Based on wide personal experience, I’d blame something around the size of a sparrow for the “deposit” on SloJoe.
Pigeon droppings are much larger.
And if a full grown bald eagle had done the deed as suggested in a meme or two I’ve seen.....it may very well have knocked the target down.
(Wonder what the SS would have done about that? Call for anti-aircraft fire?)
Okay..../jk.