Broadway’s Linda Eder rarely is alone onstage when she brings her repertoire to audiences. From within, the spirit of Lena Horne, Etta James, Judy Garland, Patsy Cline and others soar from her through the classic renditions of their most popular songs. On Thursday, her most ardent admirers — and maybe some new ones — will have a chance to hear her perform the Broadway, pops, standards, country and jazz repertoire that makes up her newest show, “Songbirds.”