In this sub-order of the Insessores the beak is short but remarkably wide in its gape (figs. 345, C, and 347), and the opening of the bill is fenced in by a number of bristles (vibrissa). This arrangement is in accordance with the habits of the Fissirostres, the typical members of which live upon insects and take their prey upon the wing. The most typical Fissirostral birds, in fact, such as the Swallows and Goatsuckers, fly about with their mouths widely opened; and the insects which they catch in this way are prevented from escaping, partly by the bristles which border the gape, and partly by a viscid saliva which covers the tongue and inside of the mouth. The group of the Fissirostres, with various additions (notably with that of the Humming-birds), is often raised to the rank of a distinct order (the Volitores of Professor Owen).
The typical Fissirostres, characterised by the structure of the beak, comprise three families - the Swallows and Martins (Hirundinidae), the Swifts (Cypselidae), and the Goatsuckers (Caprimulgidae). These three families differ in many important respects from one another, but it would be inconvenient to separate them here. The Swifts, especially, are remarkable for the peculiarity that whilst the hallux is present, it is turned forwards along with the three anterior toes. The Goatsuckers, again, hunt their prey by night, and they are provided with the large eyes and thick soft plumage of all nocturnal birds. Besides the above, there remain the two families of the Kingfishers and Bee-eaters, which are generally placed amongst the Fissirostres, though in very many respects the arrangement appears to be an unnatural one. These families are characterised by their stronger and longer bills, and by having the external toe nearly as long as the middle one, to which it is united nearly as far as the penultimate joint. In consequence of this peculiar conformation of the toes, these families were united by Cuvier into a single group under the name of Syndactyli.
Fig. 347. - Head of Goatsucker (Caprimulgus), showing the fissirostral form of beak.
The Caprimulgidae are intermediate between the Owls and the Passerine Birds. Their plumage is lax and soft, and they have a hawking flight. The eyes and ears are large, the feet short and weak, and the gape of enormous size and bordered by vibrissae. Amongst the more remarkable members of the family may be mentioned the Whip-poor-will (Antrostomus vociferus) of North America, the More-pork (Podargus Cuvieri) of Australia, and the extraordinary Guacharo Bird (Steatornis Caripensis) of the valley of Caripe in the West Indies.
The Hirundinidae have a wide gape, with few or no vibrissae, the wings being very long and the feet short and weak. All the Swallows feed upon insects, which they catch upon the wing, and they have a cosmopolitan range.
The Swifts (Cypselidae), though closely resembling the Swallows in external appearance and mode of life, have little real affinity to them. The gape is extremely wide; the wings are very long and pointed; the feet are short and weak; and the hallux is either permanently turned sideways or forwards, or can be made to assume this position at the will of the bird. The Swifts are very widely distributed in temperate and warm regions. The "edible bird's-nests" of the Chinese are made of the inspissated saliva of a Swift (Collocalia).
The Bee-eaters (Meropidae) live upon insects, chiefly upon various species of bees and wasps; but the Kingfishers live upon small fish, which they capture by dashing into the water. The common Kingfisher (Alcedo ispida) is a somewhat rare native of Britain, and is perhaps the most beautiful of British birds. Some exotic Kingfishers are of large size, and one of the most remarkable of them is the Laughing Jackass (Dacelo gigas) of Australia, so called from its extraordinary song, resembling a prolonged hysterical laugh. A very beautiful species is the belted Kingfisher (Ceryle alcyon) of North America.
The Bee-eaters are found chiefly in the warmer regions of the Old World, and their place is taken in America by the Motmots (Motnotus).
As regards their distribution in time, the Insessores are not known in rocks older than the Tertiary. The Eocene Slates of Glaris have yielded the Passerine Protornis Glarisiensis; and in the Eocene deposits of the Paris basin occur remains of the Passerine genera Laurillardia and Palaeogithalus. The Kingfishers seem to be also represented at the same period by the Halcyornis toliapicus of the London Clay. The Insessorial remains of the later Tertiary and Post-Tertiary deposits present no features of special interest.