What part of speech is a field mouse scampered by?

This phrase doesn't contain a single word that is a part of speech on its own. It's a prepositional phrase, acting as an adverb.

Here's the breakdown:

* "by" is the preposition

* "a field mouse" is the object of the preposition

The entire phrase modifies a verb (e.g., "The cat was startled...by a field mouse.").