Often a feature on late fall pelagic trips — this is a South Polar Skua (Stercorarius maccormicki). The common name is meaningful. This species nests on Antarctic coasts and offshore islands and then makes a clockwise migration around the Pacific Ocean via Japan, across to British Columbia and Washington, then down to California and back to Antarctica!
Here's a view of a different individual from the side. Most of these photos were taken on 26 October 2012.
Skuas can appear gull-like at first, but note their robust, barrel-chested appearance, the thick neck, prominent hook at the tip of the bill, overall dark coloration, and bold white wing flashes.
If seen well, you may also catch a glimpse of the strongly hooked claws on their webbed feet. The next photo isn't great, but the claws are quite visible.
Older birds often have light-colored feathers on their napes or hindnecks (they may appear golden).
I've wondered about the purpose of these white wing patches. I read one account that suggested skuas have a spread-wing display during the breeding season. Perhaps a strong white flash attracts attention or sends some sort of message?
Skuas are known for being "pirates" and "marauders" (see Rich Stallcup's wonderful description of South Polar Skuas in his book, Ocean Birds of the Nearshore Pacific). They are kleptoparasites, harrassing other birds (e.g., shearwaters or gulls) until they drop or regurgitate their food.
On fall pelagic trips when gulls are following the boat, it's not uncommon to see a skua heading straight for the flock from very far away. When within range, they start diving after the gulls (see next photo of a skua chasing an immature gull).
Skuas have an intense aura about them. Just seeing one gives you the feeling that something serious is going to happen. They're on a mission; I'm just glad I'm not the one they're focused on!