Why do birds chirp in the morning?

Scientists call birds' singing in the morning 'the dawn chorus'. It's when birds chirp in the loudest and liveliest sound.

Male birds are usually the ones chirping in the early morning to attract mates.

Male birds also chirp in the morning to defend their territory and prevent other males from invading it.

Some theories assume that birds chirp in the early morning when there's no light yet because there's nothing else they can do in the dark.

One theory says that the chirping of birds in the early morning is a signal to other birds that the singer is strong, healthy and full of energy. (See What meanings and symbolism do birds have?)

Some scientists believe that due to atmospheric conditions, the chirping of birds sound louder and clearer in the morning. However, recent studies show that the chirping is just as loud at noon or dawn. 

Male birds have distinctive tunes that vary from one another. This is used to distinguish themselves when trying to attract female mates. (See How Are Ponies and Horses Different?)