Apodiformes (Swifts and Hummingbirds)

views updated

Apodiformes

Family: Swifts
Family: Tree Swifts
Family: Hummingbirds

(Swifts and hummingbirds)

Class Aves

Order Apodiformes

Number of families 3

Number of genera, species 121 genera; 431 species

Evolution and systematics

Eighteenth-century classification kept swifts and hummingbirds well apart. Swifts and swallows were usually placed in the same family, while hummingbirds were often linked with other birds with fine bills, such as hoopoes or sunbirds. By the mid-nineteenth century, a relationship between swifts and hummingbirds was generally, although not universally, accepted. In 1892, Ridgeway wrote: "The Humming Birds and Swifts…agree in numerous anatomical characters, and there can be no doubt that they are more closely related to each other than are either to any other group of birds."

Even with a lack of fossil evidence for hummingbirds, morphological similarities, together with the results of biochemical analysis, unite modern ornithologists in treating swifts, treeswifts, and hummingbirds as monophyletic. Sibley and Ahlquist used DNA analysis to support their conclusion that the families diverged in the late Cretaceous or early Tertiary Period (65–70 million years ago). Schuchmann concurs, identifying the breaking off, from Gondwana, of a tectonic plate that became South America as the physical moment of divergence.

The Apodiformes are divided into three families. The suborder Apodi contains both the true swifts Apodidae and the treeswifts Hemiprocnidae. The latter family—whose name derives from hemi progne, meaning "half swallow"—has significant anatomical features that distinguish it from true swifts, including the presence of a non-reversible hind toe, soft feathering with down on the flanks, and the absence of a claw on the manus. The Apodidae are generally divided into two sub families. The Cypseloidinae are considered the most primitive. They do not use saliva in nestbuilding, possess two carotid arteries, and have a simple palate. Relationships among the 17 genera and species within the more advanced Apodinae are the subject of some disagreement.

Classification of hummingbirds is even more controversial. The crude division proposed by Gould in 1861 is still accepted (as of 2001). This separates the subfamily Phaethornithinae, comprising the hermits, from the Trochilinae, or typical hummingbirds.

Physical characteristics

All Apodiformes are small to very small birds. Morphologically, the most striking shared feature is the wing structure. The sternum is long and, in the case of hummingbirds, deeply keeled. The coracoid bones linking the sternum and the humerus are particularly strong. Apodiformes are unique in having shallow ball and cup sockets connecting the coracoids to the sternum. The humerus, radius, and ulna are all relatively short, but the carpal bones are exceptionally long. The total length of the "hand" bones is nearly twice that of the "arm" bones. The corresponding wing feathers are also a distinctive feature of this order. Apodiformes have 10 long primaries and normally six to seven short secondaries. In swifts, the longest primary is three times the length of the shortest secondary.

Although all Apodiform wing structures are very similar, different methods of flight mean that flight musculature is adapted accordingly. Hummingbirds' reliance on hovering flight with rapid wing beats means that they need extremely powerful flight muscles. The flight muscles make up 30% of a hummingbird's body mass. By comparison, swifts rely far more on gliding, rather than flapping, flight. The flight muscles of the Alpine swift (Tachymarptis melba), for example, make up only 16% of its body mass.

The tail of a typical Apodiform has 10 rectrices. In most swifts and some hummingbirds, it is forked, and in a number of species, the tail feathers may be longer than the body length. The tail feathers play a critical part in steering; swifts are able to make slight directional adjustments at speed, while hummingbirds tilt their tails to lift, lower, and brake. Long-tailed swifts such as needletails also use their stiffened tail feathers to provide support when clinging to vertical surfaces.

Apodiformes have small feet that are useless for walking. The genus name Apus is from the Greek a pous, meaning "without foot," but this is misleading. The small, strong, sharp claws are well adapted to clinging on vertical surfaces, with the hind toe in line so that all four toes face forward to secure a better grip. Hummingbird feet are too small and weak for walking or climbing. The birds' reliance on hovering flight, and the aerodynamic difficulties that would be posed by having larger feet, mean that tiny hummingbird feet are only suitable for perching.

In plumage, swifts and hummingbirds offer a huge contrast. Swift body plumage is usually glossy, with predominantly drab brown or black feathers. Conversely, body feathers of male hummingbirds offer a dazzling display of iridescent colors. Female hummingbird plumage is normally cryptic to aid in concealment when nesting. Males rely on bright plumage both for self-advertisement to potential mates and to warn off intruders from their territory.

All Apodiformes show similar, or shared, physiological adaptations for their extreme lifestyles. They have a relatively short gut and the caecum, which is important in plant-eating bird species, is either absent or vestigial. The three families share a unique type of malate dehydrogenase, an enzyme that catalyzes the formation of hydrogen. Erythrocytes in the blood of swifts tend to be large, enabling the birds to maximize oxygen intake from air at high altitudes. Hummingbirds have the highest known density of erythrocytes in any bird taxon, with 6.59 million per cubic milliliter. They also have the largest relative heart size and fastest heartbeat—1,260 beats per minute in the blue-throated hummingbird (Lampornis clemenciae). All these features allow hummingbirds to process large amounts of oxygen to sustain their energy-demanding flight.

At least two swift species share with hummingbirds and some Caprimulgiformes the ability to enter periods of torpidity to conserve energy. The white-throated needletail's (Hirundapus caudacutus) body temperature has been recorded dropping from 101°F (38.5°C) to 82°F (28°C) overnight. It is possible that future research will show that the white-throated swift (Aeronautes saxatalis) is not the only other swift to achieve a state of torpidity. This energy-saving mechanism exists in possibly all Neotropical hummingbirds. Nocturnal torpor may lower the body metabolism by 80–90%, with body temperature held at 64–68°F (18–20°C) and heartbeat reduced to about 50 beats per minute.

Distribution

Apodiformes are highly specialized feeders and this governs their distribution. Although hummingbirds are found from southernmost South America to Alaska, their dependence on nectar for food means that they are absent from tundra areas of Alaska and northern Canada. Swifts have successfully colonized most land areas of the planet, but their reliance on aerial plankton excludes them from the coldest parts of the Arctic and Antarctic, as well as the most arid desert areas. Food that is only seasonally available forces species breeding in north temperate areas to migrate south for the winter.

Habitat

Most swifts are highly mobile aerial feeders, hunting for insects over a wide range of terrestrial habitats, from marshy meadows to city office blocks. During the breeding season, swifts in temperate areas may travel great distances to escape severe weather—common swifts (Apus apus) have been recorded making round trips of up to 1,242 mi (2,000 km). Only a small number of Apodi are confined to particular habitats, including wholly tropical forest-dwelling species such as the whiskered treeswift (Hemiprocne comata). Most species of swiftlet appear to be sedentary, using the same caves or places nearby for roosting and nesting.

Hummingbirds have adapted successfully to everywhere in the New World where there are sources of nectar. Even in the high Andes mountains, helmetcrests and hillstars are able to exploit nectar, while adapting physiologically to extreme weather conditions.

Behavior

In behavior, swifts and treeswifts are often diametrically opposed to hummingbirds. Social organization in hummingbirds is non-existent, except for the brief mating period. Otherwise, they tend to be solitary, with males of many species aggressively defending feeding territories, often using a range of chase calls to warn off intruders. Swifts, by contrast, are usually highly gregarious, feeding, roosting, and nesting in colonies, which can be large. One Oregon autumn roost of Vaux's swift (Chaetura vauxi) had around 25,000 birds. These behavioral differences are adaptive: individual hummingbirds guard sources of nectar; swifts feeding on aerial insects and roosting or nesting on cave or cliff ledges are generally not competitive, and may benefit by sharing sometimes-limited nest caves and perhaps in locating concentrations of flying insects.

Feeding ecology and diet

Among Apodiformes, hummingbirds are the most specialized feeders. They drink nectar by probing plant corollas using an extended biforked tongue, usually in a long bill. The

specialization is such that thousands of plants are exclusively ornithophilous—dependent on hummingbirds for pollination. The birds also glean insects and spiders from leaves and flowers, an essential protein supplement to their diet. Swifts eat only insects and arachnids, almost always taken in the air. The size of prey taken is related to the size of the bird, with larger swifts taking fewer, bigger prey items. The gape is wide, with guard feathers in front of large, deep-set eyes. These feathers are moved by muscles and probably serve to cut glare from the sun.

Swifts and hummingbirds share a cervical muscular adaptation that enables them to make rapid head movements. The relatively long splenius capitus muscle enables Apodiformes to move their heads quickly to catch insects in fast flight.

Reproductive biology

Apodiformes breed during periods of peak food availability. In hummingbirds, this is usually when the greatest number of bird-pollinated plants are in flower. In swifts it is during the temperate summer or the tropical wet season when insects abound. Aerial displays play a significant role in courtship. Swifts are monogamous, with pair bonds lasting through the nesting season, as both parents tend the young. Hummingbirds are polygamous; once mated, the male plays no further part in care of the young.

Most apodiform nests are open cup constructions, held together, in the case of hummingbirds, by spiders' webs; in swifts with saliva. Because of their high dependence on flight, Apodiformes invariably build their nests with clear flightpaths in front of them. The eggs are white and oval, and most species have a clutch of one or two eggs. Some Chaetura and Hirundapus swifts are exceptional in having up to seven. Once hatched, the naked, or near-naked, young remain as nestlings for a comparatively long period in relation to the birds' size: this may be to compensate for irregular food supplies, as adults are away from the nest for long periods. Young swifts are fully independent once fledged, while hummingbirds rely wholly on the adult female for food for at least 18 days after leaving the nest.

Conservation status

Fewer than a tenth of Apodiformes are under a serious level of threat, ranging in increasing degree from Endangered and Vulnerable to Critically Endangered. Downward population trends are evident for 24 of these species. An additional 22 species are classified as Near Threatened.

Historically, none of the species in this order appears to have been regarded as any economic threat. Exploitation for financial gain, however, has been widespread and, in the case of swiftlets, continues. Numbers of Mascarene (Collocalia francica) and Seychelles (Collocalia elaphra) swiftlets have been seriously reduced by collection of nests for birds' nest soup, a dish popular in southeast Asia.

Habitat degradation and destruction are the biggest threats facing Apodiformes today, with hummingbirds in Central and South America particularly threatened. Lower montane forests are being logged and cleared to make way for such diverse activities as cattle ranching; coffee, coca, marijuana, sugarcane, and citrus plantations; mining; and charcoal. The cutting of roads through previously inaccessible areas and subsequent forest destruction are now affecting species such as the chestnut-bellied hummingbird (Amazilia castaneiventris) of Colombia and the Peruvian piedtail (Phlogophilus harterti). Human encroachment onto the upper forest slopes continues, with a number of endangered species, such as the royal sunangel (Heliangelus regalis) and violet-throated metaltail (Metallura baroni), being affected by accidental fires, which are started when vegetation below the treeline is burned off to promote fresh growth of grass for grazing. Modern machinery is enabling logging of some upper montane areas, threatening species such as the white-tailed hummingbird (Eupherusa poliocerca).

Island endemics are often at risk from introduced predators. Such a threat faces a number of Apodiformes: the Tahiti swiftlet (Collocalia leucophaeus) is probably affected by common mynas, swamp harriers, and possibly other introduced species.

The Guam swiftlet (Collocalia bartschi) is hunted by the introduced brown tree snake, and the Juan Fernandez firecrown (Sephanoides fernandensis) is killed by rats, cats, and coatis. Tiny endemic populations are also vulnerable to pressure from tourism. The caves of both the Atiu (Collocalia sawtelli) and Mascarene swiftlet have been disturbed by an increase in human visitors.

The effects of intensive agriculture on apodids are less easy to quantify. There may be visible impacts on island and restricted-range species; for example, the use of pesticides and drainage of wetlands have probably reduced the numbers of Seychelles swiftlets. But accurately monitoring wide-ranging, aerial plankton feeders is difficult.

Conservation efforts to alleviate the problems facing Apodiformes are fraught with difficulty. Many well-forested areas are relatively inaccessible by virtue of geography or political instability, making true assessments of population levels impossible. Targets set by BirdLife International for species such as the black inca (Coeligena prunellei) start with the need for full monitoring.

Even when population ranges are known, and protected areas are established, conservation attempts to safeguard key areas are often hampered by an inability to enforce legal protection. Whitehead's swiftlets (Collocalia whiteheadi) on the Mount Matutum Reserve in the Philippines are vulnerable to illegal logging, and the mangrove hummingbird (Polyerata boucardi) is threatened by cutting of mangroves in Costa Rica, even though such activity is banned.

Significance to humans

Their distinctive habits and appearance have given hummingbirds significant roles in the cultures of native American peoples for millennia. The North American Cherokee, Fox, and Creek people have stories related to the birds' speed. Numerous ingenious legends revolve around their feeding techniques; the Ge people of Brazil had a legend in which a bird sucked a biting ant out of the inside of a man's ear. The most famous hummingbird legends are associated with the Aztecs, whose most important god, Huitzilopochtli, has a name that translates as "Hummingbird from the left." This god was depicted as wearing the head of a hummingbird as his helmet. Warriors who fell in battle were believed to resurrect as hummingbirds. The Aztec king Montezuma wore hummingbird feathers as part of his elaborate headdress. Europeans continued the fascination with and desire to collect colorful hummingbird feathers, most destructively in the nineteenth and early twentieth centuries, when millions of hummingbirds were slaughtered for their feathers. Between 1904 and 1911, 152,000 hummingbirds were imported from North America to London to adorn ladies' hats. Today, direct persecution of hummingbirds is rare. Thousands of backyard feeders throughout North America testify to the birds' popularity with humans.

The word swift is derived from the Old English swifan, meaning fast. The dark, screaming, sickle-shaped common swift inspired both fear and awe in medieval Europe. In England, it was known as the devil bird. Yet in their nesting habits, many species of swift have enjoyed a physically close association with humans. The common and chimney (Chaetura pelagica) swifts rarely choose anything but artificial nest sites.

The nests of swiftlets in southeast Asia have been prized as food by humans for at least a thousand years. The trade is still enormously popular. In the 1980s, Hong Kong alone imported nests worth up to an estimated $39 million.

Resources

Books

Chantler, P., and G. Driessens. Swifts. A Guide to the Swifts and Treeswifts of the World. Sussex, U.K.: Pica Press, 1995.

Collar, N. J., et al. Threatened Birds of Asia. The BirdLife International Red Data Book. Cambridge, U.K.: 2001.

del Hoyo, J., A. Elliot, and J. Sargatal, eds. Barn owls to hummingbirds. Vol. 5, Handbook of the Birds of the World. Barcelona: Lynx Editions, 1999.

Johnsgard, P. A. The Hummingbirds of North America. Washington, D.C.: Smithsonian Institution Press, 1997.

Lockwood, W. B. The Oxford Book of British Bird Names. New York: Oxford University Press, 1984.

Long, K. Hummingbirds. A Wildlife Handbook. Boulder, CO: Johnson Books, 1997.

Sibley, C. G., and J. E. Ahlquist. Phylogeny and Classification of Birds. A Study in Molecular Evolution. New Haven, Conn.: Yale University Press, 1990.