Hint: Birds belong to the class Aves and they are characterized by the presence of feathers. The eggs of birds can be colourful or have spots.
Let us analyze the options to find out the correct option.
Option A: Birds can be omnivore, herbivore or carnivore depending on the type of species. Birds like chickens and crows are omnivores as they feed on berries, nectar, insects, small fishes, rodents, etc while some birds like snow goose, large waterfowl are herbivores as they eat both plants and animals. Therefore, this is anincorrect option.
Option B: All birds have feathers but all of them necessarily cannot fly. These are the birds which have lost the ability to fly due to evolution. Examples include Ostrich, Emu and Dodo. Therefore, this is an incorrect option.
Option C: All birds lay eggs and provide them protection. But not all of them make nests as some lay eggs on ground and some lay their eggs in other birds nests. Therefore, this is an incorrect option.
Option D: All birds lay eggs which have a protective outer layer made of Calcium carbonate. The shell is thick. Therefore, this is the correct option.
Hence, the correct answer is option (D)
Note: 1.Birds are warm blooded vertebrates.
2.All birds have feathers, hollow bones, beak and wings.
3.Ostrich are the heaviest and largest flightless birds.