APODIFORM

Chestnut-collared swift (Cypseloides rutilus) (order Apodiformes), any member of one of two groups of birds, the swifts and the hummingbirds, that are very different from one another in general appearance and way of life. The two groups, considered suborders, are Apodi, which contains two families, Hemiprocnidae for the tree swifts (also called crested swifts) and Apodidae for the true swifts (see photograph); and Trochili, which contains the single large family Trochilidae for the hummingbirds. The most obvious common characteristic of the two groups is a superb mastery of the air; both swifts and hummingbirds are agile fliers and share, in this connection, some peculiarities of wing structure. Some ornithologists believe that these anatomical resemblances are convergent (the result of the same type of natural selection acting on unrelated organisms) and that the true relationships of hummingbirds and swifts lie with other groups, not with each other. Chestnut-collared swift (Cypseloides rutilus) order of birds (Apodiformes) that comprises the swifts (see photograph) and hummingbirds. The common characteristic that has swifts and hummingbirds classified together is their mastery of aerial locomotion; they are superb fliers and share some wing modifications that differentiate them from most other birds. The hummingbirds are classed in one suborder, the Trochili, and the approximately 320 species are members of the family Trochilidae. The suborder Apodi, which contains the swifts, is divided into two families, the Apodidae for the true swifts and the Hemiprocnidae for the tree, or crested, swifts. Hummingbirds are native only to the Western Hemisphere and most species are found in the tropics of northwestern South America. Several species, including the North American ruby-throated hummingbird (Archilochus colubris), have adapted to temperate zones, and one, the rufous hummingbird (Selasphorus rufus), even ranges into Alaska. The tree swifts are limited in range, inhabiting an area from India through southeast Asia and into the East Indies. The true swifts are the most widespread members of the order, being found throughout the Old and New World wherever the land can support a large enough number of flying insects. Hummingbird shape is much the same throughout the family. The bee hummingbird of Cuba (Mellisuga helenae) is the smallest known species of bird; it is only 2.5 inches (6.4 centimetres) in length. The largest of the family is the giant hummingbird (Patagona gigas), which reaches all of 8 in. Variation occurs in the shape and size of the bill and in the ornamental plumage. The bill may be quite short, as in the bearded helmetcrest (Oxypogon guerinii), or longer than the rest of the body, as in the sword-billed hummingbird (Ensifera ensifera). It is often straight but may be downcurved, as in the sicklebills (Eutoxeres), or less frequently, upcurved. The shape of the bill is related to the flowers on which each species of hummingbird preferentially feeds. The legs and feet are small, suitable only for perching, not walking. Bright iridescent plumage, especially on the throat, is characteristic of most hummingbirds. The tails of most species are short and simple, but in a few, such as the marvelous spatuletail (Loddigesia mirabilis), they are elongated and ornate. The swifts show a much greater range in size than the hummingbirds, and members of the Old World genus Apus can weigh 30 times as much as the 0.2-ounce (5.7-gram) pygmy swiftlet (Collocalia troglodytes). The structure which shows the most variation is the tail. In the spine-tailed swifts, including the American chimney swift (Chaetura pelagica), the central shafts of the tail feathers extend beyond the flattened vane as pointed spikes. Other swifts have tails that are either squared off, slightly forked, or deeply forked (swallow-tailed). The feet of the true swifts are too weak to use for perching, and they rest by clinging to vertical surfaces with their sharp claws. In the tree swifts the feet are stronger and larger and these birds perch regularly. Adaptations to their exclusive diet of flying insects are the short weak bill and the large mouth. The swifts are limited in habitat only by the availability of flying insects. Swifts drink by dipping their bills as they fly low over a body of water. Hummingbirds, being highly specialized to subsist basically on nectar, are restricted to areas where flowering plants are available. This explains the predominant number of species in the seasonless tropics, where different plants are in flower all year. When feeding, a hummingbird hovers over a flower and extends its long tubular tongue to suck up the nectar. In addition to hovering, these birds can fly backwards and occasionally upside down. Many will also eat some insects and spiders, which they capture with their bills. Hummingbirds and swifts are both able to enter torpor, in which the metabolism slows down and the body temperature drops. This allows the hummingbirds to roost at night without exhausting their energy reserves, supporting their otherwise very high metabolism. It enables the swifts to survive long periods of fasting when aerial foraging is not possible. Courtship among swifts is completely aerial and copulation takes place while in flight. Nests of tree swifts are frequently simple cups placed on a tree limb. The palm swifts (Cypsiurus parvus) glue their nests to the underside of palm leaves with a sticky secretion produced by the salivary glands, and some species glue the eggs into the nest so the wind cannot bounce them out. The nest of one species, the edible-nest swiftlet (Collocalia fuciphaga), is constructed almost entirely of this salivary cement; these nests are the basis of the Orient's famous bird's-nest soup. Clutches range from one to six white eggs, with species from the northern regions laying the higher numbers. The Indian house swift, Apus affinus, is one of the few birds known to have two distinct breeding seasons per year; it breeds in January and again from late May to June. Incubation in the swifts is shared by both sexes. The naked young are left unattended for longer periods than are most birds as the parents forage for many small insects; however, the young swifts are basically cold-blooded (poikilothermic) during their first few weeks. Hummingbirds, unlike swifts, are usually polygamous. Territoriality is absent in some species while others may vigorously defend nesting or feeding sites. These birds are typically aggressive and take advantage of their great manoeuvrability in the air to drive off hawks and other large predators. Most courtship displays are aerial, although in those of the drably coloured hermits (Phaethornis) singing is important. In some species, leksareas of communal male displayoccur. Hummingbird nests are typically situated on a branch and composed of vegetable fibre, lichens, bark fragments, and spider webs. The female incubates her clutch of two white eggs and cares for the young alone. To feed the hatchlings she thrusts her bill deeply into a young bird's throat and regurgitates. Additional reading Two notable works on swifts are Richard B. Fischer, The Breeding Biology of the Chimney Swift, Chaetura pelagica (Linnaeus) (1958), a thorough and excellently illustrated single-species study of a New World swift; and David Lack, Swifts in a Tower (1956, reprinted 1973), a popular summary based on numerous scattered scientific papers on Apus apus. Crawford H. Greenewalt, Hummingbirds (1960, reprinted 1990), is a beautifully illustrated popular book, with emphasis on the physics of plumage iridescence and of hummingbird flight. Eugne Simon, Histoire naturelle des Trochilid (Synopsis et catalogue) (1921), is the basis of a standard classification of hummingbirds, with detailed descriptions, keys, and nomenclatorial synonymies. Kenneth C. Parkes The Editors of the Encyclopdia Britannica