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
The name 早苗鳥 can be found in old waka poetry as an alternate name for the lesser cuckoo.
The derivation is uncertain; the kanji 早苗 (rice seedlings) and 鳥 (bird) may reflect a seasonal association with rice planting.