Why is a tinamou's flight fast but difficult to sustain?

Although tinamous possess well-developed flight muscles, their ability to sustain flight is limited by their circulatory system. Specifically, the bird's heart and lungs are surprisingly small, which results in a low circulation rate that cannot support prolonged aerial activity.1

Consequently, while a tinamou's flight is swift, it is also described as clumsy. If these birds are forced to perform several flights in short succession, they quickly become exhausted.1