Rare term for the lesser cuckoo (Cuculus poliocephalus). The more common name is 時鳥 (ほととぎす).
See also: ホトトギス
卯月鳥 appears in old texts as another name for the lesser cuckoo.
卯月 (うづき, the fourth month of the lunar calendar) + 鳥 (とり, bird), referring to the cuckoo's arrival in early summer. The exact historical derivation is uncertain.