1. The eye's have it
Bird's eyes change and vary in some species by the season, sex and/or age. Red-tailed Hawks, as juves (juvenile, immature birds) have irises (the colored ring around the pupil) that are yellow, but as they mature deepen down to a reddish-brown.
2. Brooding over it
Bird eggs require heat, a constant even warmth, to incubate, for the embryo to develop into an animal capable of hatching from the egg. Almost all birds, warm-blooded animals, use body heat to incubate the eggs in their nest. But part of the point of feathers is conserve a bird's body heat. So how do they transfer it to their eggs?
... In the works.
Image ... Swanboats at rest in the Boston Public Garden.
