1 It's the dawn chorus
Scientists call birds' singing in the morning 'the dawn chorus'. It's when birds chirp in the loudest and liveliest sound.
2 To attract mates
Male birds are usually the ones chirping in the early morning to attract mates.
3 To defend their territory
Male birds also chirp in the morning to defend their territory and prevent other males from invading it.
4 It's too dark to do anything else
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.
5 It signals strength
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?)
6 The chirping is louder in the morning
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.
7 To distinguish themselves
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?)