Ok so...let me throw a curveball here. The largest known flying bird is Pelagornis sandersi. It was similar to an albatross but much bigger. It was believed to be a bird that flew over the ocean like an albatross. The skeleton we have dates from approx. 25 million years ago putting it just a tad before this mural was drawn.
But we dont really know for sure when they went extinct. What we "do" know is a similar sized bird called Argentavis Magnificnes was around to at least 10,000 years ago. So if one very large bird could survive it would seem at least possible that another species could as well.
See where im going here? Might this have been a large ocean going Pelagornis sandersi that flew of course like albatross have been known to do?
Most likely not, but I like considering the idea that he/she was actually slinging at a giant bird.