The Terns differ from the Gulls in having both mandibles of equal length instead of having the upper longer than the lower. The bill is straight, generally slender with pointed tips; the nostrils are linear ; the tail as a rule is long and deeply forked in many species, the outermost tail-feathers attenuated and greatly lengthened ; the wings are long, the first primary longest; the legs and feet are small.
The family is cosmopolitan and has been divided into a great many genera, some of which are based on very insignificant characters. For the purpose of this work I recognize six genera, a number sufficient tor all scientific purposes, but I include in the genus Sterna two forms which are sometimes separated on grounds which are of value even if they do not render the division imperative. Sterna seena is placed in a genus, Seena, by itself on account of its massive bill, whilst the Sooty Terns are placed in another genus, Onychoprion, on account of their rather different plumage and the modified webs to the toes.
Key to Genera.
A. Outermost tail-feathers longest.
a Tail short, equal to 1/3 length of wing, fork slight; webs between toes deeply emarginate…………CHLIDONIAS, p. 111.
b. Tail about equal to 1/3 length of wing. deeply forked; webs between toes strongly developed…………HYDROPROGNE, p. 115.
c. Tail long, generally more than half wing; webs between toes well developed.
a1. Bill stout, culmen slightly curved…………GELOCHELIDON, p. 116.
b1 Bill moderate, culmen decidedly curved throughout………THALASSEUS, P. 118.
c1. Bill slender, culmen straight or nearly so…………STERNA, p. 124.
B. Outermost tail-feathers short; third or fourth from outside longest……ANOUS, p. 145.