Gaviotin sudamericano (Arg, Ch), Gaviotin común (Uru), Trinta-réis-de-bico-vermelho (Br).
Bill and legs red in br plumage, black in n-br plumage. Top half of the head black, back and wing-coverts grey. Rump and deeply-forked tail white. Tail extends beyond the folded wings. Throat and breast pale greyish, rest of the underparts white. Flight feathers grey, outer primaries with outer vane slate-grey. In n-br plumage the forehead and breast are white.
A loud, nasal and high-pitched 'care, careware'; when angry screams 'CAAARE'.
Sea coast, on islands or beaches.