the egret
鷥 refers specifically to the egret, a type of white heron-like bird.
鷥 combines 絲 (silk, thread) with 鳥 (bird), likely chosen for its phonetic value and the egret's fine, silky plumage.
The top part 絲 looks like fine silk threads, and the bottom 鳥 is a bird. Imagine an egret with feathers as smooth and white as silk.
For シ, picture the egret's silk-like feathers shimmering in the sun: shimmer -> シ, and the egret's plumage glistens.