さえずる (saezuru) means “chirp” | MotiTan Japanese Dictionary

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森の中を歩くと、木の上で鳥がさえずっています。mori no naka o aru ku to ki no ue de tori ga sa e zu tte i ma su

When I walk through the forest, birds are chirping in the trees.

Describes さえずる (saezuru) as producing a series of light, lively bird calls, often heard as a pleasant song.

鳴くnaku: To make a sound naturally as part of an animal's behavior.
歌うutau: To produce a melody with the voice.

鳥が軽やかな声で鳴く。tori ga karo ya ka na koe de na ku

The native Japanese verb さえずる (saezuru, "to chirp") is an old word for lively bird calls, but its deeper word formation is uncertain.

In Japanese, さえずる (saezuru) usually describes birds, while English chirp also commonly describes a short electronic sound or a cheerful voice.