My three moggies are rather old and less active now, but in their prime two of them unfortunately were real killers. I tried the bell on collar, but she got it off. They could still stalk quite successfully with it on!
We now get a neighbours' young cat in wreaking havoc on the birds. I also use a water pistol when I get a chance.
I love cats and birds, we feed the birds on a feeding area which is quite open of cover, so any cats can be seen sneaking up. We have moorhens nesting now, and I dread to see the little black dots zooming around on the pond. I know as they get bigger, they will follow their Mum on the banks, and gradually get picked off.
When our cats were younger, we had ducklings each year, that again, were dwindling in number each day. One year we couldn't bear it, so caught the last two, and raised them, first indoors with heated pad, old teddy as Mum, use of bath etc, then in an outside run. Mum would come and sit next to them. It was a lot of time consuming work for a couple of months, but we then released them and their teddy Mum, and they eventually went back to the wild. We should have ringed them to see if they ever return.