Read about how many wild animals an outdoor cat kills in a typical year...
On a serious note: cats usually integrate well with their ecosystem and if they would really pose such a big threat, smaller mammals and brids would just migrate away. edit: I'm speaking of balanced ecosystems where cats have natural enemies, foxes and whatnot.
That's exactly what you don't want happening in an ecosystem.
The majority of birds that cats kill are frail, in poor health, or otherwise likely to have not made it through the season anyway.
[1]:http://www.rspb.org.uk/makeahomeforwildlife/advice/gardening...
I doubt that those are all frail and doomed to an early death.