The birds are vertebrates with wings and feathers so most fly. Some eat seeds and fruits, others feed from insects or of worms and the largest species, like the eagle, can eat rodents and other animals of larger size. Some, like the hummingbird, eat only pollen.
The bird flies thanks to its wings which are covered with feathers which give a great surface surface area. The feathers are very light and the central part is in fact like a tube, which gives them a rigidity while being light. Their bones are also lighter and naturally more fragile.
The birds are oviparous animals and lay eggs to reproduce. The majority builds nests where the female will lay its eggs and to brood them until the hatching. The chicks are fed by the adults until they can fly and obtain food of their own means.
Certain birds fly with difficulty or cannot fly at all like the ostrich, which can runs very quickly to escape from it's predators. They survived the natural selection without having to fly while using other means to be defend themselves or escape.
A migratory bird is a bird which spends one season in a place and when a certain period of the year arrives, often the winter, it flies away towards another area that is warmer or favorable for reproduction. The goose is an example of migratory bird which from goes away to the south when the winter arrives.
Other birds do not migrate, that is to say because they are in warm climates, or are adapted to the harsh life of the winter. The sparrows do not make migrations and remain all the winter in northern areas and survive the cold intense ones. A parrot or a parakeet would not support this kind of climate not being adapted to the cold as geese, or the sparrows.
The birds are in general are useful because they eat many insects. Some are scavangers and help to eliminate the carcasses, like the vulture or even waste, like the gulls, which eat all that seems edible. Others are tedious because they eat the grains or the fruits of harvests. Farmers make use of scarecrows, loud noises or other means to disrupt the birds and protect their crops.
Each species of bird has specific characteristics that have developed as a means of survival. The owl which sees at night night, the kingfisher which can dive to catch fish, the swallow which runs and turns at surprising speeds... all have aptitudes which theirs made it possible to survive natural selection.
