Well, I saved my pennies, sold some ukuleles and put a bid on eBay. The result is a new/old zoom for my Canon 600D. I’d read a number of stories about old sigma lens that did work with old EOS Canon cameras, but don’t work with the newer ones. Nevertheless, this one works very well with mine.
I’m still working on getting consistently clear shots at 500 mm (no mean feat at the best of times), but one or two from this afternoon’s test run show promise, at least. The main problem was the cloud cover that only occasionally let the sun through.
Anyway, here are a few samples (all the really bad ones I deleted). The best one is of the bird that never moves.