I've never researched it, but I've seen what a single outdoor cat can do... We had a big orange tabby named "Josh" who was an amazing hunter. There would be dead birds or squirrels (and more than a few rabbits) on our front porch 4 or 5 times a week for about 7 months after we brought him home... Then it slowed way down to just one or two times a week. I never did really think much about it (I was about pretty young at the time) but I believe that he did put a real throttle on the critter population in the neighborhood. Oh, and it wasn't that he was forced to hunt. He ate more cat food than any cat I've ever owned. Plus, my grandfather spoiled him with tuna or a poached egg every morning!
Mark
Don’t forget aboutdogs....dogs can kill more birds than cats do. People dont’ talk about it because they are dog people...but I know alot of dogs that will attack birds every chance they get.....raccoons/snakes/possums, etc. steal their eggs....and bigger birds will eat smaller birds. I witnessed a hawk eating a pidgeon in my own back yard last year...it was still alive. Read my page for why bird population is down.