Chapter four looks as the physiology of birds, it how they live and reproduce in different environments. Getting water and staying at the proper temperature are a challenge for birds, both large and small. Their respiratory system is more efficient compared to mammals. Birds also separately evolved a four chambered heart. Birds also evolved ways to maintain their temperature under different conditions. Their digestive system is simpler than mammals, but birds have the ability to change the size of their organs in demand, for example during migrations. This also extends to their reproductive organs, which can become active under the proper environmental and other conditions