Not saying I know this works, just sharing something for others to try who are as desperate as I have been to keep birds out of problem areas.

I have always heard that grease would keep birds away, and have tried a lot of things in the machine shed that have been suggested, such as hanging plastic snakes and owls from the rafters.  I did the rid-a-bird thing, was really a mess when it crusted over and birds kicked the stuff down in the shed and made a bigger mess.  What I have always searched for was a clean substance to use to avoid bird mess where I didn't want them.

Last spring we had birds roosting on the windowsills, made a real mess down the wall where our courtyard was, quite an eyesore.  I found some old wind chimes and hung over the windows, now have been two years bird free in that area.  Might be an accident, might be a solution.

Also had trouble with a big outside clock that had a wrought-iron frame around it.  Birds would nest in the outside frame, mess up the clock face so you couldn't read the time.  I cannot remember for sure what I sprayed on the frame, but I believe it was WD-40, no more birds that year.  This spring it dug out a new nest, first thing I found was Armor-All spray that we use to polish the dash on the car.  Been about three weeks, we see birds going to the clock with grass in their beaks, land, then turn away without nesting.

Hope these two ideas give someone ideas of ways to try to keep the birds from nesting.  I don't mind killing pest birds, but have always hated the possibility of killing a mockingbird.  (Remember the book and movie, "To Kill A Mockingbird"?  Was a big movie in the 1960's.)