Regardless of type of sunglasses you get, get ones with lenses that block blue light. Many bird species have colored oil droplets inside their retinal cells. Guess what color those oil droplets are? Yep, pretty much the same range of browns, oranges, copper colors that one finds in various "blue-blocking" sunglasses like Serengeti, Suntiger, etc. Those oil droplets do for birds what our blue blocking sunglasses do for us.