- , Marijke de Boer
Birds
There are many species of birds, but they all have wings and feathers. Bird bones are hollow. This makes them lighter in weight, costing them less energy to stay in the air. Bird feathers do not last long. They replace - molt- their worn feathers at least once a year. Some birds lose all their flying feathers at the same time and are unable to fly during that period. Others lose them one at a time.
