Bird flight

Additional net lift may come from airflow around the bird's body in some species, especially during intermittent flight while the wings are folded or semi-folded[2][3] (cf.

For specialist soaring birds (obligate soarers), the decision to engage in flight are strongly related to atmospheric conditions that allow individuals to maximise flight-efficiency and minimise energetic costs.

[6] Small birds often fly long distances using a technique in which short bursts of flapping are alternated with intervals in which the wings are folded against the body.

[3] The flight pattern is believed to decrease the energy required by reducing the aerodynamic drag during the ballistic part of the trajectory,[8] and to increase the efficiency of muscle use.

[11][12] True hovering occurs by generating lift through flapping alone, rather than by passage through the air, requiring considerable energy expenditure.

Although not a true hover, some birds remain in a fixed position relative to the ground or water by flying into a headwind.

[19] Take-off is one of the most energetically demanding aspects of flight, as the bird must generate enough airflow across the wing to create lift.

However, this technique does not work for larger birds, such as albatrosses and swans, which instead must take a running start to generate sufficient airflow.

This problem is dealt with in some species by aiming for a point below the intended landing area (such as a nest on a cliff) then pulling up beforehand.

In most species, these are lost by the time the bird is adult (such as the highly visible ones used for active climbing by hoatzin chicks), but claws are retained into adulthood by the secretarybird, screamers, finfoots, ostriches, several swifts and numerous others, as a local trait, in a few specimens.

Many small birds have a low aspect ratio with elliptical character (when spread), allowing for tight maneuvering in confined spaces such as might be found in dense vegetation.

[26] A wide variety of birds fly together in a symmetric V-shaped or a J-shaped coordinated formation, also referred to as an "echelon", especially during long-distance flight or migration.

It is often assumed that birds resort to this pattern of formation flying in order to save energy and improve the aerodynamic efficiency.

[27][28] The birds flying at the tips and at the front would interchange positions in a timely cyclical fashion to spread flight fatigue equally among the flock members.

In a 1970 study, the authors claimed that each bird in a V formation of 25 members can achieve a reduction of induced drag and as a result increase their range by 71%.

This function is most important in taking off or achieving lift at very low or slow speeds where the bird is reaching up and grabbing air and pulling itself up.

This high metabolic rate produces large quantities of radicals in the cells that can damage DNA and lead to tumours.

Birds, however, do not suffer from an otherwise expected shortened lifespan as their cells have evolved a more efficient antioxidant system than those found in other animals.

[citation needed] In addition to anatomical and metabolic modifications, birds have also adapted their behavior to a life in air.

It appears that Archaeopteryx had the avian brain structures and inner-ear balance sensors that birds use to control their flight.

[35] The presence of most fossils in marine sediments in habitats devoid of vegetation has led to the hypothesis that they may have used their wings as aids to run across the water surface in the manner of the basilisk lizards.

[36][37] In March 2018, scientists reported that Archaeopteryx was likely capable of flight, but in a manner substantially different from that of modern birds.

[45] Another "ground upwards" theory argues the evolution of flight was initially driven by competitive displays and fighting: displays required longer feathers and longer, stronger forelimbs; many modern birds use their wings as weapons, and downward blows have a similar action to that of flapping flight.

[46] Many of the Archaeopteryx fossils come from marine sediments and it has been suggested that wings may have helped the birds run over water in the manner of the common basilisk.

[47] Most recent attacks on the "from the ground up" hypothesis attempt to refute its assumption that birds are modified coelurosaurian dinosaurs.

Drag–based, and later lift-based, mechanisms evolved under selection for improved control of body position and locomotion during the aerial part of the attack.

The authors believed that this theory had four main virtues: Birds use flight to obtain prey on the wing, for foraging, to commute to feeding grounds, and to migrate between the seasons.

Flight is more energetically expensive in larger birds, and many of the largest species fly by soaring and gliding (without flapping their wings) as much as possible.

Birds that settle on isolated oceanic islands that lack ground-based predators may over the course of evolution lose the ability to fly.