a1. Tail more or less forked.b1. Tail but little more than one-third of wing, its outer feathers broad and rounded. Hydrochelidon (p. 86)b2. Tail much more than one-third of wing, its outer feathers narrow and pointed. Sterna (p. 88) a2. Tail graduated; plumage sooty brown. Anous (p. 94)
a1. Tail more or less forked.b1. Tail but little more than one-third of wing, its outer feathers broad and rounded. Hydrochelidon (p. 86)b2. Tail much more than one-third of wing, its outer feathers narrow and pointed. Sterna (p. 88)
a2. Tail graduated; plumage sooty brown. Anous (p. 94)
