Tradeoffs for locomotion in air and water

The most widely accepted theories include: As is true for any structure shaped by natural selection, bird anatomy has evolved to fit a particular species' behavior and lifestyle.

For example, birds that live in dense forests and require high maneuverability and precise landing capabilities tend to have wing shapes and body plans that reduce stability to allow the execution of fast turns and sudden accelerations.

As a result, large sea birds rely mostly on soaring flight because it allows these animals to achieve relatively continuous lift without the added metabolic cost of flapping their wings.

Consequently, birds that rely on dynamic soaring tend to have low wing loadings and high aspect ratios.

Albatrosses have the largest wingspan of any extant bird, evidence of their primary reliance on aerodynamic and slope soaring techniques to achieve their extremely long migration patterns.

[1] In contrast, thermal soaring birds, such as Rüppell's vultures, tend to have much smaller wing loadings and aspect ratios.

Because the fastest rising air occurs in the center of a thermal, these birds optimize their flight behavior by achieving very tight turning radii.

In other words, these birds tend to have smaller wings relative to body mass, which renders them less stable in gliding but gives them much more maneuverability so that they are capable of executing very tight turns.

Oscillatory modes, on the other hand, are characterized by thrust production from swiveling of the propulsive structure on an attachment point without any wave-like motion.

[7] Because BCF swimming relies on more caudal body structures that can direct powerful thrust only rearwards, this form of locomotion is particularly effective for accelerating quickly and cruising continuously.

[5][6] BCF swimming is, therefore, inherently stable and is often seen in fish with large migration patterns that must maximize efficiency over long periods.

Propulsive forces in MPF swimming, on the other hand, are generated by multiple fins located on either side of the body that can be coordinated to execute elaborate turns.

As a result, MPF swimming is well adapted for high maneuverability and is often seen in smaller fish that require elaborate escape patterns.

[5] Zebrafish have even been observed to alter their locomotor behavior in response to changing hydrodynamic influences throughout growth and maturation.

Some seabird species utilize surface feeding or plunge diving during foraging in which gravity and/or momentum is used to counteract buoyancy effects for a short period of time.

[9] As an intensification of the progressive reduction of wing size in auks, the evolution of flippers in penguins was at the expense of their flying capabilities.

Form constrains function, and the wings of diving flying species, such as the murre or pelagic cormorant have not developed into flippers.

The decreased body weight resulting from these adaptations is highly beneficial for reducing the effects of gravity, thus making lift easier to achieve.

In fact, both Brünnich's guillemots and white-winged scoters have been observed to alter their stroking behavior throughout a dive as an adjustment for changing buoyancies.

Because they have the dual role of producing thrust in both flight and swimming, wings in these animals demonstrate a compromise between the functional demands of two different fluid media.

By taking advantage of the fact that birds can freely associate any of their three locomotor modules, some pursuit divers rely predominantly on their webbed hind-limbs for thrust production during swimming and isolate wing function to aerial flight.

As a result, drag based swimming mechanisms are more often seen in birds that live in estuarine environments with more environmental obstacles that must be avoided.

[16] Some examples of birds that have lost the ability to fly in favor of an aquatic lifestyle include: The transition of predominantly swimming locomotion directly to flight has evolved in a single family of marine fish called Exocoetidae.

[20] These fish also tend to have "flatter" bodies which increase the total lift producing area thus allowing them to "hang" in the air better than more streamlined shapes.

[21] As a result of this high lift production, these fish are excellent gliders and are well adapted for maximizing flight distance and duration.

Instead of extending their duration of thrust production, monoplane fish launch from the water at high speeds at a large angle of attack (sometimes up to 45 degrees).