Birds are animals of the class aves. Whereas the Hebrew word translated "bird" in the Old Testament includes all flying things (Genesis 1:20, Leviticus 11:19), the modern scientific term includes only species characterised by having two-legs(bipedal), wings, beaks, backbones (vertebrate), laying eggs, feathers and being warm blooded.
There are several types of non-flying birds, principly penguins, ostriches and emus. Penguins are adapted to a life in a marine environment where the main food source is fish.
Some birds such as quail spend most time on the ground looking for food but can fly to evade predators.
The wing structure, feather types and mode of flight vary among birds , with some like eagles and hawks being soaring birds, using wind currents and thermals to glide without expending much energy.
Chickens, turkeys and pigeons are birds which have been domesticated by man. Chickens are used for egg production while chickens and turkeys are used for meat. Pigeons were trained to carry messages although that is less common with the availability of telephones. Pigeons are also used for meat.
A variety of other birds are kept in captivity as pets. Canaries, finches, parrots are typical cage birds kept as pets.