birds are over rated. i can walk (many though not all, they do fit through smaller holes in fences) where a cat walks, but i can't fly where a bird flys. also humans did not invent cats. cats and birds were both here long before we were. and people do feed birds too, so its not like the interference were all one sided. so personally, feral cats (and other creatures that also occasionally catch and eat birds), give me as much pleasure seeing them, as feral birds. (and as for threats to bird populations, insecticides and other agricultural chemicals are as much so as cats or foxes)
zipper and velcro could do that. a pair of gray striped brothers in a house i once lived in five years ago.