The problem with the "vermin" killing is that you are introducing a nonnative predator, who's food did you just take away? A snake? Hawk? Owl? Fox? All native. Now your introduced predator is gone and you have a vermin problem...no you have lack of natural predator problem, because while your cat was killing "vermin" and song birds he was also killing natural predators by taking their food sources. This is why this is a huge pet peeve of mine! People just dont get the scope of their doings. The drip in the ocean that becomes the tsunami on the other side of the world.I have always kept indoor cats....My one exception was we had a Bengal years back. It was F2 and had a wild side. When locked up in house he licked off his fur until bleeding and kept licking.....We had to let him out. He did great for many years but eventually was hit by a car on my super quiet street. I have to say he always hunted vermin. Mice, moles, shrews and chipmunks. Since he passed my yard is overrun with moles. Yes cats kill birds. They also kill far more rats, mice and other disease carrying vermin so they are valuable as well..