At least two subspecies:
Calonectris d. diomedea
Calonectris d. borealis -- Cory's Shearwater
The closely related Cape Verde Shearwater (C. edwardsii) is now generally treated as a distinct species. Arguments have also been put forward for splitting the remaining two races.
Two Cory's Shearwaters (Calonectris diomedea borealis) photographed on 10 September 2000 off Desertas in the North Atlantic. Photograph copyright of Fabio Olmos©, 2000.
Identification is relatively straigt forward. Most similar species are Cape Verde Island Shearwater (formally considered a subspecies) and Streaked Shearwater of the tropical and sub-tropical Pacific.
The two subspecies of Cory's Shearwater are probably identifiable at sea (reviewed in Gutierrez, 1998). The main criteria are (1) Size and structure (2) Uppersurface coloration and (3) Underwing pattern.
Figure 1. Cory's Shearwater (presumably borealis) photographed off North Carolina in August 1996. Copyright Angus Wilson©, 1996.
Where and When
The predominant large shearwater of the Mediterranean and eastern North Altantic.
C. b. borealis is the more numerous subspecies, estimated at 96,500 -136,500 pairs. Main breeding localities are in The Azores, The Berlengas (off Portugal), Porto Santo, The Canary Islands, Madeira and The Salvage Islands (Desertas). Immature C. b. borealis winter off Argentina and Brazil before migrating north to coasts of Central and North America in the northern spring.Adults show a more direct migration pattern between these wintering grounds and the breeding areas. This is the most common taxa observed off the British Isles and eastern North America
C. b. diomedea breeds within the Mediterranean on the Balearic Islands, and smaller islets off Mediterranean France, Spain, Italy, Sardinia, Corsica, Malta, Algeria, Morocco, Tunisia, Greece, Yugoslavia and Turkey. The total breeding population is estimated at 63,000 - 70,000 pairs. Some mixing and hybridization occurs. Immature C. b. diomedea winter off southern Africa before returning to the Mediterranean and European Atlantic. Adults follow a more direct migration pattern between these wintering grounds and the breeding areas.
Cory’s Shearwaters are common non-breeding visitors off the west and south coasts of southern Africa (Ryan 1997). Most are present during the austral summer, arriving off Namibia and the Western Cape Province in October and November. Cory's Shearwater (race uncertain) is regular in the Indian Ocean and Red Sea. Vagrants have been recorded in New Zealand.
Figure 2. Many of the Cory's Shearwaters (presumably borealis) seen off the eastern United States in late summer are in heavy molt. This results in irregular patches or strips of white on the uppersurface of the wings. The white comes from the exposed feathers bases. Photographed off North Carolina in August 1996. Copyright Angus Wilson©, 1996.
Bretagnolle, V and Lequette, B. (1990) Structural variation in the call of the Cory's Shearwater. Ethology 85: 313-323.
Gutierrez, R. (1998) Flight indentification of Cory's and Scopoli's Shearwaters. Dutch Birding 20(5): 216-225
Granadeiro, JP. (1993) Variation in measurements of Cory's Shearwater between populations and sexing by discriminant analysis. Ringing and Migration 14: 103-112.
Granadeiro JP, Burns MD, Furness RW (1999) Food provisioning to nestling shearwaters: why parental behaviour should be monitored? Anim. Behav. 57(3): p663-671 (Medline abstract)
Granadeiro JP, Nunes M, Silva MC, Furness RW (1998) Flexible foraging strategy of Cory's shearwater, Calonectris diomedea, during the chick-rearing period. Anim. Behav. 56(5): p1169-1176 (Medline Abstract)
Thibault, J.-C. (1994) Nest-site tenacity and mate fidelity in relation to breeding success in Cory1s Shearwater Calonectris diomedea. Bird Study 41: 25--28.
Rabouam, C., J.-C. Thibault, & V. Bretagnolle. (1998) Natal philopatry and close inbreeding in Cory1s Shearwater (Calonectris diomedea). Auk 115: 483--486.