This is an interesting article discussing a quiz bird: a large dark shearwater.
Is it a Flesh-footed Shearwater?
Is there such a thing as a dark morph Pink-footed Shearwater?
Are there Flesh- x Pink-footed Shearwater hybrids?

http://www.aba.org/birding/v44n1p56.pdf

Cheers,

Nikolas