The bell is probably the best instant solution. Bottom line is "Tis the Season".
No matter how you look at it, it is nature and they will continue to catch them. All you can do it keep an eye on them as much as pos and bang the window as much as pos when you see them down feeding and the cats are in stalking mode. Mostly anyway its an evening time activity for cats as in summer days are too hot for thier fur coats. Between 5-8pm is "hunting time" for them it seems.