crow feathers

Have you ever wondered what (biologically) makes a bird a bird?  I thought I would share this list of characteristics that are unique to birds and are not found in any other groups of animals that are alive today:

1. Feathers — all living birds have feathers.
2. Horny beak — the jaws of modern birds are covered with a horny sheath, which is much lighter than bone and helps reduce weight to aid flight.
3. Furcula — commonly called a “wishbone,” the furcula is actually two fused clavicles. Some dinosaurs also had a furcula.
4. Pneumatic bones — also called “hollow bones,” these special bones have air-filled canals that are strengthened by criss-crossed struts. Some dinosaurs also had pneumatic bones.
5. Hallux — a special backward-facing toe that helps birds perch or grip prey.  The hallux of birds that run along the ground is positioned higher up on the leg, so it doesn’t interfere with movement.
6. Air sacs — unlike mammals, birds do not have a diaphragm, so air is moved in and out of a bird’s respiratory system through pressure changes in these special sacs.  Most birds have nine.