Cory's and Scopoli's Shearwaters are now considered to be separate species
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Abbreviated extracts from their paper follow:
"Phylogenetic analyses of mitochondrial DNA sequences indicate that the three subspecies of Cory's Shearwater form reciprocally monophyletic groups and suggest that C. d. diomedea and C. d. edwardsii are more closely related to each other than either is to C. d. borealis. C. d. borealis and C. d. diomedea differ in mean size and in the typical extent of white on the inner webs of the primaries. The three taxa further differ in vocalizations. Playback studies have documented differential responses to recordings of C. d. borealis and C. d. diomedea. There are several reports of C. d. borealis within breeding colonies of C. d. diomedea but most of these involved non-breeding individuals or birds of unknown breeding status. Despite intensive monitoring of Atlantic and Mediterranean breeding colonies, reports of interbreeding by C. d. borealis and C. d. diomedea are limited. [T]he breeding grounds of these two forms are not fully allopatric and ... some mechanisms of reproductive isolation are likely to be involved in maintaining their differences. C. d. borealis and C. d. diomedea breeding in sympatry in the Chafarinas Islands differ in their feeding ecology and foraging areas during both chick-rearing and wintering periods. The Cory's Shearwater complex is best treated as three full species. [These are] Cory's Shearwater Calonectris borealis, Scopoli's Shearwater Calonectris diomedea, Cape Verde Shearwater Calonectris edwardsii."
A Cory's/Scopoli's Shearwater off South Africa. Photograph by John Graham
Sangster, G., Collinson, J.M., Crochet, P.A., Knox, A.G., Parkin, D.T. & Votier S.C. 2012. Taxonomic recommendations for British birds: eighth report. Ibis 154: 874-883.
John Cooper, ACAP Information officer, 3 November 2012