Sujatha

Hi! I am a US citizen and have applied for a I-130 for my parents on 3/31/2021. I am trying to apply for I-485 for them and it asks for their birth certificates. They were born in the 40's and 50's so they do not have a birth certificate. Can they self swear n affidavit in front of a notary and use that as secondary proof, since its not a good time to get an affidavit from India? I am also planning to get a 'Birth certificate based on passport' from the Houston Consulate (we live in TX) by sending their passports as another secondary form of evidence. Please advice if this would work or if any other suggestions for getting their birth certificates. Thanks!