Final answer:
The typical modifications of the forelimb of birds include a keeled sternum and fused carpals, adaptations critical for flight. Flightless birds have long legs adapted for running. Birds' feet and beaks are also adapted to their diets and habitats.
Step-by-step explanation:
The typical modifications of the forelimb of birds that are relevant to the question are a keeled sternum and fused carpals. These adaptations are essential for flight. The keel of the sternum provides a surface for the attachment of powerful flight muscles, which constitute a significant portion of a bird's body weight and are critical for generating the force necessary for taking off, flying, and maneuvering. The fused carpals stabilize the wing and help in the control of wing movements during flight.
Flightless birds, such as the ostrich and kiwi, have evolved long legs adapted for running, rather than flying. In contrast, other modifications such as webbed feet in waterfowl and long spreading toes in waders demonstrate adaptation to different environments and lifestyles. Flightless birds may also possess a short femur and long tarsals that support their terrestrial mode of locomotion.
Birds' feet and beaks also exhibit adaptations tailored to their specific diets and habitats, showcasing the remarkable variety within avian anatomy and the principle of form following function in evolutionary biology.