Feral birds are more of a health threat than feral cats.
West Nile, Avian flu, histoplasmosis, Chlamydiosis, Eastern Equine Encephalitis, Cryptococcosis, Allergic Alveolitis, for a start.
Presuming you mean wild birds and not some of those vast flocks of escaped macaws I've seen...
The big difference is that the birds are supposed to be there.
The cats should be in an animal shelter or someone's house. If not, they are a nuisance species and should be eliminated like coyotes, etc.