Cape Verde Shearwater

Cf. Cory’s and Scopoli’s Shearwaters.  Cory’s, Scopoli’s, and Cape Verde Shearwaters overlap widely, as Cory’s and Scopoli’s circulate nearly throughout the temperate, subtropical, and tropical Atlantic, and the three are so similar as to be indistinguishable unless viewed closely under good conditions.  They were traditionally considered to be a single species (known collectively as Cory’s Shearwater, C. diomedea), but Cape Verde is universally recognized as separate and there is compelling evidence that Cory’s and Scopoli’s, though very similar, are separate.  Genetic analysis has shown that Scopoli’s and Cape Verde are more closely related to each other than either is to Cory’s.

Scopoli’s is intermediate in size between Cory’s and Cape Verde, and differs visually in having more white on its underwings.  Typical Cory’s and Cape Verde have white underwing coverts and contrasting dark flight feathers.  On Scopoli’s the undersides of the outer primaries have white on the inner webs, forming a wedge that extends out from the coverts toward the tip.

Size:  In length measurements, Cape Verde averages about 10% smaller than Cory’s and 5% smaller than Scopoli’s.  In side-by-side comparisons, the difference between Cory’s and Cape Verde is more pronounced than the length difference alone would indicate.  Cory’s is much bulkier overall.  Within each species, however, males are larger than females, so differences in their average sizes can be swamped by internal variation.  To add confusion, Scopoli’s’ size varies across its range (eastern breeders are smaller than western breeders) and thus overlaps broadly with both Cory’s and Cape Verde.

Bill:  Cape Verde can often be identified by its bill, which is thinner, grayer, and darker overall than the others.  All show a blackish patch or ring near the tip, but on Cape Verde this tends to be larger and less distinct than on the others.  Unlike Cory’s and Scopoli’s, Cape Verde’s bill pattern varies widely.  At one end of the spectrum, some individuals have uniformly gray bills, while at the other end, some individuals have mostly pale bills with distinctly contrasting tips, as on Cory’s and Scopoli’s.