Feral cats use the minimum energy to obtain food. They are excellent mouse hunters, but hunting birds is an inefficient use of their precious energy since healthy birds tend to simply fly away. While house cats who are allowed outdoors do, indeed, tend to hunt birds, ferals eat leftovers and catch small ground mammals and insects. Many people think that ferals are a nuisance and a danger to their pets and should therefore be eliminated. Ferals do get into garbage occasionally.
People also complain that they make noise when mating and fighting, and have endless kittens.
Another complaint is that feral cats attack pets and spread disease to them. In fact, since ferals are wild, they prefer not to interact with us or with our pets at all. Instead, they establish their own territories geographic areas that will support them. While some of them are certainly quite sickly, the majority tend to be in good health a lot healthier, in fact, than some of the house cats I see. The sickliest of cats that are picked up and brought in to veterinarians tend to be abandoned house pets who are not used to looking after themselves.