Cory's shearwater (Calonectris diomedea)

Also known as: Atlantic shearwater, Mediterranean Cory’s shearwater, Mediterranean shearwater, North Atlantic shearwater, Scopoli’s shearwater
GenusCalonectris (1)
SizeLength: 45 - 48 cm (2)
Wingspan: 100 - 125 cm (2)
Weight560 - 1060 g (2) (3)

Classified as Least Concern (LC) on the IUCN Red List (1).

Cory’s shearwater is a large, brown and white seabird with a strong, distinctive flight, gliding above the ocean with only occasional deep beats of its long, often bowed wings (3) (4) (5) (6) (7). The upperparts of the body are grey-brown in colour, with paler edges to the feathers of the back, mantle and upper tail, while the underparts are white. The underside of the tail is dark, and the undersides of the wings are white, with dark front and rear edges and a dark tip (3) (4) (6) (7). There may sometimes be a pale crescent at the base of the tail (2) (6) (7). The head is also greyish-brown in colour, and the long, stout, strongly hooked beak is yellow with a dark tip (3) (4) (6) (7). As in other Procellariiformes (shearwaters, albatrosses and petrels), Cory’s shearwater has prominent, tubular nostrils and a keen sense of smell (4). The male Cory’s shearwater is larger than the female (3), and juveniles resemble the adult (2). The calls of this species include a long wail and a gull-like ia-gowa-gow (6).

Cory’s shearwater can be distinguished from other, similar species (such as the great shearwater, Puffinus gravis, and the streaked shearwater, Calonectris leucomelas) mainly by its large, robust build, its more uniformly pale underparts and the more uniformly coloured head (2) (3) (6) (7). Three subspecies were previously recognised, Calonectris diomedea diomedea, Calonectris diomedea borealis and Calonectris diomedea edwardsii, which differ mainly in size (2) (3). However, C. d. edwardsii is now classified as a distinct species, the Cape Verde shearwater (Calonectris edwardsii) (8) (9). The subspecies C. d. diomedea is sometimes given the name ‘Scopoli’s shearwater’, but it has not been given separate species status (10).

Cory’s shearwater breeds mainly on islands and cliffs in the Mediterranean (C. d. diomedea), although large colonies are also found in the eastern Atlantic, on the Canary Islands, Salvage Islands, Berlengas Islands and the Azores (C. d. borealis) (2) (4) (5) (7) (8). This species has also been recorded further north in Europe (6) (8). It is a migratory species, with birds from the Mediterranean wintering mainly off South Africa and probably reaching into the south-west Indian Ocean, while the Atlantic population winters off the east coasts of North and South America (2) (4) (5) (8).

A bird of the open ocean, Cory’s shearwater is rarely seen near land, returning to barren offshore islands only to breed (2) (4). Nesting occurs on cliffs, caves, rocky slopes or boulder fields, usually in areas of sparse vegetation (2) (4) (6) (8), and nesting burrows may also be dug in soft ground (11). This species forages over relatively warm waters (4) (5), often where a meeting of different water types brings food to the surface (4).

Cory’s shearwater feeds mostly on fish, squid and crustaceans, and will also take scraps and offal around fishing boats (2) (4) (5) (8) (12). Prey is either seized from the water’s surface or from just below (2) (4) (5), and hunting often occurs at night (2) (5). This species can swim underwater using the wings for propulsion, but rarely dives great than ten metres deep (11). It will also forage close to groups of feeding dolphins, tuna or seals, taking advantage of prey driven near the surface by these underwater predators (4) (5) (12).

Cory’s shearwater breeds in colonies (2) (5), often with other seabirds (4). The nest is build at the end of a burrow or rock crevice, which may measure up to one metre in length (2) (3) (4) (5), and there can be strong competition for suitable nesting sites (3). This species is believed to be monogamous, and breeding pairs will often occupy the same nest site over successive years, reinforcing the pair bond by sitting close together and preening each other (4) (5). The breeding season usually runs from March to November (5) (11), and egg-laying is highly synchronised, with most birds in a colony laying at around the same time (3) (4). Cory’s shearwater lays a single, white egg, either on bare ground or on top of small stones and shells. The male and female take turns at incubation, in ‘shifts’ of 6 to 8 days, and the egg hatches after 52 to 62 days (2) (4) (5). The chick is covered in grey-brown down (2), and both the chick and the adults are able to defend the nest by regurgitating a noxious stomach oil onto intruders (4). The adult shearwaters care for the young for up to 100 days (4). Prior to fledging, the provisioning rates may decline, and near to fledging the chick may reject feeds and be abandoned by the adults (13). Cory’s shearwater does not usually begin to breed until 7 to 13 years old (2).

Cory’s shearwater is a widespread and abundant seabird and is not currently considered at risk of extinction (8). However, the species may face a number of threats. Cory’s shearwater breeds mainly in heavily populated areas, putting it at risk from habitat alteration, exploitation for food and for fish bait (2), disturbance at breeding colonies, and pollution of nearby waters (5). It is also under threat from introduced mammalian predators, such as the black rat (Rattus rattus), which is the most destructive predator of seabirds in the Mediterranean, often feeding on chicks (14) (15). In addition, Cory’s shearwater is often caught accidentally on longline fishing gear (16) (17). The loss of its partner can cause the remaining member of a breeding pair to desert the nest, so affecting breeding success, while this species’ long lifespan, late maturity and low reproductive rate mean that even a small increase in adult mortality can cause quite significant population declines (16).

Human exploitation of Cory’s shearwater has for the most part now ceased, and some of its breeding islands have been declared reserves, from which introduced predators such as rats may eventually be removed (2). However, regular monitoring of Cory’s shearwater colonies has been recommended, to identify any declines (2) (17), while shearwater mortality in longline fishing gear also needs to be monitored (16). Fortunately, simple measures such as setting longlines only at certain times of the day can go a long way to reduce the accidental catch of this and other seabirds (16).

To find out more about the conservation of seabirds, see:

Authenticated (15/09/10) by Justin Hart, Biologist, Cetacean Watching Lda.

  1. IUCN Red List (August, 2010)
  2. del Hoyo, J., Elliott, A. and Sargatal, J. (1992) Handbook of the Birds of the World. Volume 1: Ostrich to Ducks. Lynx Edicions, Barcelona.
  3. Brooke, M. (2004) Albatrosses and Petrels across the World. Oxford University Press, Oxford.
  4. National Audubon Society - Cory’s Shearwater (August, 2010)
  5. Kaufman, K. (2001) Lives of North American Birds. Houghton Mifflin Harcourt, Boston, Massachusetts.
  6. Peterson, R.T., Mountfort, G. and Hollom, P.A.D. (1993) Collins Field Guide: Birds of Britain and Europe. HarperCollins Publishers, London.
  7. Tove, M.H. (2000) Guide to the Offshore Wildlife of the Northern Atlantic. University of Texas Press, Austin, Texas.
  8. BirdLife International (August, 2010)
  9. Integrated Taxonomic Information System (ITIS) (August, 2010)
  10. Heidrich, P., Amengual, J.F. and Wink, M. (1998) Phylogenetic relationships in Mediterranean and North Atlantic shearwaters (Aves: Procellariidae) based on nucleotide sequences of mtDNA. Biochemical Systematics and Ecology, 26(2): 145-170.
  11. Thibault, J.C., Bretagnolle, V. and Rabouam, C. (1997) Calonectris diomedea Cory's shearwater. BWP Update, 1: 75-98.
  12. Granadeiro, J.P., Monteiro, L.R. and Furness, R.W. (1998) Diet and feeding ecology of Cory’s shearwater Calonectris diomedea in the Azores, north-east Atlantic. Marine Ecology Progress Series, 166: 267-276.
  13. Warham, J. (1990) The Petrels: Their Ecology and Breeding Systems. Academic Press, London.
  14. Igual, J.M., Forero, M.G., Gomez, T., Orueta, J.F. and Oro, D. (2006) Rat control and breeding performance in Cory’s shearwater (Calonectris diomedea): effects of poisoning efforts and habitat features. Animal Conservation, 9(1): 59-65.
  15. Thibault, J.C. (1995) Effect of predation by the black rat Rattus rattus on the breeding success of Cory’s shearwater Calonectris diomedea in Corsica. Marine Ornithology, 23(1): 1-10.
  16. Belda, E.J. and Sanchez, A. (2001) Seabird mortality on longline fisheries in the western Mediterranean: factors affecting bycatch and proposed mitigating measures. Biological Conservation, 98(3): 357-363.
  17. Granadeiro, J.P., Dias, M.P., Rebelo, R., Santos, C.D. and Catry, P. (2006) Numbers and population trends of Cory’s shearwater Calonectris diomedea at Selvagem Grande, Northeast Atlantic. Waterbirds, 29(1): 56-60.