Hi Geoff,
Have just returned from two weeks enjoying seabirds in Galapagos and Peru [amazing - happy to pass on details to anyone interested] and thus join this debate late in the day after the key issues have been identified [especially the constructive comments from Paul and Mike, and your own further analysis].
I fully agree that the bird is definitely not a Wedge-tailed Shearwater and further agree that in the context of Hong Kong it is most probably a Bulwer's Petrel.
I have seen a good number of Wedge-tailed Shearwaters in Australia on several visits and refreshed my jizz on a recent pelagic out of Wollongong earlier this year when good numbers of Short-tailed and Wedge-tailed passed close to the boat. Compared to Short-tailed, Wedge-tailed is significantly larger and heavier with broader-based wings and a more powerful but languid flight style compared to the more frantic flight of Short-tailed. Whilst I agree with you that sizing an isolated seabird can be difficult, the jizz is conclusive. The wings are far too narrow and too long in relation to the body length, and the head and bill far too small with too little projection in front of the wings not just for Wedge-tailed Shearwater but for any shearwater.
However, all these features are spot on for a petrel. Most of your photos show clearly the small head and indicate a small bill, and the last two show in addition the wing shape and wing/tail ratio. Your description of the flight style being lazy with long glides on angled wings is again dead right for a petrel in light wind conditions.
The bird appears all dark [other than the paler upperwing bar which you refer to but which I cannot see clearly] which fits Bulwers [although dark phase Wedge-tailed Shearwaters can also show paler upperwing bars but are eliminated on jizz].
The last issue is whether there are any confusion species. The slightly larger but similar West Indian Ocean Jouanin's Petrel is a possibility but highly unlikely compared to Bulwers which is known to breed in South and East China Seas and expected to return from the south to its breeding areas in late April and May. Other possible dark petrels are generally larger and heavier with heavier heads and bills and broader wings.
So there it is, a good candidate for Bulwer's Petrel. Well done again for spotting the difference, getting the photos and triggering the debate,
Mike
PS I declare my interest in Graham's Champagne competition, but assure you this has not biased my views!!
