The Gannet is one of our largest seabirds, and they nest in large colonies. They are white, with large black wingtips. They actually start life dark all-over (as in the second photo) and gradually swap dark feathers for white. Gannets are spectacular divers. they fold their wings back and enter the water at high speed, travelling underwater for while, chasing the fish.
This isn't a great photo, as Gannets are normally seen far out at sea. If you look from Castle Point, there are often flocks of around half a dozen flying in a line passing by.
