Could be worse... the robin that visited my parent's windows every spring would also poop as he attacked, then drag his wingtips through it so he painted "lovely" patterns up the window. You might be able to interrupt his obsession by covering all windows for a few days.
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You might consider taping up hawk silhouettes on each window... I've also seen spider web designs suggested... They sell those as window clings, but I'm thinking you could also print them on acetate and tape them up, and they'd be (relatively) unobtrusive.
Or if you want the real thing, here's a link... looks like it's a couple more bucks per pair from Amazon, but you can get them in 1 day, and if they don't deter your robin you can return them within 30.
https://www.amazon.com/dp/B000HHLJOC/
