There's no scientific evidence that letting cats outdoors cause a decline in bird populations: and that's according to the UK charity that campaigns for the protection of birds[1].
The majority of birds that cats kill are frail, in poor health, or otherwise likely to have not made it through the season anyway.
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...