godan verb (-su), intransitive verb
start singing; begin to chirp; break into song
Used for birds, insects, or animals beginning to vocalize. Often implies a sudden or noticeable onset.
朝になると、小鳥が一斉に鳴き出す。
In the morning, the small birds all start singing at once.
セミが急に鳴き出した。
The cicadas suddenly started chirping.