nest, rookery, hive, cobweb, den
栖 centers on a place where creatures settle and live: a nest for birds, a rookery for birds, a hive for bees, a cobweb for spiders, or a den for animals. All meanings share the idea of a dwelling or shelter made by an animal.
栖 combines 木 (tree) and 西, which likely acts as a phonetic component. The character originally referred to a bird's nest in a tree, and later extended to other animal dwellings.
A tree 木 on the left and 西 on the right: picture a bird building a nest in the branches of a tree as the sun sets in the west. The nest is a home for the bird.
For セイ, imagine a bird carefully laying eggs in its nest and saying 'say' (セイ) as it settles in. The bird's 'say' cues the reading セイ.
Takasu Special Needs School (older name style)