Accipiters use these adaptations to help them maneuver in flight, particularly while they are hunting small birds. The other members of the genus accipiter that can be found in North America are the Cooper's hawk and the northern goshawk.
Each year many sharp-shinned hawks migrate south from Canada and can be seen at hawk watches in the US such as Hawk Ridge in Duluth, MN, where all of these sharpies where caught and banded. Not all sharpies migrate south however, since the number of people who have put up feeders has increased so has the number of sharpies that have stayed further north.
Since the sharpies are mainly eating birds, feeders attracting song birds in the winter become a convenient feeding ground for sharpies, and Cooper's hawks as well.
You can tell the ages of the birds pictured above by the color of their eyes. The first bird pictured is a hatch year bird. When sharpies first hatch they have yellow eyes which darken with age. So the bird in the second and third pictures would be a second year bird, since its eyes have changed from yellow to orange. The bird in the final pic is at least a third year bird. As you can see the eyes have become completely red. Once they reach this point the eyes will no longer change, so it is pretty much impossible to tell a sharpies age, once they reach adulthood, just by looking at the eyes.