First of all, congrats to Michael on getting a cormorant picture. We've got them here on the Oregon coast and they're one of the hardest birds to photograph not only because they're dark but because the tend to stay at distance only coming to land on remote rocks or steep cliffs. Not easy to get close to!
But a key factor here (aside from the obvious one of exposure) is to shoot raw and use a converter that gives you good shadow detail. Here's an example of a dark bird (crow, that unlike the cormorant is very approachable) processed with default settings by various converters.
http://www.lashier.com/home.cfm?dir_cat=20417
Note that this was done with C1's "film standard" tonal curve and "extra shadow" would have brought out even more detail.
- DL