wagtail
鶺 refers specifically to the wagtail, a small bird known for its long tail that bobs up and down.
鶺 combines 脊 (spine, back) with 鳥 (bird), likely referring to the wagtail's distinctive back-and-tail movement or its slender shape.
The left side 脊 looks like a spine, and the right side 鳥 is a bird. Picture a bird with a very flexible spine that lets it wag its tail constantly: that's the wagtail.
For セキ, imagine a wagtail perched on a stone seat (セキ) by a stream, bobbing its tail: seat -> セキ.
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