pigeon
鵻 centers on a pigeon or dove, a specific kind of bird. The meaning is narrow and concrete, with no strong abstract extensions.
鵻 combines 鳥 (bird) with 隹, which is itself a pictograph of a short-tailed bird. Together they reinforce the bird meaning, specifically a pigeon or dove. The exact historical development is uncertain, but the character likely originated as a specialized bird name.
The left side is 鳥 (bird), and the right side is 隹 (another bird shape). Picture a pigeon with a short tail like 隹, cooing on a branch.
For スイ, imagine a pigeon cooing softly while sitting on a swing: swing -> スイ, and the pigeon sways gently.