(~s, warbling, ~d)
1.
When a bird ~s, it sings pleasantly.
The bird continued to ~...
A flock of birds was already warbling a cheerful morning chorus.
VERB: V, V n
2.
If someone ~s, they sing in a high-pitched, rather unsteady voice.
She ~d as she worked.
...singers warbling ‘Over the Rainbow’.
VERB: V, V n