I wondered why this might be, so asked Google. There was a fascinating article on the website wired.com by Mary Bates, which explains this phenomenon. She writes:
"You may have noticed a cacophony of birdsong in the wee hours of the morning ... it can start as early as 4.00 am and last several hours. Birds can sing at any time of day, but during the dawn chorus their songs are often louder, livelier, and more frequent. It's mostly made up of male birds, attempting to attract mates and warn other males away from their territories.
But why choose the hours around sunrise to sing? There are a number of theories, and they're not necessarily mutually exclusive.
One idea is that in the early morning, light levels are too dim for birds to do much foraging. Since light levels don't affect social interactions as much, it's a great opportunity to sing, instead.
Another idea is that early morning singing signals to other birds about the strength and vitality of the singer. Singing is an essential part of bird life, but it's costly in terms of time and energy. Singing loud and proud first thing in the morning tells everyone within hearing distance that you were strong and healthy enough to survive the night. This is attractive to potential mates, and lets your competitors know you're still around and in charge of your territory. ...
Although dawn songs don't carry farther, they are clearer and more consistent, and this could be even more important. Individual males have their own signature songs, with slight variations that identify them to their neighbours. If you're a male trying to attract a mate or defend your territory, it's more important to let your fellow birds know that it's you singing, than it is to be heard over a long distance. Singing in the morning leads to a more consistent signal and makes it more likely that other birds will be able to identify the singer correctly."
So now we know! Whatever the reason, it is a wonderful way to wake up - to a sky filled with dawn's early light, and that glorious singing. I am grateful.