The top speed for airplanes could be limited by physical rules of aerodynamics, but the birds' flight is much more complex, breaking those rules. Many think that the fastest speeds at which the smallest insects and the largest aircraft can fly are strongly connected to their weight and their wingspan.
A team led by animal ecologist Thomas Alerstam at Lund University in Sweden gathered radar data on the flight speeds of 138 bird species, varying in size from songbirds weighing under an ounce (28 g) to 20-pound (9 kg) swans.
The team discovered that the difference in speed between small and large birds was less significant than the rules of aerodynamics could predict. The researchers believe evolution forces birds to employ speeds that better fit their lifestyles. For example, large birds may fly slower than expected due to the fact that high speed can result inconvenient, "especially during takeoff, maneuvering and landing," Alerstam explained.
Related species were found in most cases to fly at comparable speeds no matter their weight or wing span, supporting the idea that evolution determines speed. It was also proved that birds of prey had slower flight speeds than believed, while songbirds were faster than predicted.
It is not clearly known why the bird flight is far more complex than expected, but several factors are involved in this, the most important appearing to be the wing's shape and the way they flap their wings.
These subtle differences and the forces behind them "could inform engineers on what special kinds of flight are needed for aircraft for special purposes, such as small flying robots carrying surveillance cameras into dangerous zones," Alerstam said.
A research made at the same university matched earlier results of a research at Brown University, showing that bat's flight is even more sophisticated than that of the birds.
"Aerodynamic forces generated by bat wings during flight are far more complicated than those of birds. Bat wings are more flexible" said Sharon Swartz, an evolutionary biologist at Brown's University.
"The materials-the skin and bone-are more stretchy. The bones actually bend when the bat is flying. At slower speeds, flexible wings seem to have advantages."