I go in and out of my front door all the time. When I first saw the spot of mud on the window (the big black spot in the photo), I didn't really think about what it was or how it got there.
I only thought, "I'll have to get out there and clean that window."
See the bird next to the spot?
Sayornis phoebe, the Eastern phoebe.
Phoebes are flycatchers, and will fly off a perch, snatch a flying bug, and fly back to the perch.
They are tail-waggers, which means they drop their tail feathers and jerk them back up, over and over.
They are named after their call: phoebe! phoebe!, which they repeat endlessly and at length, until you are ready to blast them with your Airzooka in exasperation.
(I was mildly surprised that when our son learned to talk, he did not call them "Shut up, phoebe"s. Because that's what I mostly call them: "Shut up, phoebe!")
Mud nests about 5 inches in diameter. The ledge is just over an inch wide. This fact will not stop a pair of phoebes from plastering the entire top of the ledge with mud and bits of moss in the vain hope that maybe the ledge just needs a little encouragement to grow wider.
I'm hoping the paper I stapled over the top of the door will discourage them. I love birds, but even if the ledge was wide enough, I'm not giving up my front door to a tyrant flycatcher!