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, the eggs hatch on their own. The current theory about birds is that the earliest birds did just that when they were cold-blooded creatures living in warm places. (27) 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. (28) 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. (29) 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 (30) 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 os-preys 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.
26．What is the main topic of the passage?
27．What evolutionary change in birds led to nest building?
28．According to the speaker，where were the first bird’s nests located?
29．What is the killdeer’s nest in parking lot an example of?
30．According to the speaker，what is a possible reason that birds began to build nests in trees?