sing, chirp, warble, chatter
囀 centers on the light, musical sounds of small birds: singing, chirping, warbling, or chattering. It describes the varied, often rapid vocalizations birds make.
囀 combines 口 (mouth) with 轉 (turn, revolve), suggesting a mouth producing turning, warbling sounds. The exact historical development is uncertain, but the phono-semantic structure is plausible.
A mouth 口 that turns and rolls 轉 like a bird's song: imagine a bird's beak opening and closing rapidly, producing a continuous, turning warble.
For テン, picture ten tiny birds lined up on a branch, all chirping at once: ten -> テン, and their combined chatter fills the air.