Humans, like 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 egg-laying process.
Animals that give birth to live young are called viviparous animals. Viviparous translates to reproduction within the mother’s body. Animals that give birth by laying eggs are called oviparous. Generally speaking, this translates to animals whose embryos develop outside the body and inside the eggs. The animals that belong to this group are most fish, amphibians, reptiles, insects and, of course, birds.
The reason birds give birth through egg laying 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 that way. Birds developed from dinosaurs which, as far as we know, were land-bound. The first birds that developed from the dinosaurs did not have the ability to fly. However, as bird species evolved, 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 they can be light enough to be sustained by the winds. One of the main reasons birds can fly is that they are very light, much lighter than a mammal of the same size.
As is evident in all viviparous mammals, as the fetus grows inside the mother, the mother becomes heavier and heavier. If birds were viviparous, it wouldn’t be long before the combined weight of the mother and fetus would make the mother too heavy to fly. This would prevent him from being able to escape from predators, search for a lot of food, etc. However, if the fetus can be allowed to grow outside the body, into an egg, the problem is solved.
Therefore, as soon as she can, the female will get rid of the egg she carries inside, depositing it in a nest where she can incubate and hatch. This method of procreation, however, carries its own set of risks. Since the fetus is outside the body, the male and female partners must work together to protect the egg. This means that someone must always be around to prevent predators from taking advantage of a missing parent to raid the nest and steal the eggs.