Scopoli shearwater?

[Rommert]

Zoals jullie weten waren Roef en ik op Madeira en nu schijnt het dat daar in een grote groep Kuhlsen een Scopoli shearwater is gefotografeerd. Deze is dus later gedetermineerd op de foto's van de groep. Wat vinden jullie ervan?
Hieronder het mailtje en de link.

Hello all,

Hope you are all well.

Guess what we found out? We saw (but didn't notice) a Scopoli shearwater on the 1st day of our Zino's petrel pelagic expedition! The only problem is that we only discovered it 2 months later... Have a look at the photo and the story about its identification: http://www.madeirabirds.com

All the best,
Catarina & Hugo

[Rommert]

I am certainly not an expert on cory's (sub)species identification - I've seen both types but never studied them in detail- so I may be completely wrong here. However I am not yet quite convinced that these pictures unequivocally show diomedea (scopoli's). Both birds pictured clearly show some white in the exposed parts of the under primaries, but is this really enough to exclude with confidence borealis? Some pictures of diomedea that I looked at (from NE Spain, http://www.rarebirdspain.net/arbsi027.htm) seem to show really a lot of white, the white/black ratios (in length) on the part of under pp exposed beyond the under pc are about 3:1 or more. On the Madeiran birds I would estimate it to be about 1:1 on Roef's photo and a bit less on Hugo/Catarina's (perhaps even closer to 1:2; note that the curvature of the primaries foreshortens the outer part of the feathers relative to the inner parts). Does this really fall outside the range of borealis? On the aforementioned webpage by Richard Gutierrez I read that there is variation in amount of white in both forms, and this seems not to have been fully and quantitatively described yet - at least RG comments on studies being just started on this in the Canaries. Has any such work, measuring or photographing the extent of white in birds caught in the breeding colonies, been done on Madeira?
Though presented as less useful characters, id texts also state that scopoli's has a slenderer bill (and this does seem to be visible on some photographs I've looked at), slightly slenderer head and neck shape, and slightly paler plumage on those parts. I can't say I find the two candidate scopoli's birds clearly different from the surrounding birds in those respects. Hugo/Catarina's bird even strikes me as rather heavy-billed.
Did Ricard Gutierrez give more details on why he thought that the bird was a diomedea?