Answer: The wasps you see may be checking out the birdhouse to see if it's a good place for them to nest. Typically, nesting birds will defend their nests so I suspect the house is empty and the wasps are just checking out the neighborhood. Since spring is prime time for bird nesting, you'll want to discourage wasps and encourage birds. Try to look inside, without causing too much trouble in case birds are nesting. If there are no birds, you can discourage wasps by either lining the inside of the roof with aluminum foil, or by rubbing a bar of hand soap on the inside of the roof (either will annoy wasps but not affect birds). If there are birds nesting, just leave it to them to defend their own nest.
Good luck to you and your nesting birds!
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