No less remarkable is the performance of the Frigate-bird (_Fregata_), a tropical species allied to the Pelicans and Boatswain-bird, and to our own more familiar Cormorants and Gannets. It might well be called a marine Swift, having excessively short legs and small feet, and a wonderful expanse of wing. As with the Swifts, of course most of its time is spent on the wing; the feet are only useful for supporting the body when ashore, they are never used for walking, at any rate, for more than a few steps. The wings afford the only means of locomotion. Our knowledge of these birds when under the stress of sexual excitement we owe to Dr. C. W. Andrews, who had the good fortune to study the species known as the Great Frigate-bird (_Fregata aquila_) during his task of surveying Christmas Island (Indian Ocean). “About the beginning of January,” he remarks, “the adult males begin to acquire a remarkable pouch of scarlet skin beneath the throat; this they can inflate till it is nearly as large as the rest of the body, and a dozen or more of these birds sitting on a tree with outspread, drooping wings and this great scarlet bladder under their heads are a most remarkable sight. When a hen bird approaches the tree the males utter a peculiar cry, a sort of ‘wow-wow-wow-wow,’ and clatter their beaks like castanets, at the same time shaking their wings. When they take to flight the air is allowed to escape from the pouch, but occasionally they might be seen flying with it partly inflated.” Here again there can be no doubt about the purpose, or perhaps one should say the stimulus, of this strange performance. This pouch, I have been enabled to ascertain from dissection, is not formed by inflating the gullet, but, as in the case of the Prairie-hen, by the enlargement of the air-sacs of the neck. These air-sacs, which are present in all birds, are only enlarged to further the ends of sexual display in a few species, and, curiously enough, these are in no way related one to another. The Adjutant storks, it may be remarked in this connection, have used the air-sacs which are fed by the nasal system instead of those fed by the lungs, as in all the species so far described. When deflated this pouch forms a quite inconspicuous conical swelling in front of the neck; under the stimulus of excitement, it awakens as it were into activity, and is suddenly transformed into a great red or red-and-black bag, encircling the neck and projecting far downwards in front of it, only to be deflated an instant later with a speed which leaves one gasping.