It was a little unfortunate that fog became a factor once we reached the Nantucket Shoals area. However, we did have reasonable views of a handful of Cory's Shearwaters just before the fog effectively closed our day. Two of these birds, I felt, showed an underwing pattern approaching that of Scopoli's Shearwater (C.d. diomedia), whilst others/most showed characteristics closer to Cory's Shearwater (C.d. borealis).
Cory's/Scopoli's Shearwater - This bird doesn't show a clean cut demarcation between the white coverts and the dark underside of the primaries. However, the primaries appear mostly dark and I'd be inclined to think this was closer to borealis than diomedia.
possible Scopoli's Shearwater - most of the underwing of this bird appears rather white, especially on the primaries. This bird has a closer appearance to those photographed within the breeding range of diomedia.
possible Scopoli's Shearwater - dorsal view of the same bird.
Some amazing shots of Scopoli's Shearwaters fron Italy can be found at Daniele Occhiato's web page.