Contact the Philadelphia Community Cats Council trap/neuter assistance. Unfortunately, if you just round up the cats and euthanize them, more will take their place within weeks. The best option is to get them all fixed (which will reduce the howling...). I know for a fact there are folks in this area that are actively trapping and spay/neutering the feral cats. Hopefully you'll see a change soon! See http://www.phillycats.org/