巣: a nest built by swallows and a warehouse used as a den
Read 巣 as a swallow nest and a den for stray cats. Compare building a nest with an existing warehouse having become a place used by animals.
A small 巣 built by swallows
巣 (す) means nest or den. 「軒下にツバメが小さな巣を作った。」 means “Swallows built a small nest under the eaves.” Swallows are the builders, and under the eaves is the stated location.
作った reports the past act of building the nest. This location describes this example; it does not make the eaves a required location for every bird's nest.
An existing warehouse as a den
「古い倉庫が野良猫の巣になっている。」 means “The old warehouse has become a den for stray cats.” The warehouse is an existing space described as having become a den for stray cats.
野良猫の relates the den to the cats. The word 巣 does not establish that the cats built the warehouse, and the example supplies no judgment about crime or pests.
The building action and the resulting use of a space
ツバメが小さな巣を作った reports swallows building a nest; 倉庫が巣になっている describes the warehouse's state as a den. Distinguish the created nest from the existing space used as shelter.
Both are called 巣, so the word is not restricted here to things birds built themselves. Read the builders or the space alongside the verb in each sentence.
Does 古い倉庫が野良猫の巣になっている say that the cats built the warehouse?
Show the answer and why
No. It describes an old warehouse that has become a den for stray cats. Distinguish 作った for the swallows building a nest from なっている for the warehouse's state. 巣 is not restricted to objects birds have built.