Hummingbirds
Hummingbirds are birds native to the Americas and constituting the biological family Trochilidae. They are the smallest of birds, most species measuring 7.5–13 cm (3–5 in) in length. The smallest extant hummingbird species is the 5 cm (2.0 in) bee hummingbird, which weighs less than 2.0 g (0.07 oz). The largest hummingbird species is the 23 cm (9.1 in) giant hummingbird, weighing 18–24g (0.63–0.85 oz).
They are known as hummingbirds because of the humming sound created by their beating wings, which flap at high frequencies audible to humans. They hover in mid-air at rapid wing-flapping rates, which vary from around 12 beats per second in the largest species, to in excess of 80 in some of the smallest. Of those species that have been measured in wind tunnels, their top speeds exceed 15 m/s (54 km/h; 34 mph) and some species can dive at speeds in excess of 22 m/s (79 km/h; 49 mph).
Aerodynamics of flight
A female ruby-throated hummingbird hovering in mid-air
A trail of wake vortices generated by a hummingbird's flight discovered after training a bird to fly through a cloud of neutrally buoyant, helium-filled soap bubbles and recording airflows in the wake with stereo photography.[85]
Hummingbird flight has been studied intensively from an aerodynamic perspective using wind tunnels and high-speed video cameras.
Two studies of rufous or Anna's hummingbirds in a wind tunnel used particle image velocimetry techniques to investigate the lift generated on the bird's upstroke and downstroke.[86][87] The birds produced 75% of their weight support during the downstroke and 25% during the upstroke, with the wings making a "figure 8" motion.[88]
File:Hummingbird feeding closeup 2000fps.webm
Slow-motion video of hummingbirds feeding
Many earlier studies had assumed that lift was generated equally during the two phases of the wingbeat cycle, as is the case of insects of a similar size.[86] This finding shows that hummingbird hovering is similar to, but distinct from, that of hovering insects such as the hawk moth.[86] Further studies using electromyography in hovering rufous hummingbirds showed that muscle strain in the pectoralis major (principal downstroke muscle) was the lowest yet recorded in a flying bird, and the primary upstroke muscle (supracoracoideus) is proportionately larger than in other bird species.[89]
Because of their flying technique, these birds no longer have an alula, while the alula digit has evolved to become absent.[90]
The giant hummingbird's wings beat as few as 12 beats per second[91] and the wings of typical hummingbirds beat up to 80 times per second.[92]
As air density decreases, for example, at higher altitudes, the amount of power a hummingbird must use to hover increases. Hummingbird species adapted for life at higher altitudes therefore have larger wings to help offset these negative effects of low air density on lift generation.[93]
A slow-motion video has shown how the hummingbirds deal with rain when they are flying. To remove the water from their heads, they shake their heads and bodies, similar to a dog shaking, to shed water.[94] Further, when raindrops collectively may weigh as much as 38% of the bird's body weight, hummingbirds shift their bodies and tails horizontally, beat their wings faster, and reduce their wings' angle of motion when flying in heavy rain.[95]
High-speed capture sequence of two velvet-purple coronets sparring near a hummingbird feeder in Ecuador
Diet and specializations for food gathering
Lesser violetear at a flower
File:Hummingbird.ogv
Hummingbird in Copiapó, Chile: The apparent slow movement of its wings is a result of the stroboscopic effect.
For nutrition, hummingbirds eat a variety of insects, including mosquitoes, fruit flies, gnats in flight or aphids on leaves and spiders in their webs.[117][118][119][120] The lower beak of hummingbirds is flexible and can bend as much as 25 degrees when it widens at the base, making a larger surface for catching insects.[119] Hummingbirds hover within insect swarms in a method called "hover-hawking" to facilitate feeding.
To supply energy needs,
hummingbirds drink nectar, a sweet liquid inside certain flowers. Like bees, they are able to assess the amount of sugar in the nectar they drink; 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, requiring hummingbirds to meet their nutritional needs by consuming insects.
Hummingbirds do not spend all day flying,
as the energy cost would be prohibitive; the majority of their activity consists simply of sitting or perching. Hummingbirds eat many small meals and consume around half their weight in nectar (twice their weight in nectar, if the nectar is 25% sugar) each day.[121] Hummingbirds digest their food rapidly due to their small size and high metabolism; a mean retention time less than an hour has been reported. Hummingbirds spend an average of 10–15% of their time feeding and 75–80% sitting and digesting.
Because their high metabolism makes them vulnerable to starvation, hummingbirds are highly attuned to food sources. Some species, including many found in North America, are territorial and will try to guard food sources (such as a feeder) against other hummingbirds, attempting to ensure a future food supply for itself. Additionally, hummingbirds have an enlarged hippocampus, a brain region facilitating spatial memory used to map flowers previously visited during nectar foraging.[123]
Hummingbird beaks are flexible and their shapes vary dramatically as an adaptation for specialized feeding.[15][16] Some species, such as hermits (Phaethornis spp.) have long bills that allow them to probe deep into flowers with long corollae. Thornbills have short, sharp bills adapted for feeding from flowers with short corollae and piercing the bases of longer ones. The sicklebills' extremely decurved bills are adapted to extracting nectar from the curved corollae 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 a hummingbird feeds on nectar, the bill is usually opened only slightly, allowing the tongue to dart out and into the interior of flowers. Hummingbird bill sizes range from about 5 mm to as long as 100 mm (about 4 in).[124] When catching insects in flight, a hummingbird's jaw flexes downward to widen the gape for successful capture.