Rare literary name for the lesser cuckoo (Cuculus poliocephalus), a bird associated with melancholy and transience in classical Japanese poetry. The common modern name is ホトトギス.
See also: ホトトギス
The mujōchō is a bird often composed about in old waka poetry.
Another literary kanji spelling for the lesser cuckoo, also read ほととぎす. 無常鳥 is even rarer and emphasizes the Buddhist concept of impermanence (無常).
From 無常 (mujō, 'impermanence') + 鳥 (chō, 'bird'), referring to the bird's call as a reminder of the transience of life in Buddhist-influenced classical poetry.