GREAT SHEARWATER – (Ardenna gravis)
The Great Shearwater is a pelagic (open sea) seabird measuring some 50 cm (20 in.) long. It has a dark brown head and upper parts, and white under parts with a washout patch of brown on the belly. There is a also another brown patch at the shoulders. The eyes and bill are black, and the legs and feet are pinkish. The bill upper mandible ends with a hook. The under side of the wings is white with brown edges. Both sexes are similar.
Shearwaters are part of the Procellariidae family, which includes seabirds with a ‘tubenose’ bill. This highly specialized bill is made of plates and the nostrils are inside one of them in the shape of a ‘tube’. This family of birds drink seawater, and they have glands in their bill to extract the salt from the water. Their nostrils also have a self-defensive feature – when threatened the great shearwater can spit out a foul-smelling oil from that organ.
As for the name ‘Shearwater’, it stems from the way the bird flies over water, holding their long, narrow wings straight near the tip of the waves, almost ‘shearing’ them. The Latin name ‘Ardenna’ was given to this species by Italian naturalist Ulisse Aldrovandi in 1603, and refers to a ‘seabird’. The Latin adjective ‘gravis’ means ‘heavy’.
The diet of the great shearwater consists of small fish, squid and crustaceans from schools near the water surface. They can dive and swim under water to pursue their prey. They will also feed near whales and dolphins, when those mammals bring schools of fish near the surface. And they also follow fishing vessels for scraps of fish. They can feed in huge flocks, and are knows to fight noisily over pieces of food.
Great shearwaters breed in large colonies on isolated rocky islands (see next paragraph), digging burrows around one meter (3 ft) long with a chamber at the end, at a higher level. They build their nest in that chamber, lay one egg, and only visit the nest at night to deter predators.
The great shearwater does not breed on PEI, and its occurrence has been recorded as ‘uncommon’ for summer and fall, most likely following storms, as this is a bird of the open seas. This shearwater breeds on small islands in the South Atlantic Ocean, such as Gough and Tristan da Cunha. It migrates north during the southern winter, and ends up in the North Atlantic Ocean. It is one of the rare species with this ‘reverse’ migrating route.
Conservation: although numbering in the millions, adults and eggs of the great shearwater are harvested on their breeding grounds for human consumption at ‘unsustainable levels’. Being a species that builds an underground nest, it is highly vulnerable to feral predators such as cats and rodents. Mice are present on some of their breeding islands and exact a toll. In spite of those threats, the species is currently listed as of ‘least concern’.