Birds and mammals began to carry their young inside their bodies until they were ready to enter the world. However, birds could not do that and still fly so they changed to building nest for their eggs. One bird would always stay back to guard the eggs and keep them warm and protected until they hatched. The mammals, on the other hand, kept their babies inside them until they’re ready to come out. When the babies of birds and mammals were born, the parents would feed and protect them until they were big enough to survive on their own.