thrush
鶇 refers specifically to a thrush, a type of bird. The meaning is narrow and concrete, centered on this particular bird species.
鶇 combines 東 (east) and 鳥 (bird). The character likely originated as a phono-semantic compound, with 東 contributing the sound and 鳥 indicating a bird. The exact historical development is uncertain, but the structure is typical for bird names.
A bird (鳥) associated with the east (東): picture a thrush singing at dawn in the eastern sky.
For トウ, imagine a thrush perched on a toe (トウ) of a giant statue facing east: toe -> トウ, and the bird sings from that high spot.