zwitschern
to chirp
The typical high, short sound that small birds such as sparrows or finches make.
Am Morgen hört man die Vögel vor dem Fenster zwitschern.
In the morning, you can hear the birds chirping outside the window.

to chirp / tweet
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to chirp
The typical high, short sound that small birds such as sparrows or finches make.
Am Morgen hört man die Vögel vor dem Fenster zwitschern.
In the morning, you can hear the birds chirping outside the window.

'Zwitschern' (to chirp) is an onomatopoeic word. The sound of the word imitates the noise it describes—the high, short sounds made by small birds.
Ich liebe es, im Frühling aufzuwachen, wenn die Vögel zwitschern.
I love waking up in spring when the birds are chirping.
In its main meaning, it is always small birds that 'zwitschern' (chirp). Typical subjects are 'die Vögel' (the birds), 'die Spatzen' (the sparrows), or 'die Finken' (the finches).
Ein kleiner Spatz zwitscherte fröhlich auf dem Dach.
A little sparrow chirped happily on the roof.
You can recognize the modern, colloquial meaning from the subject of the sentence. If a person (e.g. 'er' (he), 'die Journalistin' (the journalist)) 'zwitschert' (tweets), it means publishing a message on social media.
Der Politiker zwitscherte gestern Abend seine Reaktion auf die Debatte.
The politician tweeted his reaction to the debate last night.
Using 'zwitschern' (to tweet) for social media messages is colloquial. It comes directly from the name of the platform 'Twitter', whose logo is a bird.
Sie zwitschert mehrmals täglich über ihr Leben als Künstlerin.
She tweets several times a day about her life as an artist.
to sing
'singen' and 'zwitschern' can both describe the sounds made by birds. 'singen' is more general and sounds more like a beautiful, melodious sequence of tones. You can use it for many birds. 'zwitschern' is especially fitting for small birds and means short, high, bright sounds. The words can be confused because you can often hear both from birds. If you want to describe birdsong generally or beautifully, 'singen' fits. If you mean many short, high sounds, 'zwitschern' is more precise.

Im Busch zwitschern kleine Spatzen den ganzen Nachmittag.
Small sparrows chirp in the bush all afternoon.
Am Morgen singen die Vögel vor meinem Fenster.
In the morning, the birds sing outside my window.