Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Birds need a light-weight body in order to stay aloft. Even so, flying is hard work, and flight muscles need a constant supply of oxygen- and nutrient-rich blood. The organ systems of birds are adapted to meet these needs. Answer: to stay aloft
Why do birds need a light-weight body?