Which of the following adaptations is critical for flight in birds?

Lighter bones are crucial for flight in birds because they reduce the overall body weight, making it easier for birds to achieve and maintain flight. This adaptation, often referred to as pneumatic bones, allows for the necessary lift and maneuverability while consuming less energy. The hollow structure of these bones still provides the required strength to withstand the stresses of flight.

Other adaptations, while beneficial for birds in various aspects, do not directly contribute to the mechanics of flight in the same fundamental way. For instance, a warm-blooded metabolism supports sustained activity levels and allows birds to thrive in various environments, but it is the physical structure that specifically enables them to fly efficiently. Similarly, beak structure is largely related to feeding adaptations and ecological roles rather than directly facilitating flight. Elongated limbs can aid in certain types of movement but are not as critical for flight as the adaptation of lighter bones. Thus, lighter bones are the key anatomical feature that enables birds to fly effectively.