Hi Andy (et al),
Per chance do you know what criteria they are using to rule out Pycroft's Petrel please?
This brief article has a few pointers http://www.nzseabirds.com/Recognise_...ks_Petrels.pdf
All the best
Hi Marcus, that's an excellent link. I've bookmarked it, thanks. This paper in Western Birds also covers those two species plus Stejneger's and De Filippi's. It all highlights that the ID shouldn't be taken lightly. Cook's is the expected gray summer pterodroma but we shouldn't make any assumptions.
My field experience is zero, but looking at a few of the photos of the flocks (such as this series here), head pattern, collar, bill shape would seem to tend towards the features of Cook's.
If the amount of white visible on sitting birds is also a reliable feature (Hadoram note in your link), the sitting flock in the photo above also fits better for Cook's too.