Southampton swept to the top of the Championship as manager Nigel Adkins celebrated a year in charge in style.
Rickie Lambert opened the scoring from the penalty spot after he was fouled by Steven Caldwell, before Guly do Prado converted a Frazer Richardson cross.
Richardson pulled the ball back for Adam Lallana to sweep home the third and Chris Wood grabbed a consolation for Birmingham from 20 yards.
Substitute Richard Chaplow wrapped up the win from Lallana's pass.
A club-record 13th consecutive home win made it Saints' best start to a season in 103 years.
It has been an impressive year for Adkins, who found the club lying 22nd in League One when he replaced Alan Pardew 12 months ago this week but has now won 38 of his 54 matches in charge.
Birmingham, meanwhile, have taken just one point from matches following a Europa League outing so far this season and are down in 14th place.
Lambert scored a hat-trick last weekend and made no mistake from the spot in the 10th minute to open the scoring, firing in off the post.
The hosts doubled their lead 10 minutes later with a slick move, full-back Richardson breaking clear down the right to cross for Brazilian forward Guly to sweep the ball first time past Boaz Myhill.
The third goal, in the 34th minute, was almost a carbon copy with Richardson allowed to reach the by-line before cutting the ball back for Lallana to tuck away.
Wood forced a sharp save from Kelvin Davis with a free-kick before the break, but scored at the start of the second half with a fierce effort from the edge of the area.
Stephen Carr then drove against a post and Davis denied Marlon King as Birmingham briefly threatened an unlikely comeback.
But in the 78th minute Lallana fed substitute Chaplow to tuck away the fourth.