http://cbp.gov/xp/cgov/travel/id_visa/legally_admitted_to_the_u_s.xml
Q: What Documents Must You Present?
A: A U.S. citizen must present a passport if traveling from outside of the Western Hemisphere (The Western Hemisphere includes North, Central, and South America). If traveling from inside the Western Hemisphere, any proof of U.S. citizenship that clearly establishes identity and nationality is permitted, such as a birth record or baptismal record.
Hope that helps. :thumbsup: