Humans, as do most mammals, give birth to live young. Many other species such as reptiles and some fish also give live birth. Birds, however, always give birth through the process of laying eggs.
Animals that give birth to live young are called viviparous animals. Viviparous translates to reproduction inside the body of the mother. Animals that give birth by means of laying eggs are called oviparous. Loosely this translates to animals whose embryos develop outside of the body and within eggs. Animals that fall into this group are most fish, amphibians, reptiles, insects, and of course, birds.
The reason that birds give birth through the laying of eggs is rooted in how they developed as a species. Birds are one of the few non-insect animals that have the ability to fly. But they didn’t start out like that. Birds developed from the dinosaur which, as far as we know, were earthbound. The early birds that developed from dinosaurs did not have the ability of flight. As the species of birds evolved, however, they gained certain abilities and lost others. In order for birds to fly they had to lose density and weight. Otherwise, there is no way that they could be light enough to be supported by the winds. One of the prime reasons that birds can fly is that they are so lightweight, much lighter than a mammal of the same size.
As is apparent in every viviparous mammal, as the fetus grows inside the mother, the mother becomes heavier and heavier. If birds were viviparous, it would not be too long before the combined weight of the mother and fetus resulted in the mother being too heavy to fly. This would prevent her from being able to escape predators, to hunt for many foods, and so on. If, however, the fetus can be allowed to grow outside the body, in an egg, the problem is resolved.
This is why, as soon as she can, the female bird will get rid of the egg inside her by laying it in a nest where it will be allowed to incubate and hatch. This method of bearing young, however, comes with its own set of risks. Since the fetus is outside of the body, the female and male pair must work together to protect the egg. This means that someone must always stay close by to prevent predators from taking advantage of a missing parent by raiding the nest and stealing eggs.
Source by Ken Lawless