I have never understood how a birth certificate proves that YOU are anything - there's no photo (not that any of us look like our selves at birth), and nothing really to prove that you are the subject of the birth certificate.
In France, the only thing that proves your French nationality is your carte d'identité. A passport is not proof of French nationality, only the carte d'identité does that. And now they are making your re-prove your nationality when you go to renew your carte d'identité (every 10 years). If you were born outside France, you have to produce a certificate of nationality - fortunately, I still have all my paperwork handy from when I took French nationality, but I still have a few years to go on my first carte d'identité and I can hope that they'll be over with this silliness by then.
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