那只夜莺在树枝上婉转地歌唱。

The nightingale ___warbled sweetly on the branch.

小女孩用颤抖的声音唱生日歌。

The little girl ___warbled the birthday song with a trembling voice.

清晨的鸟儿在树梢上欢快地歌唱。

The birds ___warble cheerfully in the treetops at dawn.