From an ecosystem perspective, cats are very successful hunters (the record for % of hunts that end in success is held by a small African wild cat^), but if there's a healthy coyote population in the vicinity, feral cats quickly become scarce (as do any overly fat squirrels).
^ https://youtu.be/nl8o9PsJPAQ
Crows and ravens predate on baby birds and nestlings, but again this is a dynamic ecosystem norm. A backyard birdfeeder for songbirds tends to draw bird-hunting hawks too, which is fine, they're incredibly acrobatic and watching them hunt is quite the show - plus, natural (unsubsidized) predation removes the chaff from the wheat, improving the songbird population genetics. Young hawks in turn may be hunted by owls.
That's nature for you - and an argument for not letting pets roam outdoors unsupervised. Additionally, letting cats eat wild animals tends to lead to internal parasites, fleas, viruses etc.