Birds' bones are hollow, and their bones may weigh less overall than their feathers. The energy needed for taking off and sustaining flight means that birds have to eat a large amount of calories for their size, and consume them up to 50 times faster than humans; Carter may have been aware of this when giving Fevvers her massive appetite. As she notes, the tail of a bird is also crucial in balance, steering and braking.Fevvers bases her early study of flight on watching pigeons on her windowsill. As she comes to realise, however, her body has few of the features which allow birds to fly easily over long distances.
A bird's wingspan corresponds to its overall size and weight. Scientists believe that the physiology of humans would prevent them from flying with any size of wing, but there was a flying dinosaur, the quetzalcoatlus, which weighed a similar amount to an adult human male and flew with a wingspan of around 15 metres. Even so, it is thought that the dinosaur could only soar, and not take off. To have the strength and lung capacity for take off, the quetzalcoatlus would have needed a chest at least one metre in diameter.