The reflective rolls that Ty mentioned seemed to work best for me. I trialed a long sheet between fence posts. It did well, with wind being the only issue, ripping the material away from the staples along the bottom. I put in a board that I had nearby to hold it, then stapled again and it stayed, but did lose some reflectivity, so I'd likely make up specific frame pieces to pin it down better in the future. No issues with birds, as it's reflective, but not in a mirror way.
I also did try a mirror, but it really didn't seem to brighten things up much. And I did purchase two first surface mirrors, but they're really small (all I was willing to spend) and so don't give the reflectiveness the big rolls do.