It's probably a mud-dauber wasp nest. Do you see any wasps flying in or out of it? If not, then it's a bird's nest. I can't really tell how big it is by the photo--but it's one of those two things. The hole will be sealed at night if it's a bird.

mudd dobber built that

This thing appeared on one of the plants outside my house around a 1-2 weeks ago. My aunt was also talking about how the hole "reseals" itself at night.