Calonectris Shearwaters

Genus Calonectris

Presently three species of this genus are recognized.

  • Streaked Shearwater Calonectris leucomelas
  • Cory's Shearwater Calonectris diomedea with two subspecies:
    • Scopoli’s or Mediterranean Shearwater Calonectris d. diomedea.
    • Cory’s or Atlantic Shearwater Calonectris d. borealis
  • Cape Verde Shearwater Calonectris edwardsii

Evolution and taxonomy

All species of this group are northern hemisphere breeders. Formerly two species were recognized, but during the last decades there was a tendency to split the Cory's shearwater into three separate species, which was not supported by all authors. Penhallurick and Wink (2004) returned to the two species concept and proposed to consider Cory's and Scopoli's again as subspecies of diomedea. They did not include the Cape Verde Shearwater in their analysis, but list it as a third subspecies of diomedea. A recent study by Gómez-Diaz et al. (2006) on the phylogeography of Calonectris, based on molecular and morphometric data, doesn't support this vision. They consider edwardsii as a separate species. This study shows that the Calonectris species complex is an early split from the shearwater lineage, about 13.8 to 9 mya (Penhallurick & Wink, 2004, Gómez-Diaz et.al 2006). Leucomelas diverged about 3 mya, which coincides with the emerging of the Panama landbridge, separating the Atlantic and Pacific populations. The Atlantic population started to split up much more recently, about 1 mya. It is estimated that edwardsi separated from the Mediterranean diomedea population about 0.7 mya.
Fossil remains of several Calonectris species are known from South Africa, South Carolina (C. krantzi) and Bermuda (C. wingatei) and date back to 0.5 and 0.4 mya. (Olson 1985a, 1985b, 2008)

Distribution

The Streaked Shearwater breeds mainly around Japan and migrates during the winter to the south-western Pacific, as far as northern Australia.
The Palearctic group contains the Cory's, Scopoli's and Cape Verde shearwater. The first breeds mainly on the Macaronesian Islands (Canaries, Madeira, Azores) and the Berlenga's off Portugal. The borderline between the Atlantic and Mediterranean subspecies lies close to the Strait of Gibraltar and corresponds with the Almerian-Oran Oceanic Front, being the real boundary between the Atlantic and Mediterranean surface waters. True Cory's have been found breeding in Almería on the Mediterranean coast of Spain. Some interbreeding has been reported, and interesting is the fact that in recent years the Mediterranean race established a small colony at the French Atlantic coast west of Bordeaux (DBA, WP reports 2006).
The Cape Verde Shearwater is an endemic of the Cape Verde Islands and doesn't move over great distances.
The winter distribution and migrating of the Cory's follows a route to South and North America. The Scopoli's mainly follows the African coast and concentrates near South Africa.

Behaviour

Due to their close relationship the behaviour of all taxa is very similar. Their dominant flight pattern is typical for larger tubenoses: a few slow and shallow wing beats, followed by a glide close to the surface of the sea in normal stuations. Depending on the wind conditions they can vary between this way of flying to dynamic soaring like a small albatross. The foraging strategy is mainly surface seizing of small fish and squid. Although not being the most aquatic species, diomedea is capable of diving to a maximum of about 15 meter and leucomelas at least 5 to 7m (Oka 1994, Matsumoto 2005). When diving the wings are folded to a certain extend and wings and feet are used for propulsion simultaneously. Under water Calonectris species are not very agile compared with other shearwaters. Like most other procellariiforms Calonectris species are not good walkers.
Calonectris shearwaters are burrow breeders, using natural cavities in volcanic rock, remains of old buildings, but also dig their own burrows when nescessary.

Osteology

The skulls are very similar in structure. All of them having long bills with nostrils of about 1/5 of its length and a somewhat ‘flat’ cranial profile.
The skull of the Streaked Shearwater averages a bit smaller with a more slender bill than in Cory's and Scopoli's. The ramphotecal coating is greyish horn colored instead of the yellow bills of its Atlantic and Mediterranean counterparts.
Skulls of both diomedea subspecies overlap in measurements, but borealis averages larger and differs by being more robust. Sexes do overlap too, but in in borealis and diomedea there is a marked difference in average size, males being larger. Both have yellowish bills with blackish markings at the tip.
The bill of borealis often said to be heavier built than in diomedea. After calculating the bill ratio (culmen length :  height at nostrils)  of over 50 Cory's and about 20 Scopoli's (from museum specimens)  this appears to be true for most  - but not all - males. The females just have bigger bills (own research). When comparing the the cranium of both species the Mediterranean Scopoli's is slightly more slender built than the Cory's.
The skull of the Cape Verde Shearwater is much smaller and has a darker brownish bill, also with blackish markings at the tip. All Calonectris shearwaters have a whitish apex to their bill tips.

Bill length and height (various sources)

Species

Male

 

Female

 

 

length

height

length

height

leucomelas

45.5 – 55.0

16.3

45.0 – 51.0

 

diomedea

49.0 – 58.9

17.1 – 19.4

41.5 – 54.5

 

borealis

51.0 – 59.0

19.8 – 22.9

48.8 – 58.8

17.0 – 20.9

edwardsii

41.0 – 49.0

 

39.0 – 46.0

15.6 – 18.6



Streaked Shearwater Calonectris leucomelas, Kamakura, Honshu, Japan.
Culmen: 51.9 mm; total: 106.9 mm, adult male

Scopoli’s Shearwater Calonectris diomedea diomedea, Mediterranean Sea.
Culmen: 53.4 mm; total: 108.9 mm, adult male.

Cory’s Shearwater Calonectris diomedea borealis, Pico, Azores.
Culmen: 58.1 mm; total:  118.6  mm, adult male.

Cape Verde Shearwater Calonectris edwardsii, Location unknown.
Culmen: 41.7 mm; total: 90.2 mm, unsexed adult, probably female.

Skull measurements (unsexed) (min-max; average)

 

C.d. diomedea (n=11)

C. d. borealis (n=5)

Total length

102.4 - 110.3; 107.3

104.9 - 113.8; 110.9

Postorbital width (pow)
= maximum

34.1 - 36.8; 35.4

36.6 - 39.7; 38.8

Ratio total length : pow

2.9- 3.1; 3.0

2.8 - 2.9; 2.9

References