It is now generally recognized that the Gulls and Terns with their allies, constituting the Order Gaviae, are nearly related to the Limicolae. The resemblance between the two groups is shown in almost every detail of their anatomy, and it is even a question whether they should not, as has been proposed by some writers, be united into one order. Some points of similarity are well known; for instance, the fact that the eggs of Gulls and Terns so closely resemble those of Plovers that a not inconsiderable proportion of the eggs sold in Europe as " Plovers' eggs " have been laid by Terns. Even as regards the webbed feet, to which the Gaviae owe their inclusion in the Cuvierian Natatores, it may be observed that some Limicoline types, like the Avocet, have webs developed between the toes to very nearly the same extent as Hydrochelidon amongst the Terns.
In the present order the bill is generally of moderate length, the feet webbed, the hind toe small (occasionally wanting), raised above the plane of the anterior toes and not united with them by web. The wings are long, and there are 11 primaries, but the terminal one is very short and inconspicuous; fifth secondary wanting. Tail-feathers 12. Oil-gland tufted. Spinal feather-tract well defined on the neck by lateral bare tracts, and forked on the upper back ; the dorsal apterium well developed. An aftershaft present.
The skull is schizognathous and schizorhinal; vomer well developed ; no basipterygoid processes; nostrils pervious. Cervical vertebras 15. Furcula U-shaped. Two carotids; caeca present, but small and functionless in Gulls. The ambiens is found in all except Rhynchops the femoro-caudal, semitendinosus, and accessory semitendinosus are always present; the accessory femoro-caudal is present in Sterna and Rhynchops, wanting in Larus and Stercorarius.
Eggs double-spotted. Nest none or a scanty structure of grass. The young are covered with down when hatched, and able to run, but they are fed by the parents for some days.
Scarcely any two writers agree as to the classification of the members composing the present order. Apart from the question as to whether the Auks and their allies (Alcidae) should be placed here or should form a separate group, a question that does not affect the present work, for no species of the Auk family is Indian, it is doubtful whether the Skimmers (Rhynchops) and the Skuas (Stercorarius) should be regarded as subfamilies of Laridae or distinct families, and the separation of the Terns as a subfamily from the Gulls has more weight of authority than evidence of structural distinction in its favour*.
The two families of Gaviae are thus distinguished:—
Bill without a cere ; claws moderately curved, not sharp ; caeca rudimentary…………………Laridae, p. 297.
Bill with a cere; claws strong, much curved and sharp ; caeca long…………………Stercorariidae, [p. 328.