For the Birds: Why Birds Can Fly

Warbler chasing bug by Maggie Bond

By Christine Southwick

With a downward stroke of its wings and an upward push of its legs, a bird becomes a magical being — easily doing something that no other creatures can do. It becomes airborne; soaring, gliding, thermalling, turning, plummeting, diving, escaping, hunting, mating. They can travel amazing distances (“as a crow flies”) during migration, or soar, roll, and plunge just for fun.

Feathers define which entities are classified as birds. Almost all birds can fly; some can even swim using their wings to propel themselves through the water.

Feather barbs and barbules

What makes birds able to fly? Feathers are the key, followed by bone structure, combined with musculature adaptation, and a constant source of oxygen.

Feathers are truly amazing! Feathers have evolved into strong, light-weight and flexible structures, using a hollow shaft to support vanes which branch out from the shaft. Barbs and barbules on the vanes hook together to create the strong aero-dynamic shapes needed to fly. When you see a bird preening its feathers, it is hooking together the barbs and barbules that have become separated, and it is water-proofing its feathers.

Chickadee silhouette showing flight feathers
by Christine Southwick

To be able to fly, weight is an obstacle that must be overcome, either by creating enough lift or by reducing the weight ratio. Bird bones, unlike solid human bones, have hollow spaces reinforced by crisscrossing struts which can withstand the stresses of taking off, flying, and landing without added weight. The bones in the wing are surprisingly similar to the ones in our arm and hand, but with the joints fused so that there are fewer joints, reducing wing weight, and requiring less energy to move the whole wing as a unit. A strong but lightweight beak instead of a heavy jaw with teeth shaves off even more weight.

Bewick's wren by Doug Parrott

The forward thrust needed to fly is created by super-hero-sized pectoral flight muscles. Birds have a fused collarbone, and an enlarged breastbone (keeled sternum) which these flight muscles use as their fulcrum.

Flying requires a constant follow of oxygen, not the expansion/contraction of a diaphragm-stoked system. Birds have several air sacs which maintain a fixed volume of oxygenated air constantly flowing through them in one direction. Air sacs account for about 15% of a bird’s total body volume, about double the ratio of human lung volume.

These adaptations allow birds to spread their wings and effortlessly fly, leaving us in awe, envy, and earthbound.