Many egg-laying animals merely lay their eggs and leave. Turtles for instance, the eggs hatch on their own. The current theory about birds is that the earliest birds did just that when they were coldblooded creatures living in warm places. However when they became warm-blooded creatures living in cold places they had to remain on the eggs to keep them warm. The process we call incubation. For this they needed a place to nest. Very likely the first nests were just primitive depressions scrape into the ground. Even now many species still lay eggs in this sort of crude nests. In fact every spring a mother killdeer lays hers in some pebbles along the edge of the parking lot just outside this building. Primitive nests on the ground were fine for some birds but others began to elevate their nests in branches perhaps to avoid predators. These early elevated nests were probably lose platforms of sticks and twigs -- the types still built by ospreys and most Arians today. The latest evolvement in nest, the most recent version, so to speak, is the cup-shaped nest. This is the one we regard today as the typical bird's nest, you know, like a robin's nest.