Here is what RSPB have to say on the subject of cats and wildlife:
"It is likely that most of the birds killed by cats would have died anyway from other causes before the next breeding season, so cats are unlikely to have a major impact on populations. If their predation was additional to these other causes of mortality, this might have a serious impact on bird populations.
Those bird species which have undergone the most serious population declines in the UK (such as skylarks, tree sparrows and corn buntings) rarely encounter cats, so cats cannot be causing their declines. Research shows that these declines are usually caused by habitat change or loss, particularly on farmland."
Our cat currently doesn't go out as she is too young and stupid (would definitely end up going for a dip), we have tried to put a collar on her but it doesn't stay on for more than a day so no chance of her wearing one when she eventually goes wandering. She is a cat, cats hunt, I'm fine with that. Humans on the other hand are far more cruel in my opinion.