Where most of the storm-petrels we see on an Antarctic expedition are Wilson’s, there are some others we can see. From those, the Black-bellied Storm-petrel is the most common. I have the feeling I see them mostly around the South Shetland Islands. When, somewhere in the second afternoon of the crossing of the Drake Passage, I start to see Black-bellied Storm-petrels, I know we’re almost there (usually to great relief of some of the passengers).
From above, the Black-bellied Storm-petrel looks almost identical to the Wilson’s with the same dark brown plumage, (a bit less obvious) cream-coloured wingbar and white rump. When given a good look, one can see that the legs of the Black-bellied Storm-petrel do not protrude beyond the tail, which they do in Wilson’s. However, the difference in their underside makes ID a lot easier. Strangely enough, I always tell people to look for a white belly when looking for Black-bellied Storm-petrels. Where (the more common) Wilson’s have a uniformly brown underside, Black-bellieds have a white body and white underwing coverts. The name ‘Black-bellied’ comes from a variable thin dark line across their belly, which separates it from the White-bellied Storm-petrel.
On the slightly northern routes, e.g. towards the Falkland Islands or South Georgia, there is the chance on finding a third species of storm-petrel, the Grey-backed Storm Petrel. This, much rarer species is often connected to rafts of kelp. So it pays off to keep an eye out for this one when you see some kelp floating around. Like their name says, Grey-backed Storm-petrels are best identified by their grey back.
Like I said, there is also a White-bellied Storm-petrel, but this one is rarely seen on Antarctic cruises. Its distribution is a bit further north again, and it’s usually seen in the southern parts of the Atlantic, Pacific or Indian oceans. I’ve seen a few when visiting the Tristan da Cunha group in the South Atlantic. To complicate things more, there are also Black-bellied Storm-petrels lacking the black line over their belly, but that’s a whole different discussion.