Cat suddenly hunting

[BluGreenBlu]

Our year old boy has suddenly developed a taste for hunting birds. He’d only ever caught one before and now suddenly we’ve had three in the past two days- my husband just watched him pounce on one in the garden, so I know he’s not picking them off a nest somewhere.
I suspect I know the answer but is there anything we can do to rein him in a bit?

Keep him in? Our old neighbour used to get really upset by our previous cat hunting, but while I don't much like it myself, it's just what cats do if you let them out. We keep ours in overnight which probably helps, and I know people say to get a collar and a bell but I'd be too worried our boy would injure himself even if he tolerated it. (We also do "mouth check" prior to admission to the kitchen via the utility.)