Ciuffo

This story of the vault is not clear to me. Of the new born I have, three seemed to have a vault, one died immediately after few days drawned, but I do not know yet if the two survivors are male or feminae. One seems to be a male, since is identical to the father, and the other to the mother, both seem healthy and with no problems, but they are just 1 month and half old. Other thing I noted is that this vault was very evident when they were jusy born, but now is disappearing under the feathers. I cannot see the vault on yours, either, I only see feathers. Is it detectable only if you touch them?