Help

Our two youngest Kittens are full grown now and became two natural born killers. Every morning they come home with prey they catched during the night. Yesterday morning alone there were four dead birds on our doormat and this morning one bird still alive. They have small bells around their neck but nevertheless it makes no difference. Especially redwings are easy prey for them. I think they hunt as a joint venture, leaving young birds without a chance.
What can we do, besides keeping the cats in house during the night?