Ages 6 to 12 years. Although there are over 9,000 species of birds in the world, each develops from a single-celled egg, is incubated, hatches, and grows to adulthood. Some bird life cycles involve migration. The book focuses on the various stages and explains: differences in the length of time birds incubate their eggs and care for their young; the development of a chicken embryo and how a chick hatches; dangers to nesting habitats, the effects of pollution, and how these affect the life cycle of birds.