HOW BIRDS FLY

Feathers

Birds' feathers act somewhat like propellers when the bird flaps its wings.  Feathers called primary fathers act like the oars on a boat, propelling the bird forward and upward as the wing is stroked down.  Secondary feathers provide more thrust on the return stroke.

Wings

The shape and size of a bird's wings affect how it flies.  The wingspand of a birdsis known as the aspect ratio.  Small birds such as woodpeckers and hummingbirds have small wings giving them better maneuverability and quicker acceleration (they have small wings allow them to flap more times in a given period of time).  Birds of prey like hawks have large wings, allowing them to maintain higher speeds for longer times (the size of the wings provides more thrust per flap).

Internal anatomy

Birds bodies' are very efficient to reduce weight and drag.  The bones of flying birds are hollow, and they have light jaws.  The gizzard, an organ in the bird's digestive system, acts as ballast to help keep the bird steady while flying.  Female birds have only one very small ovary.

The circulatory system of the bird is unique, as well.  The strongest muscles are located in the breast of the bird and have an overabundance of blood vessels to allow more blood flow to these muscles (the reason that the meat is dark).  The lungs are set up very differently from humans.  There are a series of air sacs throughout the birds body that extract oxygen from the air much more efficiently to power the muscles.  The hearts of birds are much larger and stronger than those of mammals (relative to their body size).