Are cats just ruthless killers?
Letting your pet cats outdoors is a controversial topic (and apparently a cultural issue - here in the States, we lean more towards keeping them inside, and the Brits think we're nuts!). Does it prevent behavior problems? Maybe -- but I have to say I have PLENTY of behavior clients with indoor/outdoor cats who fight with other cats, urinate or spray inside the house, or have aggression or attention seeking issues. So letting cats go outdoors is not the panacea for all feline behavioral ills as some might have you believe (I've previously written about some reasons to keep your cats indoors).
A new book "Cat Wars" might have you thinking that cats are the only source of avian woes (I've also written on this topic before for The Dodo - so don't forget about humans, squirrels, raccoons and other animals that make life rough on songbirds).