There are four regularly-occurring shearwaters in the Gulf of Maine: Great (the most numerous), Cory's, Sooty, and Manx. Telling them from one another can be tricky, especially when the boat is rocking and/or you're actively barfing. I took a whale watch boat out of Boothbay Harbor this week and got pretty good looks at all four species, and so wanted to take a moment to compare them for you.
Shearwaters are encountered offshore, usually seen resting on the water, skittering awkwardly out of the way of your tour boat, or flying incredibly gracefully low over the water. I think it was that old philosopher, Wikipedia, who said it best: "These tubenose birds fly with stiff wings and use a "shearing" flight technique (flying very close to the water and seemingly cutting or "shearing" the tips of waves) to move across wave fronts with the minimum of active flight."
Sooty Shearwater
These guys are the easiest so I'm getting them out of the way first. Sooty Shearwaters are all brown. They're the only all brown ones. Their underwings are flashy silver, which may help in certain lighting conditions or when you see one far away, but their all-brownness is the thing. They're about the same size as Greats.
Great Shearwater
The Great (not Greater!) Shearwater is the most common shearwater seen in the Gulf of Maine, oftentimes by a factor of 20 or so to 1. It's the default species, and so getting a good mental image of their size, plumage, and structure is important to use as a baseline to compare other species.
The most notable field mark, I've found, is the white on their head. Both in flight and on the water, Greats have white feathering that wraps most of the way around their neck, giving them a capped appearance. For me, that cap is the first thing I look for when seeing a shearwater: if it's got a cap, it's a Great.
Cory's Shearwater
These shearwaters are bulkier than Greats, without a capped appearance and all-brown on top. Their yellow bill, if you can see it, is a cherry on top. On our recent trips these birds numerous, and for some stretches about equalling the number of Greats.
There are two subspecies of Cory's in the Atlantic: the borealis subspecies (the expected on in the Gulf of Maine), and the diomedea subspecies, known as Scopoli's Shearwater. The easiest way to separate the two is by the amount of white in the underwings, with Scopoli's showing more white in the primary feathers near the tip of the underwing.
Manx Shearwater
Manx Shearwaters are the smallest regular shearwater in the Gulf. They are, oh I dunno, 2/3 the size of a Great? That feels about right. Though they have white on the face, it's much more limited than on a Great, and doesn't really give them a capped appearance in flight (thought it does on the water a little bit). They're a darker brown, close to black. The bottom photo shows a Manx taking off above a Great, making for a helpful comparison.
