first day of singing/chirping (for a species in a given year)
Technical term used in phenology and nature observation; refers to the first date in a calendar year when a particular bird, insect, or other singing animal is heard. Often contrasted with 終鳴日 (last day of singing).
See also: 終鳴日
The first day of singing for the Japanese bush warbler this year was March 15th.
The Japan Meteorological Agency observes not only the cherry blossom blooming date but also the first day of cicada singing.
Compound of 初 (first) + 鳴 (singing/calling) + 日 (day). A straightforward technical coinage.