The fact that several species in Australia are now extinct kinda shows cats do harm bird populations. Cats are usually an invasive species and hundreds of them in an area can decimate local wildlife. Overcrowding only kill birds when there are too many, while cats will always kill birds. There are definitely places where it matters more, like on small islands, but in general any invasive species can massively shift ecosystems.

[–] gmtom@lemmy.world 2 points 3 weeks ago

Cats are a recent addition to Australia. Cats have been in the UK fir thousands of years.