巣

1. Origins, structure, and basic meaning

• Structure: Written 巣 and read す, it denotes an animal's nest and, by extension, a place repeatedly occupied as a den.
• Basic usage: It appears under eaves as a swallow nest and in an old warehouse used by stray cats.

2. Common pitfalls and similar expressions

• Built nest versus occupied den
  • The word can name a constructed nest or more broadly a place that animals make their base.