the tailor-bird the working bird
鴂 is a rare kanji referring specifically to the tailor-bird, a small bird known for sewing leaves together to build its nest. The meaning is narrow and ornithological, with no broader conceptual center.
鴂 combines 夬 (likely a phonetic component) with 鳥 (bird) to name a specific bird species. The exact historical development is uncertain.
The left side 夬 looks like a hand holding a needle, and the right side 鳥 is a bird. Picture a tailor-bird using its beak like a needle to sew leaves together.
For ケツ, imagine the tailor-bird making a small cut (kets) in a leaf with its sharp beak before sewing: kets -> ケツ.