the coast of Brazil to their breeding grounds in the Southern Atlantic. The Greater Shearwaters fly about 32,000 kilometers (20,000 miles) in this large migratory loop around the Atlantic Ocean. When they take off from the water, they face into the wind and run on the water. The wind helps them to become airborne. Sometimes they eat too much and then they can't fly! The Greater Shearwater migration root follows the deep ocean currents that rise to the surface. They follow these currents because their food is concentrated there.

The volcanic islands where the Greater Shearwaters breed have lots of soft soil which the birds can easily dig through to build their nesting burrows. These burrows are usually three feet long. Millions of nesting pairs can be found in the breeding colonies, and interestingly enough, they reuse the same nests year after year. They lay only a single egg and both parents take care of the egg during this time. Eggs take about 55 days to hatch and while the chicks are growing, the parents visit the nest only after dark to avoid predators. The citizens of Tristan da Cunha, the main island in this group, are allowed to visit the breeding islands to harvest eggs and chicks for food.

Greater Shearwaters eat squid and fish. In feeding, they depend on their sense of smell. They have a small opening at the bottom of their bill for smelling and therefore are called tubenose seabirds. As they fly, they smell food and then use their sight to find it in the ocean. They feed at the surface, but also make shallow dives into the ocean for food. Greater Shearwaters often follow in the wakes of fishing boats to feed on the discards and they have violent, noisy fights over the discards. They often feed in large numbers near these ships and the coast.

The biggest environmental threat to the Greater Shearwaters is in swallowing small bits of plastic debris found in the ocean. On Gough Island, the house mice often eat the chicks. The Greater Shearwater population seems to be doing well, but as the ocean populations of fish decreases due to overfishing, the Greater Shearwaters may have trouble in the future finding the food they need for these long migrations.

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