Owls

Owls are infrastructure. A hidden society needs a mail system that does not use the ordinary post, and the books answer with birds that know where to find a person who is trying not to be found. Students bring them to school. The castle has an owlery because the joke has to live somewhere.

They are animals, not metaphors you can skip. A snowy owl is also a companion. The species list is less important than the job: news arrives on wings, and it can arrive when you wanted a quiet morning.