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Why is the pectoralis muscle important for bird flight?

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A zebra finch (Taeniopygia guttata) with a bright orange beak, gray head, orange cheek patch, and black-and-white striped tail perches on a branch. — Bird A brightly colored zebra finch (*Taeniopygia guttata*) perched on a branch.

The pectoralis muscle is essential for bird flight because it serves as the primary engine for the downstroke, which provides the necessary power for locomotion.1

In birds, the large pectoral muscles extend from the sternum (breastbone) to the lower surface of the humerus. When these muscles contract, they pull the wing downward. The sternum features a prominent median keel that provides the necessary surface area for the attachment of these powerful flight muscles, allowing birds to generate the force required for both hovering and propulsive flight.12

While the pectoralis is responsible for the downstroke, it works in coordination with other muscles to facilitate the full range of motion needed for flight.1