I blogged about a grackle (a female grackle, brown where this male is glossy black) who had adopted our handwashing sink as a toilet and was leaving heaps of grackle-poop there on a daily basis. We set up various keep-aways, including a mosquito net which was quickly pooped through and plastic bowls which were a bit more successful. She abandoned us for a while, but returned to old habits from time to time.
A couple of weeks ago, she came roaring back, up to her usual tricks. And Patti Moore noticed that she was cleaning odd streaks and scratches off the mirror over the sink and theorized that our grackle had seen her reflection, attacked it, and pooped in triumph or sullen defeat (hard to guess whether she'd have thought she won).
This was an easy theory to test. We covered the sink and mirror in black plastic for a couple of days: no poop. Then we just covered the mirror: no poop. Then we took the mirror away and have been poop-free for many days now.
Brilliant deduction, Patti! Thank you for freeing our grackle from her rivalry and for freeing us from the results.