Hummingbirds are New World birds of the the family Trochilidae. They are among the smallest of birds, with some no more than three inches in length, and the largest measuring at roughly five inches. The smallest extant bird species is the one-and-one-third inch long bee hummingbird,which weighs less than 2.5 grams.
Hovering in midair through high speed wing-flapping featuring nearly fifty strokes per second, they make a humming sound audible to humans. They are capable of flying as fast as 35 miles per hour, both frontwards and backwards. They have extremely high metabolisms, such that with the exception of insects, hummingbirds while in flight have the highest metabolism of all animals, a necessity to support the rapid beating of their wings during hovering and fast forward flight. Their heart rate can reach as high as 1,260 beats per minute, with a breathing rate of 250 breaths per minute. During flight, oxygen consumption per gram of muscle tissue in a hummingbird is about 10 times higher than that seen for elite human athletes. Hummingbirds, which are continuously hours away from starving to death, consume more than their own weight in nectar each day, and to do so they must visit hundreds of flowers. They can make use of ingested sugars to fuel hovering flight within 30–45 minutes of consumption, oxidizing sugar in flight muscles rapidly and reserving their limited fat stores to sustain them overnight or to power migratory flights.
The dynamic range of metabolic rates in hummingbirds requires them to have superpowerful kidney function that can be slowed or stopped when it is undergoing water deprivation, a physiological mechanism unique to hummingbirds.
Prior to nonstop flight across hundreds of miles during migration, a hummingbird will store fat as a fuel reserve, doubling its weight by as much as 100 percent, increasing potential flying time over open water.
Hummingbirds are capable of conserving energy when inactive or not foraging for food by going into torpor, a hibernation-like, deep sleep state known as torpor needed to prevent energy reserves from falling to a critical level. During night-time torpor, body temperature falls from 104 degrees Fahrenheit to 40 to 154 degrees Fahrenheit, with heart and breathing rates both slowed dramatically. Body mass declines throughout nocturnal torpor, amounting to about 10% of weight loss each night. Their hormone, corticosterone, is one signal that arouses a hummingbird from torpor.
There are 338 recognized species of hummingbirds, which fall into nine main clades, the Topazes, Hermits, Mangoes, Brilliants, Coquettes, Patagona, Mountain Gems, Bees, and Emeralds, defining their relationship to nectar-bearing flowering plants and the birds’ continued spread into new geographic areas.
All hummingbirds depend on flower nectar to fuel their high metabolisms and hovering flight. The various species have made evolutionary adaptations to coordinate bill shape to flower shape. Hummingbirds appear to have originally evolved from from insectivorous swifts (family Apodidae) and treeswifts (family Hemiprocnidae) about 42 million years ago, based in part due to a key adaptative mutation of an altered taste receptor that enabled hummingbirds to seek nectar.
In traditional taxonomy, hummingbirds are placed in the order Apodiformes, which also contains the swifts. However, some taxonomists have separated them into their own order, the Trochiliformes. Hummingbirds’ wing bones are hollow and fragile, making fossilization difficult and leaving their evolutionary history poorly documented. Most, but not all, scientists theorize that hummingbirds originated in South America. Hummingbirds exhibit gender dimorphism, in which males are smaller than females in small species, and males are larger than females in large-bodied species. The extent of this sexual size difference varies among clades of hummingbirds. In some clades males and females are similarly sized. In many clades, females have longer, more curved beaks favored for accessing nectar from tall flowers.
Sexual size and beak differences likely evolved due to constraints imposed by courtship because mating displays of male hummingbirds require complex aerial maneuvers and are costly in terms of energy. Males tend to be smaller than females, allowing conservation of energy to forage competitively and participate more frequently in courtship. Thus, sexual selection will favor smaller male hummingbirds. To augment courtship, many male hummingbirds have plumage with bright, varied coloration. By merely shifting position, feather regions of a muted-looking bird can instantly become fiery red or vivid green. The outer tail feathers of male Anna’s hummingbird (Calypte anna) vibrate during courtship display dives and produce a loud chirp. When courting, the male ascends some 35 meters before diving over an interested female at a speed of 88 feet per second, equal to 385 body lengths/second, producing a high-pitched sound. This downward acceleration during a courting dive is the highest reported for any vertebrate undergoing a voluntary aerial maneuver, resulting in gravitational forces, the level of which equals or exceeds the g-force acceleration that causes near loss of consciousness in fighter pilots during flight of fixed-wing aircraft in a high-speed banked turn.
Hummingbirds can sing at the same frequency as the tail feather chirp, but its small syrinx is not capable of the same volume. The sound is caused by the aerodynamics of rapid air flow past tail feathers, causing them to flutter in a vibration which produces the high-pitched sound of a courtship dive.
Female hummingbirds tend to be larger, requiring more energy, and their beaks longer to access preferred flowers. Female hummingbirds tend to have longer beaks that allow for more effective reach into crevices of tall flowers for nectar. Thus females are better at foraging, acquiring flower nectar, and supporting the energy demands of their larger body size.
Female hummingbirds construct small cup-shaped nests on the branch of a tree or shrub, often using spider silk and lichens to bind the nest material together and secure the structure. Use of silk allows the nest to expand as the young hummingbirds grow. Two white eggs are laid and incubation lasts 14 to 23 days, depending on the species, ambient temperature, and female attentiveness to the nest.The mother feeds her nestlings on small arthropods and nectar by inserting her bill into the open mouth of a nestling, and then regurgitating the food into its crop.
Hummingbird flight has been studied intensively from an aerodynamic perspective using wind tunnels and high-speed video cameras. The birds produce 75% of their weight support during the downstroke and 25% during the upstroke, with the wings making a “figure 8” motion. Hummingbird hovering has been estimated to be 20% more efficient than performed by a helicopter drone.
Male rufous and broad-tailed hummingbirds (genus Selasphorus) have a distinctive wing feature during normal flight that sounds like jingling or a buzzing shrill whistle. The trill arises from air rushing through slots created by the tapered tips of the ninth and tenth primary wing feathers, creating a sound loud enough to be detected by female or competitive male hummingbirds and researchers up to 300 feet away.
Behaviorally, the whistle serves several purposes, including announcing the gender and presence of a male bird, providing audible aggressive defense of feeding territory and an intrusion tactic, enhancing the alerting of a threat, and favoring mate attraction and courtship.
Hummingbirds are specialized nectarivores and are tied to the ornithophilous flowers upon which they feed. Like bees, they are able to assess the amount of sugar in the nectar they eat; they normally reject flower types that produce nectar that is less than 10% sugar and prefer those whose sugar content is higher. Nectar is a mixture of glucose, fructose, and sucrose, and is a poor source of nutrients, so hummingbirds meet their needs for protein, amino acids, vitamins, minerals, etc. by preying on insects and spiders.
Hummingbirds tend to be interested in flowers of a color wavelength ignored by nectar interested insects, which thereby reduces nectar robbing. Hummingbird-pollinated flowers also produce relatively weak nectar (averaging 25% sugars w/w) containing a high proportion of sucrose, whereas insect-pollinated flowers typically produce more concentrated nectars dominated by fructose and glucose. Hummingbirds eat many small meals and drink with their tongues by rapidly lapping nectar. Their tongues have tubes which run down their lengths and help the hummingbirds drink the nectar.
Hummingbirds are restricted to the Americas from south central Alaska to Tierra del Fuego, including the Caribbean.
Most hummingbirds of the U.S. and Canada migrate southward in fall to spend winter in Mexico, the Caribbean Islands, or Central America. A few southern South American species also move north to the tropics during the southern winter. A few species are year-round residents of California and southwestern desert regions of the USA. Among these are Anna’s hummingbird, a common resident from southern Arizona and inland California.

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