Magnetic Bird Poop
I've seen quite a few bikes with bullet hole stickers on the fenders. Personally, I don't care much for them, but I can understand ...

https://www.bikernewsonline.com/2006/01/magnetic-bird-poop.htm
But, I just don't understand bird poop stickers. Actually, these are magnets. I don't think any biker would put these on their bike, and I don't see anyone putting them on their cage.
But it would be a great gag to pull on one of your buddies!