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Movement in the Air
Movement in the air is better known as flying. Most birds fly from place to place and many of them fly to catch their food. Swallows can be seen at the end of a summer day, flying high over houses in towns and villages. They fly very quickly and often change direction. They are hunting for insects which they eat as they fly.
Hawks and eagles fly high in the sky using their very good eyesight to find their prey on the ground. Owls hunt at night. They have large eyes and very sensitive hearing which helps them find their prey in the dark.
How Birds are Adapted to Flight
Birds are vertebrates and have a bony skeleton inside their bodies. Their bones, however, are very light and some of them are hollow. This makes it easier for them to fly. Their arm bones are long and thin. These bones have flight feathers attached to them. Only birds have feathers; it is one of their characteristics.
Birds need large muscles to move their wings up and down. These muscles are attached to the wings at one end and to a special bone, called the keel bone, at the other end. The muscles work in antagonistic pairs.
When one muscle contracts the wings is moved up. When the other muscle contracts it pulls the wing down. The keel is really a part of the breastbone or sternum.
The feathers overlap each other in a very special way. When the wings push down during flying (the downstroke), the feathers stay close together and do not let air move between them. When the wings are pulled up (the upstroke), air is able to move between the feathers. This means that birds are able to gain height with the downstroke but they do not lose it with the upstroke. Also, the wings are held out straight during the downstroke, which pushes against a lot of air. Birds bend their wings during the upstroke to move them through less air.
Feathers are also very light and the quill of each feather is hollow. The barbs of the feather are joined together by very small barbules. Birds spend a lot of time making sure that the barbs are joined to each other. This is important as it permits them to fly properly. You can often see birds using their beaks to put the barbules in place. This is called preening.
All of these things help the bird to stay in the air. We call them adaptations.
Some birds, such as eagles and vultures, have very large wings. They are able to fly high in the sky and make circles. They do not seem to move their wings very much. This is called gliding. They can do this because they find places where heat is rising up from the land. The heat makes the air move upwards. This is called a thermal. The air which is moving upwards pushes against the large wings of these birds and keeps them in the air. The birds hardly need to move their wings at all.
Other flying animals
Many insects fly and some of them are carnivores. The dragonfly hunts other flying insects. A few mammals fly. The bat is a mammal and its arms have very long bones which are joined by a thin layer of skin to form its wings. They eat flying insects, such as moths. Bats cannot see well in the dark so they use sound to find their prey.
Most flying insects have two pairs of wings. Their wings are very thin and transparent. They are made of a strong substance called chitin, the same material which makes their exoskeleton. The wings of insects are small compared to the size of their bodies. This means that their wings have to beat (move up and down) very fast to keep them in the air. To do this insects have muscles inside their hard outer covering. One set of muscles contracts to move the wings up. The other set contracts to move them down. Air cannot move through insect wings. The insect has to change position of its wings during the upstroke to stop the upstroke pushing the insect down in the air.
Some insects, such as the housefly, have only one pair of wings. The second pair has been replaced by tiny structures which help it keep its balance when flying. In beetles, the second pair of wings has changed into covers for the first pair. These covers protect the wings when the beetle is not flying.