There are 4 US Post Offices in Callahan County serving a total of 12,762 residents equating to 3,191 residents per post office. It's estimated that approximately 20,370 packages are handled across the 4 county post offices annually. Callahan County is a county located in the U.S. state of Texas. As of the 2010 census, the population was 13,544. Its county seat is Baird. The county was founded in 1858 and later organized in 1877. It is named for James Hughes Callahan, a soldier in the Texas Revolution. Callahan County is included in the Abilene, TX Metropolitan Statistical Area."