first animal sound of the year; first seasonal animal sound
Rare term for the first vocalization of an animal in the new year or a new season, such as the first cry of a bush warbler in spring.
In haiku, the first song of the bush warbler is sometimes composed as a sign of spring's arrival.
Compound of 初 (first) and 声 (voice, sound). The exact historical derivation is uncertain, but it follows a common pattern for seasonal firsts in classical Japanese.