In places where cats are native (pretty much the whole old world/Europe, Africa & Asia) it’s mostly killing old or weak birds who can’t fly as well as the fitter, younger birds - they’ve evolved together to get better at hunting/evading each other.

In places where small cats are not native (the new world/Australian & the Americas - especially North America compared to South though due to more convergent evolution in South America) the birds haven’t co-evolved with the cats so don’t see them until it’s too late and they’re already caught regardless of how fit or healthy they are.

It’s a classic case of native species being adapted to deal with native species, and when the invasive species comes it just eats the native one because its skillset is different to the native predators/herbivores/plants/literally any life