For internatnal flights, for sure
A birth certificate is acceptable identification for a child.
You actually need a Passport Book and Documents for the baby or a child in order to fly out of the state or Country. The rules are simple. Even if you going to fly out of the country, you must always bring the Passport for your child so that the Airline Service will have the ability to let you travel over the place and get inside of the plane. The birth Certificate will not work, that's only that document of when your child was born in the country not to travel. Only Citizenship, Passport, and Travel document will help you and your child to get anywhere you want and need. Without them even if you can't or unable to compare, they will not let you in and travel in the plan.
www.tsa.gov
I think even for babies you are going to beed a passport. You can call the airport for sure.
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