The birdsong is known to be a type of vocalization done by birds. There are different reasons why a male songbird will utter the birdsong. Some do it because they are showing other birds that they own the territory. Others will do it just to show their character. The sounds emitted by the bird is usually very melodious to the ears of human beings.
People usually know if there is a nighthawk nearby based on the sounds that it makes. This is the answer at letter D. Nighthawks are not afraid to show that they are guarding their territory whenever they have to. Their sounds are usually sharp and are just pleasing to listen to.