Hello. Before we can even attempt to determine gender, we need to know which type of Finch you have. I am assuming these are Finches from a pet store or breeder and not wild, outside Finches, yes? If you got them from a store, surely they know which type they are and can tell you or else there was a sign telling which type. Most likely they are Zebra or Society Finches but in any case, we have to know what they are first. If you can find out for me, we can go from there. Thanks, Patricia