How do you attach your latex rubber doves to appearing or vanishing cages? What do you place in the bottom hole?

Lou, I tie a very thin monofilament line to the perch, pass it through the hole and using a needle, up through the back. The line is attached to the top of the cage. I found that using two lines side by side will keep the "bird" sitting upright on the perch. I don't put anything in the hole. It isn't necessary since the bird will appear to be sitting on the perch when produced.