A、Their flying ability improved greatly. B、They became warm-blooded. C、They began to lay eggs. D、They changed their migration pa

Many egg-laying animals merely lay their eggs and leave. Turtles, for instance, and horseshoe crabs, the eggs hatched and the little ones are their own. The current theory about birds is the earliest birds did just that when they were cold-blooded 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.  For this, they needed a place--a nest.  Very likely, the first nest was just a primitive hole scraped in the ground.  Even now many species still lay eggs in this sort of crude nest. Primitive nests found 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 loose platforms of sticks and dried leaves. The type is still built by some birds today. The latest development in nest, the most recent vision, 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.