Originally Posted By: Trav
If its bathroom vent just leave them alone if they are nesting, if you force them to leave or screen it over the chicks will die.
They wont do any harm to your house like rodents.Once they are gone (they will go when the chicks fledge) screen it over.
THIS^
Small birds won't be there for long, their chicks will be flying within two or three weeks, depending on species. Once you see that happen and they've left the nest for good, then just pull out the old nest and cover up the opening with a screen to prevent future issues.
I agree with Astro, it's probably a bathroom vent so a little extra bird poop for a couple weeks won't hurt anything.