The supposed impact on bird life seems possibly quite overstated. Extensive personal experience suggests that (at least the cats that select me to live with) do not catch many birds at all.

Of the current cat with free-ish outdoor access: Last year birds accounted for 3.6 percent of all recorded catches (5/138). This year birds are 1.7 percent of all recorded catches (1/58). And all of the birds I've seen caught first knocked themselves out striking windows, or flew inside the house via open door and injured themselves trying to escape in panic. The healthy ones don't let cats near them at all.

Of the prior cat, zero birds over 9 years. Of the cat before that, maybe 1 or 2 birds over 18 years.

Of course anecdotes aren't data etc, but, the impact of data is its story, and anecdotes direct and constrain the collection and analysis of data.