First,
Second, I agree with huntercf
(They are just) hoping you will just cave and get rid of them because someone else doesn't like chickens.
Have you ever had a creditor nagging you on the phone and sending threatening letters? I have...over $4.53--no joke. They keep harrassing you until you send them money. The city has only sent one letter. Obviously somebody has complained. You say that you keep 4 Bantams. I'll bet there are people in your neighborhood that have 4 parakeets, or 4 lovebirds, or 4 guinea pigs, or 4 cats, or 4 dogs--nobody thinks that THEY are breaking a zoning law. If, in fact, you are not zoned for chickens, YOUR actions to keep yours will change the zoning particulars as a precedent, which is how our laws work in the U.S. (That's how you win cases in court, by bringing up examples to prove your case.
Third, JUST like we are all now on forums when our animals get sick, instead of CALLING OUR VETERANERIAN, how about taking this, a legal issue to your ATTORNEY. This is a GREAT time to fight city hall. We in Illinois who have a bankcrupt government, a little worse than the bankcrupt government in Pennsylvania, are finding that "city hall" cannot afford as much manpower as they could when times were better and they could waste a lot of money, like they used to do. If you fight them, you are small change and are not worth a lot of trouble to them. So...hire a lawyer and bring them a lot of trouble! If you do, and he/she files a suit, you will be able to keep your birds until it is settled, one way or the other. If you win, great. If you lose, great, Sue them.
Oh, regarding the $4.53? I started calling their 800 number and reminded them that every letter they send costs them $.44 and I am happy to be on HOLD, so that they can pay their phone company for it. I told them that I will call them and wait on hold EVERY TIME I receive a letter. You must be stubborn--good luck in the good fight.