Of fossil forms referred to the Family, Necrobyas harpax and N. rossignoli are described from the Eocene of France, together with Otus (i.e. Asio) and Bubo; the latter genus and Strix occur in the Lower Miocene of the same country, Strix also in the Malta caverns and in the Mare aux Songes in Mauritius, Nyctea at Torquay and in France, Bubo in Wyoming, Badiostes in Patagonia.
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The Sub-Order Caprimulgi consists of the Nightjar or Goat-sucker group, with the Families Caprimulgidae, Podargidae, and Steatornithidae, of which the latter contains only the remarkable Guácharo. The Caprimulgidae may be divided into the Sub-families (1) Caprimulginae and (2) Nyctibiinae. Authorities disagree as to the exact relationship of these birds to their allies, here classified as Coraciiformes; but that all are allies is certain, while both in appearance and habits Nightjars are decidedly Owl-like.
Apart from the Steatornithidae, the skull is flattened, the eyes are large, the beak is short and extremely broad, being hooked and toothed in the Nyctibiinae and occasionally decurved in the Caprimulginae and Podargidae; the gape is enormously wide, and is in many cases provided with stiff bristles, which in Aegotheles have long lateral filaments. An appearance of great size is given to the head by the loose plumage. The feet are fairly strong, with the digits somewhat united basally; the anteriorly scutellated metatarsi vary from comparatively long and bare in Nyctidromus and the Podargidae to very short and feathered in Nyctibius. The outer toe of the Caprimulginae has only four joints, and the mid-toe has a pectinated claw, while in the Podargidae and some Caprimulginae the hallux is partially reversible. The pointed wing has ten primaries, sometimes much elongated (p. [418]), and eleven or twelve secondaries; the tail may be square, rounded, graduated, or forked, and has ten rectrices, occasionally lengthened or even racquet-tipped (loc. cit.). The furcula is U-shaped, the tongue short; the slit-like nostrils are basal and overhung by a membrane and feathers in Podargus and Batrachostomus, whereas they are open and near the tip of the bill in Aegotheles, but soft, tubular, and often elongated in the Caprimulginae. The syrinx is bronchial, sometimes tending to tracheo-bronchial; the aftershaft is rudimentary; the adults have down only on the unfeathered tracts, while the nestlings have a thick covering of it, which is generally buff or grey, but white in Podargus and Batrachostomus.
The length varies from about twenty inches in Nyctibius and Podargus to seven or eight in Caprimulgus parvulus and Phalaenoptilus nuttalli. The characteristically soft plumage shews an intricate mixture of brown, grey, fawn, black, and white, and is ordinarily barred and minutely freckled, but frequently patched or spotted with white; it is, however, impossible in a limited space to describe the species in detail, though it may be noticed that several have reddish nuchal collars; and some exhibit rufous and grey phases–unless, as may be the case, the rufous forms are females–while others from arid districts have a protective coloration of a more or less sandy hue. In the Podargidae large powder-down patches occur laterally on the rump, in the Nyctibiinae on the breast and sides. The sexes are often alike, the young either resembling the female, or assuming the full plumage at once. Lyncornis, Otophanes, and Batrachostomus have head-tufts like those of some Owls, the constituent feathers in the last genus being bristle-pointed.
Nightjars are found in most parts of the world, while the northern species habitually move southwards for the winter, Podager and Chordiles, at least, flocking in August and September. The most typical forms are distinctly crepuscular, and pass the day–as our British bird does–quiescent on the soil, or upon some post or fence, often concealing themselves below shrubs or herbage, or in hollow stumps. At such times they will almost permit themselves to be trodden upon before rising from the ground, and sit with their eyes closed; on branches the body is ordinarily placed lengthwise, but on thin palings or wire this is of course impossible. Nyctidromus exhibits more terrestrial habits, and walks instead of shuffling; the American Bull-bat (Chordiles virginianus) hawks in the full glare of the sun. The more diurnal species frequently rise to a considerable height in the air, sailing backwards and forwards with an easy, flapping motion, descending with undulating swoops, or remaining momentarily poised aloft, and then darting suddenly upon their prey; the flight of their nocturnal allies is weaker and more lowly, being jerky, twisting, and erratic. Some forms, if not all, when inspecting an intruder turn the head almost completely round. The vibrating sound often accompanying the passage through the air may be produced by the wings coming into contact, as is the clapping noise occasionally heard; but the cause is not certainly ascertained, nor is that of the far-resounding churr uttered by the male of the Common Nightjar while stationary. The birds are, however, often quiet on the wing, and steal upon the listener noiselessly with the mouth widely opened. The voice is generally hollow, but is described in various cases as a "croak," a "loud shrill cry," a "sad whistle," a "jarring note," or a "moan"; while the American Whip-poor-Will (Antrostomus vociferus), Chuck-Will's-widow (A. carolinensis), and Poor-Will (Phalaenoptilus), as well as the Tasmanian More-pork (Podargus cuvieri), are so called from the sounds they rapidly utter. The second of these is said to be silent when breeding, contrary to the habit of our Nightjar. The food consists as a rule of insects, and especially beetles, captured in the air; but the Podargidae are asserted to pick Phasmidae and Cicadidae off the trees, and even to eat fruit–as Steatornis does–or mice.[[236]]
Most Nightjars make no nest, but lay one or two white, yellowish, or pinkish eggs, beautifully marbled or scrawled with black, gray, brown, or violet, on the ground in open spots, frequently shaded by trees, ferns, or gorse. More rarely lichen-covered rocks or flat house-tops are chosen. Phalaenoptilus has white eggs, like those of the Podargidae, among which Podargus makes a flat, loose structure of twigs and grass upon some branch to contain its complement of three, and Batrachostomus deposits one on a peculiar pad of brown or greyish down, which is fixed to a bough and is at times based on a little bark, lichen, moss, or leaf-refuse.[[237]] Aegotheles lays from three to five in hollow trees, the parent hissing if caught upon them. Eggs of Ae. wallacii are stated to shew pale streaks. Nyctibius appears to breed in hollows of branches or stumps, and not on the ground.[[238]] Nightjars sit very closely, and are said to remove the contents of the nest if disturbed; the young, though hatched helpless, quickly learn to escape from danger; while the parents occasionally feign lameness to divert attention from them. The males sometimes incubate.
The superstitious of all classes are inclined to view these birds with dread, a fact due to their nocturnal habits and Owl-like aspect, coupled with their strange utterances and sudden apparitions. The Indians of Central and South America think that they portend serious evil, but refuse to kill them; while in England gamekeepers and others are only too ready to shoot them under the unfair designation of "Night-hawk."
Fam. VIII. Caprimulgidae.–Of this group some eighty species occupy nearly the whole globe, except the coldest parts, the Eastern Pacific Islands and New Zealand.
Sub-fam. 1. Caprimulginae.–Caprimulgus europaeus, the Nightjar, Goatsucker, or Fern-Owl, visits Britain for the summer, and extends from Europe and North Africa to South Mongolia in Asia, reaching North-West India and South Africa in winter. C. ruficollis of South-West Europe and the neighbouring portions of Africa has once occurred in England, as has C. aegyptius of North-East Africa and West Asia. The genera Heleothreptus of Brazil and Argentina, and Macrodipteryx of Tropical Africa, contain respectively one and two members, remarkable for the extraordinary elongation of the remiges in the male. H. anomalus has the first six primaries curved inwards, the seventh, eighth, and ninth prolonged–especially the eighth; M. vexillarius, the Pennant-winged Nightjar, has the same three feathers produced, but the ninth in particular; M. macrodipterus has the ninth alone extended, with long bare shaft and racquet-like tip: and this is at times erected when the bird is sitting on the ground. Scotornis climacurus of the north of Tropical Africa, the four species of Hydropsalis, inhabiting South America southwards to Argentina, and the three of Macropsalis, ranging from Panama to Bolivia and South-East Brazil, have enormously elongated rectrices, the median pair being highly developed in the first-named, the whole number in the second, and the lateral pair in the last. These long feathers seem to impede the flight but little, though Hydropsalis constantly opens and shuts its tail in the air.