Not really Antarctic birds, but ones that we see in the northern parts of the Drake Passage, close to South America or on our way to the Falklands and South Georgia are two species of shearwater: the Great and the Sooty Shearwater. These birds have enormous migrations and can even be seen in the North Atlantic (in fact, I once saw a Great Shearwater between Greenland and Iceland on the last trip of the Arctic season and only a few weeks later, I saw another one in the Drake on the first trip of the Antarctic season).
Sooty Shearwaters breed on the Falkland Islands (and other places in the Southern Hemisphere) are brown and have a characteristic white ‘flash’ on the underwing. Great Shearwaters breed on Tristan da Cunha and the Falkland Islands and is a but larger as the Sooty, have a pale underside and a dark cap. They often fly with stiff wings, ‘bouncing’ over the waves, making use of the wind close to the surface. Especially Sooty Shearwaters can be seen in quite large groups.