Why do tinamous usually walk or run instead of flying?

Tinamous are highly adapted for ground dwelling and primarily walk or run to navigate their environment. While they possess well-developed flight muscles, they are not built for sustained flight. Their circulatory system, specifically the relatively small size of their heart and lungs, is insufficient to support prolonged activity in the air. Consequently, if forced to make several flights in quick succession, tinamous become exhausted rapidly.1

Because of these physiological limitations, they prefer to escape notice by standing motionless or slipping away quietly through cover. They typically only resort to flight when they are almost stepped upon, and even then, their flight is described as clumsy, albeit swift.1