Flight is an ability that humans envy greatly, and the dream of flying is one of humanity's greatest dreams. From the story of Mozi making a "wooden kite" recorded in Han Feizi's "Outer Reserves" to the first flight of the Wright brothers in 1908, humans have realized their dream of flying with the help of tools. Humans have only been flying in the sky for about a century, while birds have been flying in the sky for 150 million years. Why can birds fly?
Currently, it is widely believed that birds originated from theropod dinosaurs in the Late Jurassic period 150 million years ago. Due to survival pressures such as foraging and avoiding danger, the structure of the forelimbs of bird ancestors gradually changed, and early feathers appeared, enabling them to glide. The new mode of movement greatly expanded the survival space of birds, and the advantage of a vast aerial ecological niche prompted birds to evolve towards adaptive aerial survival, eventually allowing them to take to the skies.
Flight requires powerful wings to beat the air and generate lift and thrust. Unlike other flying animals such as insects and bats, bird wings are composed of regularly arranged hierarchical feathers that can meet the aerodynamic requirements of flight by constantly changing the shape, size, and angle of the wings relative to the body. When the tail wing of a bird is spread open, it acts like a fan, helping to steer during flight, change direction, decelerate, and land.
Flight requires weight reduction. The heaviest existing flying birds weigh about 15 kilograms, such as the great bustard, griffon vulture, and pelican. To reduce weight, birds' teeth disappeared, their coccyxes atrophied, and many parts of their skeletons fused or became hollow. Bird skeletons account for only 5% to 6% of their total body weight, compared to about 20% of the human skeleton's weight. In addition, the right ovary and fallopian tubes of birds atrophied, and even their bladders disappeared, allowing feces and urine to be excreted as soon as they are formed, never remaining in the body.
Flight also requires a large amount of energy. To adapt to flight, birds evolved a complex air sac system connected to their lungs, making their respiratory systems highly efficient. In addition, the volume of their hearts expanded, blood flow accelerated, and metabolism increased, providing the energy needed for bird flight.
Bird flight is the result of long-term evolution and natural selection. Except for ostriches, penguins, kiwis, and emus, the vast majority of birds existing on Earth today can fly. Of course, due to adaptation to their respective environments, the flying abilities and flight methods of different birds vary.