Nest plugs come in all forms and contain a variety of materials. The USDA has a pdf of various nest plugs you might encounter. We know that mason bees use mud and so to wasps- but add them to the nest in a different format. Other bees use masticated leaf material or a mix of plant materials. Each plug tells a different story.
Click here for a printable page of different nest plugs
mason bee mud plugs
Leafcutter bee leaf plug.
