Why do birds sing?

 
Two reasons: to attract a mate or declare their territorial rights.
The number of birds singing is a sure sign that spring is approaching. Recently I have taken to recording them  on the Merlin app both in the garden and locally while out waking.
Birds Recorded Using Merlin App:
Thrush
Robin
Oystercatcher
Dunnock
Willow Warbler
Blackbird
Wren
Chiffchaff
Chaffinch
Goldfinch
Blue Tit
Greenfinch
Nuthatch
Starling
Jackdaw
Siskin
Sparrow
Great Tit
Wood Pigeon
Crow
Blackcap
Bullfinch

If you have never used the Merlin app  to identify birdsong I would recommend it. Its free and easy to use.