 Hummingbirds are birds of the family Trochilidae, and are native only to the Americas. They are known for their ability to hover in mid-air by rapidly flapping their wings 15–80 times per second (depending on the species). Capable of sustained hovering, the hummingbird also has the ability to fly backwards, being the only group of birds able to do so[1]. Hummingbirds may also fly vertically or horizontally, and are capable of maintaining a position while drinking nectar or eating tiny arthropods from flower blossoms. Their English name derives from the characteristic hum made by their wings.
Hummingbirds are small birds with long, thin bills. The bill combined with an extendable, bifurcated tongue, has evolved in order to allow the bird to feed upon nectar deep within flowers. The bill can be opened wide, and the lower half (mandible) also has the ability to flex downward to create an even wider opening, facilitating the capture of flying insects in the mouth rather than at the tip of the bill.
The Bee Hummingbird (Mellisuga helenae) is the smallest bird in the world, weighing 1.8 grams (0.06 ounces) and measuring about 5 cm (2 inches). A typical North American hummingbird, such as the Rufous Hummingbird (Selasphorus rufus), weighs approximately 3 g (0.106 ounces) and has a length of 10–12 cm (3.5–4 inches). The largest hummingbird is the Giant Hummingbird (Patagona gigas), with some individuals weighing as much as 24 grams (0.85 ounces) and measuring 21.5 cm (8.5 inches).Most species exhibit conspicuous sexual dimorphism, with males more brightly colored and females displaying more cryptic coloration. [3] Iridescent plumage is present in both sexes of most species, with green being the most common color. Highly modified structures within certain feathers, usually concentrated on the head and breast, produce intense metallic iridescence in a rainbow of colors. 
Hummingbirds feed on the nectar of plants and are important pollinators, especially of deep-throated, tubular flowers. Like bees, they are able to assess the amount of sugar in the nectar they eat; they reject flower types that produce nectar which is less than 12% sugar and prefer those whose sugar content is around 25%. Nectar is a poor source of nutrients, so hummingbirds meet their needs for protein, amino acids, vitamins, minerals, etc. by preying on insects and spiders, especially when feeding young.Most hummingbirds have bills that are long and straight or nearly so, but in some species the bill shape is adapted for specialized feeding. Thornbills have short, sharp bills adapted for feeding from flowers with short corollas and piercing the bases of longer ones. The Sicklebills' extremely decurved bills are adapted to extracting nectar from the curved corollas of flowers in the family Gesneriaceae. The bill of the Fiery-tailed Awlbill has an upturned tip, as in the Avocets. The male Tooth-billed Hummingbird has barracuda-like spikes at the tip of its long, straight bill.
The two halves of a hummingbird's bill have a pronounced overlap, with the lower half (mandible) fitting tightly inside the upper half (maxilla). When hummingbirds feed on nectar, the bill is usually only opened slightly, allowing the tongue to dart out and into the interior of flowers. Hummingbirds do not spend all day flying, as the energy costs of this would be prohibitive. In fact, they spend most of their lives sitting, perching and watching the world. Hummingbirds feed in many small meals, consuming up to their own body weight in nectar and insects per day. They spend an average 10%-15% of their time feeding and 75%-80% sitting, digesting and watching. Obtaining this much food requires a lot of work. Scientists have recorded a Costa's Hummingbirds making 42 feeding flights in 6-5 hours, during which time it visited 1,311 flowers. Hummingbirds are found only in the Americas, from southern Alaska and Canada to Tierra del Fuego, including the Caribbean. The majority of species occur in tropical Central and South America, but several species also breed in temperate areas. Only the migratory Ruby-throated Hummingbird breeds in continental North America east of the Mississippi River and Great Lakes. The Black-chinned Hummingbird, its close relative and another migrant, is the most widespread and common species in the western United States, while the Rufous Hummingbird is the most widespread species in western Canada.
Most hummingbirds of the U.S. and Canada migrate south in fall to spend the northern winter in Mexico or Central America. A few southern South American species also move to the tropics in the southern winter. A few species are year-round residents in the warmer coastal and interior desert regions. Among these is Anna's Hummingbird, a common resident from southern California inland to southern Arizona and north to southwestern British Columbia. The Rufous Hummingbird is one of several species that breed in western North America and are wintering in increasing numbers in the southeastern United States, rather than in tropical Mexico. Thanks in part to artificial feeders and winter-blooming gardens, hummingbirds formerly considered doomed by faulty navigational instincts are surviving northern winters and even returning to the same gardens year after year. Individuals that survive winters in the north may have altered internal navigation instincts that could be passed on to their offspring. The Rufous Hummingbird nests farther north than any other species and must tolerate temperatures below freezing on its breeding grounds. This cold hardiness enables it to survive temperatures well below freezing, provided that adequate shelter and feeders are available.
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