Hummingbirds
Hummingbirds
Hummingbirds are small, often tiny, birds of the Americas, named after the noise made by their extremely rapid wingbeats. There are 328 species of hummingbirds, which make up the family Trochilidae.
Hummingbirds are spectacularly beautiful birds, because of the vivid iridescence of their feathers. They are such accomplished fliers that they can aggressively drive away much larger, predatory birds. Invariably, people who have had the opportunity to watch hummingbirds regularly develop an admiration for these lovely sprites.
Hummingbirds are widely distributed in the Americas, occurring from Tierra del Fuego in the south, to the subarctic of Alaska and Canada in the north. However, the greatest richness of hummingbird species occurs in the tropics, especially in forests and associated, disturbed habitats where flowers may be relatively abundant.
Biology of hummingbirds
Hummingbirds have small, weak legs and feet, which are used only for perching, and not for walking. Hummingbirds only move about by flying, and they are extremely capable aerial acrobats. Although diminutive, hummingbirds can fly quickly over short distances, at up to 31-40 mph (50-65 km/h). Hummingbirds can fly forwards, backwards, and briefly, upside down. These birds are also very skilled at hovering, which they typically do when feeding on nectar from flowers. Hovering is accomplished using a figure-eight movement of the wings, and a relatively erect posture of the body.
Because of their small size, the tiniest hummingbirds must maintain an extraordinarily rapid wingbeat rate of 70 beats per second to stay aloft. However, the larger species can fly with only about 20 beats per second. The flight muscles of hummingbirds typically account for 25–30% of the body weight, compared with an average of 15% for other birds.
Also as a direct result of their small body size, hummingbirds have a high rate of heat loss from their bodies. This is because small objects, including small organisms, have a relatively large surface area to volumeratio, and lose heat more rapidly from their surface than do larger-bodied animals. This relatively high rate of heat loss, combined with the fact that hummingbirds are very active animals, means that they have a very high rate of metabolism. Consequently, hummingbirds must feed frequently, and relatively voraciously, to fuel their high-energy life style.
When weather conditions make it difficult for them to forage, for example, during intense rain or cool temperatures, hummingbirds may enter a state of torpor. This involves becoming inactive, and reducing and maintaining a relatively low body temperature, as a means of conserving energy until environmental conditions improve again.
Most hummingbirds feed on nectar, obtained from flowers. Feeding is usually done while hovering. All hummingbird species have long, slender bills that are specialized for this mode of feeding. The bill of the swordbill hummingbird (Ensifera ensifera ) is straight, and is as long as the bird’s body and tail, about 4 in (10 cm). This sort of extremely developed feeding device is adapted to extracting nectar from the base of long, tubular flowers, in particular, certain species of passion flower (Passiflora spp.) that have corolla tubes about 4.3 in (11 cm) long. A few species of hummingbirds, for example, the white-tipped sickle-bird (Eutoxeres aquila ), have downward curving beaks that are useful for probing flowers of other shapes.
Hummingbirds also have a long tongue that can be extended well beyond the tip of their bill. The tongue of hummingbirds has inrolled edges, that can be used to form a tube for sucking nectar.
Some tropical plants occur in a mutualistic relationship with one or several species of hummingbird, that is, a symbiosis in which both species receive a benefit. The advantage to the hummingbirds occurs through access to a predictable source of nectar, while the plant benefits by pollination. While feeding, the hummingbird will typically have its forehead dusted with pollen, some of which is then transferred to the receptive stigmatic surfaces of other flowers of the same plant species as the bird moves around while foraging. Hummingbird-pollinated flowers are usually red in color, and they have a tubular floral structure, with nectar-secreting organs at the base.
While nectar is the primary food of hummingbirds, they are also opportunistic predators of small insects, spiders, and other arthropods. This animal food is an important source of protein, a nutrient that is deficient in sugar-rich nectar.
The largest species of hummingbird is the giant hummingbird (Patagona gigas ) of montane habitats in the Andes, up to 0.7 oz (20 g) in body weight and 8.5 in (22 cm) long, although about one-half of the length is the elongated tail of the bird. The smallest hummingbird is the bee hummingbird (Mellisuga helenae ) of Cuba, with a body length of only 1 in (2.5 cm) and a weight of 0.07 oz (2 g). This is also the world’s smallest species of bird. Most other hummingbirds are also small, typically about 2 in (5 cm) long.
Hummingbirds can be spectacularly colored, especially the males. Most of the coloration is not due to the presence of pigments, but to iridescence. This is a physical effect associated with prism-like microstructures of the feathers of hummingbirds and some other birds. These break light into its spectral components, which are selectively segregated into brilliant reds, pinks, blues, purples, and greens through absorption processes, and by the angle of incidence of light. The most vivid colors of hummingbirds are generally developed by the feathers of the head and throat, which are prominently displayed by the male to the female during courtship flights. The male birds of some species of hummingbirds also develop crests, and intricately long tail feathers.
Even the smallest hummingbirds can be quite aggressive against much larger birds. Because of their extraordinary mobility, hummingbirds can successfully chase away potential predators, such as small hawks and crows. Hummingbirds are also aggressive with other hummingbirds, of the same or different species. This can result in frequent and rowdy fights as territorial claims are stated and defended at good feeding stations.
North American hummingbirds
Compared with the tropics, relatively few species of hummingbirds breed in North America. By far the most widespread and common species is the ruby-throated hummingbird (Archilochus colubris ). This hummingbird occurs over most of eastern and central North America south of the boreal forest, and is the only species in the east. This species can be fairly common around gardens and other disturbed habitats where wildflowers are abundant, especially red-colored flowers, as is the case of most hummingbirds. The ruby-throated hummingbird is a migratory species, which spends the winter from the southern tip of Florida to Central America.
KEY TERMS
Iridescence —A non-pigmented coloration caused by the physical dissociation of light into its spectral components. The feathers of some birds, including hummingbirds, can develop spectacular, iridescent colors. These colors disappear if the physical structure of the feathers is destroyed by grinding.
Many other species of hummingbirds occur in southwestern North America. The most common and widespread of these species are the broad-tailed hummingbird (Selasphorus platycercus ), rufous hummingbird (S. rufus ), Anna’s hummingbird (Calypte anna ), and black-chinned hummingbird (Archilochus alexandri ). Several other species also occur in more southwestern parts of the United States, some of them barely penetrating north from Mexico.
Conservation of hummingbirds
In the past, hummingbirds were hunted in large numbers for their beautiful, iridescent feathers, which were used to decorate the clothing of fashionable women. Sometimes entire, stuffed birds were used as a decoration on hats and as brooches. Fortunately, this gruesome use of hummingbirds in fashion has long passed, and these birds are now rarely hunted.
Today, the greatest risks to hummingbirds occur through losses of their natural habitat. This is an especially important problem for the many species of hummingbirds that breed in mature tropical forests. This ecosystem type is being rapidly diminished by deforestation, mostly to create new agricultural lands in tropical countries. Nearly 30 hummingbird species are considered critically endangered, endangered, or vulnerable by the World Conservation Union (IUCN), and the vast majority of these are birds that inhabit forests at least partly. Since 1987, all members of the hummingbird family have been listed in Appendix II of the Convention on International Trade in Endangered Species, meaning that trade in live birds is regulated and not allowed without a permit.
In most places where they occur, hummingbirds are highly regarded as beautiful creatures and pleasant birds to have around gardens and other places that are frequented by people. The presence of these lovely birds is often encouraged by planting an abundance of the red, nectar-rich flowers that hummingbirds favor. These birds will also avail themselves of artificial nectar, in the form of sugary solutions made available at specially designed feeders that are hung around homes and gardens. Of course, only those species of hummingbirds that frequent relatively open, disturbed habitats will benefit from this type of management. The many species that only breed in forests can only be sustained by preserving extensive tracts of that natural ecosystem type.
Resources
BOOKS
Brooke, M., and T. Birkhead, eds. The Cambridge Encyclopedia of Ornithology. Cambridge: Cambridge University Press, 1991.
del Hoyo, J., A. Elliott, and J. Sargatal. Handbook of the Birds of the World. Vol. 5, Barn-owls to Hummingbirds. Barcelona: Lynx Edicions, 1999.
Forshaw, Joseph. Encyclopedia of Birds. 2nd ed. New York: Academic Press, 1998.
Greenewalt, C.H. Hummingbirds. New York: Dover Press, 1991.
Johnsgard, P.A. The Hummingbirds of North America. Washington, DC: Smithsonian Institution Press, 1997.
Tyrell, E.Q., and R.A. Tyrell. Hummingbirds: Their Life and Behaviour. New York: Crown, 1985.
Bill Freedman