I've owned a couple of mirror lenses, which I sold when I could afford the FD 600/4.5. The biggest problems they had were low contrast relative to a refracting lens and double-line bokeh in situations that are pretty common in bird photography--water bird wading in reeds, bird in a tree surrounded by branches. A light 500mm lens is attractive for flight shots against a blue sky (bad bokeh not being an issue), but f:8 is on the slow side for that purpose.