As the bird flies through the air by beating its wings, air flows over the wing. Bernoulli said that because of how the wing was shaped and how the air flowed over it, air pressure would be lower on top of the wing than on the bottom. This is lift, and it allows the bird to fly. Flying is difficult to do and it takes a lot of energy. Beside having wings, you need to have some very powerful muscles to operate them.
Birds have very large chest muscles called pectorals which are used to flap the wings.
When we eat a chicken filet sandwich, we are actually eating the pectorals. Besides having powerful muscles, birds have several neat tricks to make them lighter. Powerful jaws and teeth are very heavy, so birds have beaks instead. Mammal bones are very heavy, so a bird's bones are nearly hollow to reduce weight. Finally, birds lay eggs instead of giving birth to live birds.