The main flight muscles in birds are the pectoralis and the supracoracoideus. The pectoralis powers the downstroke, and the supracoracoideus powers the upstroke. Together they account for roughly 15 to 25 percent of a bird’s total body mass. Insects and bats also fly, but they use different muscle arrangements to get airborne.
The Pectoralis: Powering the Downstroke
The pectoralis is the largest muscle in a flying bird’s body, making up about 10 percent of body mass on its own. It originates from the keel, a large bony ridge that projects downward from the breastbone, and from the furcula (wishbone) at the front of the chest. It inserts on the upper arm bone near the shoulder. When it contracts, it pulls the wing forcefully downward and backward, generating the thrust and lift that keep a bird in the air.
This muscle produces impressive power. In pigeons, the pectoralis generates about 51 watts per kilogram of muscle during level flight and roughly 119 watts per kilogram during takeoff from the ground. That spike during takeoff makes sense: launching off the ground demands far more force than cruising at altitude.
The pectoralis is built mostly from fast-oxidative muscle fibers, the type that contract quickly but also resist fatigue. In pigeons, about 85 percent of the pectoralis is composed of these fibers. This composition is what allows birds to sustain wing beats over long distances. Migratory species take this even further, fueling their pectoralis primarily with stored fat, which provides more energy per gram than carbohydrates or protein. Some migrants fly thousands of miles nonstop without eating or drinking.
The Supracoracoideus: Raising the Wing
The supracoracoideus sits beneath the pectoralis, also anchored to the sternal keel. It is much smaller, roughly one-fifth the mass of the pectoralis, or about 2 percent of total body weight. Despite its position on the underside of the chest, it raises the wing. It accomplishes this through a clever anatomical trick: its tendon passes upward through a bony channel called the triosseal canal, formed where three bones of the shoulder meet. This channel acts like a pulley, redirecting the muscle’s pull so that a contraction from below lifts the wing from above.
This pulley system is unique to modern birds. Their dinosaur ancestors lacked the bony triosseal canal, meaning the supracoracoideus originally served a different function (pulling the arm forward rather than lifting it). The evolution of this channel was a key step in the development of powered flapping flight.
The fiber composition of the supracoracoideus varies more between species than the pectoralis does. Some birds pack it with fast-oxidative fibers for endurance, while others rely more on fast-glycolytic fibers suited for short, explosive bursts. The difference tracks with lifestyle: a duck that flies steadily for hours needs endurance fibers, while a pheasant that rockets off the ground and glides needs burst power.
Smaller Muscles That Fine-Tune the Wing
The pectoralis and supracoracoideus handle the heavy lifting, but a suite of smaller muscles at the shoulder, elbow, and wrist controls how the wing moves through each stroke. The coracobrachialis and subcoracoideus help the pectoralis retract the wing during the downstroke. The deltoideus assists the supracoracoideus in elevating the wing. The scapulohumeralis caudalis and latissimus dorsi stabilize the shoulder joint and assist with both strokes.
Further down the wing, muscles at the elbow and wrist unfurl the wing at the start of the downstroke and fold it back in before the upstroke. This folding and unfolding changes the wing’s surface area and angle throughout the stroke cycle. Other muscles rotate the wing at the elbow and wrist in opposite directions, creating a twist along the wing’s length that lets the bird fine-tune its angle of attack. This is similar to how a propeller blade is angled differently at the tip than at the hub.
How Insect Flight Muscles Differ
Insects use a fundamentally different system. Most flying insects rely on indirect flight muscles that never touch the wings at all. Instead, these muscles deform the shape of the thorax (the middle body segment), and the wings respond because they’re hinged to the thorax wall.
Two sets of indirect muscles work against each other. Dorsal-ventral muscles run from the top of the thorax to the bottom. When they contract, they pull the roof of the thorax down, which levers the wing tips up. Dorsal-longitudinal muscles run front to back, like bow strings. When they contract, they arch the thorax roof upward, snapping the wing tips down. The alternating contraction of these two muscle groups produces the wing beat cycle.
Some more primitive fliers, like dragonflies and cockroaches, use a different approach. Their downstroke is powered by direct flight muscles that attach through ligaments directly to the wing’s hinge. These insects can control each wing independently, giving dragonflies their remarkable ability to hover, fly backward, and change direction almost instantly.
The most striking difference between insect and bird flight muscles is how they’re controlled. In dragonflies and other insects with synchronous (neurogenic) muscles, every single contraction requires a nerve impulse. This limits wing beat frequency to roughly 10 to 50 beats per second, because nerves need a brief recovery period between signals. Flies and bees, by contrast, use asynchronous (myogenic) muscles that contract automatically whenever they’re stretched past a threshold. The nervous system sends a “start” signal, and then the two opposing muscle sets trigger each other back and forth, no further nerve input needed. This allows wing beat frequencies of 500 to 1,000 beats per second in some species.
How Bats Compare to Birds
Bats are the only mammals capable of powered flight, and their muscle arrangement shares some features with birds. Like birds, bats have a keeled breastbone where a large pectoralis muscle attaches, and the pectoralis drives the downstroke. The biceps also plays a role, folding the wing at the elbow during each downstroke.
The key difference is that bat wings are far more flexible than bird wings. A bat’s wing membrane is stretched between elongated finger bones, and dozens of small muscles in the arm, forearm, and even the wing membrane itself adjust the wing’s shape in real time. Tiny muscles called plagiopatagiales originate from the skeleton and insert directly into the wing membrane, allowing bats to reshape their wing surface during different aerodynamic conditions. Because so many muscles contribute to flight, the pectoralis accounts for a smaller share of total flight energy in bats than in birds.
Why the Keel Matters
In both birds and bats, the enlarged keel of the breastbone is what makes powerful flight possible. This bony ridge dramatically increases the surface area available for muscle attachment. A bigger keel supports bigger flight muscles. Flightless birds like ostriches and emus have flat breastbones with no keel, which is one reason they cannot fly even though they still have pectoralis muscles. Penguins are an interesting exception: they retain a keeled sternum because they “fly” underwater, using the same pectoralis-driven downstroke to propel themselves through water instead of air.
Hummingbirds represent the opposite extreme. They have proportionally massive flight muscles relative to body size and beat their wings at roughly 34 times per second, far faster than most birds. Their pectoralis generates lower force per stroke than a pigeon’s, but the extraordinary stroke rate compensates, allowing them to hover in place and even fly backward.