None of the proposed collars, bells etc. deal with the larger issues of environmental destruction as they are geared to “prevent kills.” Birds’ behavior changes when predators are in the area. They may be less successful nesting because instead of finding food they’re guarding nests or watching out endlessly for cats (and dogs have the same effect!) and may not even nest in areas that have been successful in the past. Even without death they are impacted and their behaviors change. These items are to me just a way for humans to feel better about the destruction they could personally prevent. I’m horrified when the people doing this are scientists but maybe I need to stop assuming they would somehow have higher standards for their own personal behaviors.
If you need your cat to go outside to feel better somehow, build an outdoor exclosure with chicken wire and netting that prevents bird entry. The cats can play without wanton unnecessary destruction and disease spreading.
Other thoughts: Having cats outdoors is often terrible for the cat’s health as far as diseases that couldn’t otherwise be contracted if they had been kept indoors. Also, they may be attacked and suffer greatly at the paws/claws of other cats, dogs, and all manner of wildlife. Picture a society where pet and feral dogs are free to roam around without rabies vaccinations, defecating in your garden, near your well source, terrifying animals and humans and causing traumatic road hit situations with cars, while killing and heavily impacting the behavior of native (and sometimes seriously threatened) wildlife. Would you be okay with this taking place in your yard?
This is me treading lightly and trying to keep my emotions in check. I have very strong feelings about this issue. As someone with plenty of health issues the thought that a neighbor’s choices could sicken me or destroy my property makes me just as irate as if they were intentionally dumping toxins in my water and I believe it should be dealt with similarly.