The guy next door adopted two shelter cats and kept them in all summer as they got used to their new lives. He let them out about 2 weeks ago and the result has been carnage. The ground nesting birds I watched all summer have been slaughtered. I'd say numbers are down by 75%. They also picked off a female mourning dove. The usual - well fed cats.
A similar thing happened when at my old house, the neighbour's cat had surgery that kept it out of action for a number of weeks. My yard had a rich bird life, but once the cat was out, it had a lot of blue jay feathers blowing around. I liked those jays - they were neat creatures.
I imagine the local bobcat will seek them out, which will be a sad ending for the wandering pets. I like cats, but they are invasive thrill killers.
My house cats stay in. Smart cat owners around me keep their cats in because the coyotes take care of any that are left out, as well red tailed hawks and one really big owl. My cats are all rescues, all fixed, relatively spoiled and they like to watch fish. I also added wide window sills for lounging and watching birds on the back waterfall. They are enjoyed.
There is one feral in the neighborhood, she is spayed, she is smart, she is 4 years old and was spayed at about 4 months of age. She mainly gets mice rats and voles, I do keep her fed. Trouble is people dump housecats out here. They usually show up on my porch, get neutered and found a home. My bird feeder is in my pond area in the back yard and my dogs are very large, it is a no-cat zone. Don't have too much trouble with her birding, very rarely she gets a little one at one of the front waterfalls. But I do like finding the rats and voles she takes out, because they do a lot of damage.