Coming back from breakfast today, I noticed spatters of mud on the screen door. This is odd, as the door is recessed, three feet from the nearest mud, and out of the wind. There was also mud on the concrete. So I looked up.
Some enterprising swallow (probably the fork-tailed one that's been yelling at me when I leave for work) is building a nest right above my front door; there's already a good mud foundation on the brick, about 10 feet up.
This explains why the cat is having random attacks of "Bird! Die now! Why can't I get through the glass and kill you?" when she looks out the window that covers the aerial approach to the front door.
Given that swallows like to return to nesting sites, this may have to be dealt with soon, before they complete the nest and use it for a month.