Instead of requiring birth city, make it optional.
Birth certificates do not always give the city of birth. Many birth certificates only list county. Requiring this information means creating additional records be kept because the annotated birth certificate should be what is on the birth certificate, not what you verbally ask a parent.
The birth city is optional already.
The only required Birth Location field is the the Born Outside US field. We understand that not everyone has this information. It does help in matching so if you have it please provide, but if you don't that is why we made optional.