Why don't birds get pregnant?

["Philip Veerman" <>]

Well, what a dumb question. Because they lay eggs. One could suggest the period 
of production of eggs is like short pregnancy. For most birds, the demands of 
flight would be significant force against retention of eggs in the body for any 
more than the minimal time. However quite possibly in many millions of years, 
some birds such as descendants of modern penguins may evolve into a viviparous 
lifestyle. Just like whales evolved from animals that probably originally had a 
breeding system similar to what seals do today. I wonder what sea-snakes do.

Philip