There's actually info for that on the web:
https://www.google.fr/search?q=Why+do+birds+sing%3F&oq=Why+do+birds+sing%3F&aqs=chrome..69i57j69i60&sourceid=chrome&ie=UTF-8
quotes from easy to find web pages:
songbirds make up almost half of the world’s 10,000 bird species including warblers, thrushes, and sparrows.
Bird song is a very specialised form of bird call that, unlike all the quacks, honks and tweets we hear throughout the year, serves one function only – to ensure the breeding success of the singer, to indicate clearly that the singer is healthy and fit and ready to breed.
It's largely a boy thing, designed so that other females of the same species are attracted and males of the same species are repelled.
Bird song is most highly developed in a group of birds called passeriformes. Don't worry, we are surrounded by them, wrens, robins, blackbirds, song thrushes are all passeriformes. Some tropical birds are know to duet, with the female singing too.
Females need to know this if they are to choose the right partner. Then, the song has to say something about the health of the singer. A long, loud song for instance indicates a certain amount of stamina, a bird in good condition.
Similarly with repertoire. It appears that in some species, a wide variety of sounds in a song is especially attractive to the ladies. Many species even mimic other birds' songs just to increase their repertoire, and it's not unknown for other sounds, such as cats' calls, to be included as well.
As bird song is part of the breeding cycle, most birds sing in the breeding season. This means that birds will start singing in late January and stop singing in July.
They are prompted to start singing, it appears, by increasing daylight (more light sets their little hormones racing and in response, they sing).
They stop singing when they start moulting, usually July time, as the last thing you want to do when your feathers start falling out and you are not quite as quick off your perch is to advertise your presence to predators.
Some birds, of course, sing in winter, robin being the best known, but this is linked to defending feeding territories rather than breeding. Interestingly, in winter, female robins sing as well as males.
Just as there is a seasonal cycle to bird song there is also a daily cycle, with the most intense period being at first light, the so called 'dawn chorus'. Why sing so intensely at dawn though?
First of all, male birds may die overnight if they have not been able to feed well during the previous day.
So, first light is a time when when birds can announce their survival, and this advertises something of their feeding abilities to potential mates - and of course, it's important that females know if males can find food or not, especially when they will be relying on this after their chicks have hatched.
Also, a side product is that birds looking for territories can hear where the spaces are and move in.
Secondly, many females lay eggs at first light and immediately after this are at their most fertile. This is a time when males need to fend off other potential suitors, and as we now know, song is a key way they do this.
Hence the dawn chorus.
