Viviparity In Early Adulthood

Why is it that reptiles like lizards and snakes are capable of laying eggs as well as performing live births but birds can only lay eggs? As with most biological phenomena the mystery of why birds do not give birth to live young can be explained once again by Darwin and his theory of evolution. As per the norm with evolution this was a matter of life or death, survival of the fittest and adapting to changing environments over millions of years and many generations.
Viviparity and oviparity. What?
Hundreds of years ago the great Greek philosopher and scientist, Aristotle, first described the terms viviparous (to give birth to live young) and oviparous (egg-laying) animals. It is a popular misconception that viviparity is restricted to only
Scientists have done research and consulted with fossil evidence and found a correlation between past climates and the convergent evolutionary move toward viviparity. Viviparity and egg retention served a thermoregulatory process; a way for the squamates to maintain optimal embryo development temperatures because laying the eggs would have left them exposed to unfavourable temperatures. If the embryo of an animal is not developing at the correct temperature their development could take longer, negatively affecting the mother or the embryo could die. Squamates are cold blooded and need to find warmth, for example by bathing in the sun they are able to maintain the temperatures at which their bodies are at. This is but one of the reasons that viviparity evolved. Another reason for the evolution of viviparity is for the protection and safety of the eggs to ensure succession of offspring. By laying eggs as opposed to retaining them they are not only exposed to the elements but also to possible predation. Viviparity allows the mother to save energy in terms of protecting a clutch of eggs from predators; the eggs are also safe from microbial attack, drying out and direct UV