Why Do Birds Lay Eggs?

[Staff]

Why Do Birds Lay Eggs?

LiveScience.com: Life's Little Mysteries - Why Do Birds Lay Eggs?
excerpt from above:

Mammals and other groups have evolved the ability to keep embryos within the reproductive tract until development is complete. One of the challenges of this baby-making method called viviparity is that babies are heavy to carry around.

Scientists suggest viviparity has yet to evolve in any species of bird because mother birds can’t fly with weighty developing embryo passengers.

A second problem with viviparity is...