This bird is a South Polar Skua (Stercorarius maccormicki) and I only know this because much better birders than me (tenuously) identified the bird in the field and made a confirmed ID based on the pictures take on the boat!
This is a rare sighting, and as this species 'shows little interest in vessels' I suppose we were very lucky to get such good views.
I am no expert, but as far as I can tell, one of the key ID features that separate this species from the more frequent Brown Skua is the contrast in colour between the underwings and the body. The South Polar Skua also has a 'finer bill and smaller head'.
Its at times like this I am thankful for experts!
I have to say it was an impressive bird - having the bulk and attitude you would expect from a bird that lives over the Souther Ocean.
As ever, to join in with WBW click on the link below. SM