Source: SCI AM
Bird legs appear disproportionately thin due to the body being mostly covered by feathers, creating a visual illusion. Birds, descendants of theropod dinosaurs, shifted their leg orientation and muscle distribution as they evolved for flight. Balancing on two legs required changes in body structure, including a shift in the center of mass towards the front of the body.
Over time, bird legs evolved with fewer bones, consolidated muscles in the thighs, and a unique tendon system for gripping perches. Flightless birds have more substantial legs adapted for walking. The slender legs of songbirds reflect their reliance on flight rather than walking. Despite their unusual appearance, bird legs contribute to their charm and agility, underscoring their evolutionary history and adaptation for flight.
Read the full story HERE: https://www.scientificamerican.com/article/why-do-birds-have-such-skinny-legs/