RP75

They may well have been referring to the actual visa application part of the process, as that's when you will need to provide the beneficiaries birth certs. For the I-130 they're not required though. 
 

This official uscis document gives very clear instructions for documentation specific to each situation
 
https://www.uscis.gov/sites/default/files/document/forms/i-130instr.pdf
 
You would do well  to study this carefully. It will answer not only this question but many others , and ensure your application is complete and accurate and less likely to get rejected or receive an RFE