There was some discussion about prions on last weekend's pelagic trip off Wollongong, followed by many photos taken of Prions (but I managed only a handful). I thought I'd start the ball rolling and hopefully others will post some of their photos.
After looking closely at the birds at sea and now at a computer I'm confident that a considerable proportion of prions seen were Antarctic Prions (a new bird for me, so I was a novice).
Attached are two separate individuals that show the diagnostic characters well. Relative to Fairy Prion these are:
Bill shape (elongated and slightly 'bowed')
Less extent of black pigmentation on tail
Greater extent of grey on sides of breast
Something that I also noticed at sea was the difference in the contrast of the 'M' on the upperwing. In Antarctic Prions it was less distinct, as seen in the photos. While in Fairy the 'M' along with the long dark tail tip was conspicuous in flight. For me it helped pick them out from the crowd at sea and then focus in on looking at the other features more closely as they wizzed past. This difference is almost certainly due to timing of moult (and hence plumage wear) - see the bottom image showing the bird in primary moult. I wonder if this difference in timing may be a helpful ID clue during this time of year?