Tell that to the bards
Poets have always loved birds and their songs. There is something so evocative about birds singing in the gardens and woods. From Chaucer to Shakespeare, they all wrote of birds and birdsong as lifting the spirits, heartening and enchanting us.
The flowers appear on the earth; The time of singing birds is come, And the voice of the turtledove is heard in our land; says the Song of Solomon.
"Blithe spirit, bird thou never wert" said Shelley of the sky lark.
Keats wrote an ode to the nightingale.
Both these odes are full of wonder and awe at the artistry of these avian songsters. I wonder if poets would still feel such pleasure in birdsong if they knew what it was all about.
Bird calls and songs have a definite purpose, after all. They are announcements to other birds. Defining and defending territory, advertising for a mate, deterring predators and signals of alarm.
Mynahs have a call when they are startled and take wing. To me it sounds exactly like "I am outta here!", a very quick musical trill. It is not a song, though. It's a warning, there is a human here, clear out.
Bulbuls have a call that is very insistent and comes in two or sometimes three parts. 'Pick pick, pick pick', they say, and repeat it with more urgency, 'Pick pick pick, pick pick pick pick'; and then comes the explosive denouement of 'Pickacho'.
Though almost never seen, the koel's 'cou, cou', repeated higher and more frantic, is the most familiar to us all.
The magpie robin, my favourite singer in birds, is almost operatic in its songs in the breeding season. It has to be, because the larger the repertoire of its songs, the better the chances of finding a mate.
When they are warning each other to keep off, demonstrating their suitability to mates, when they are establishing their territories or giving alarm, birds don't know or care that we thrill to their song. But like Terry Pratchett says in one of his trademark footnotes:
It's hard to be an ornithologist and walk through a wood when all around you the world is shouting: 'Bugger off, this is my bush! Aargh, the nest thief! Have sex with me, I can make my chest big and red!'
It is a good thing nobody told this to Keats and Shelley. :D