Birmingham overpowered struggling Southend to go five points clear at the top of the Championship table.
DJ Campbell put Birmingham ahead after eight minutes, taking Gary McSheffrey's pass to score from 18 yards.
Stephen Clemence doubled the lead seven minutes before half-time, shooting into the ground and over Darryl Flahavan.
McSheffrey's free-kick made it three after 54 minutes and Radhi Jaidi's header rounded things off six minutes before the end.
"If we had got a good start today then the tired minds and tired legs from our midweek Carling Cup quarter-final defeat might have gone away.
"But when you go down within five minutes then the tired legs and minds get worse.
"We got punished today. Yes we were tired but they are a good side."
"We are on course and have a chance. Fifteen players have gone since last season and we have brought in a lot of younger players.
"It has been a huge turnaround, so we are delighted it has clicked and it can only get better.
"It was a potential banana skin coming here because we are top and they are bottom, so it was a good win."
Southend: Flahavan, Hunt, Sodje, Clarke, Hammell, Campbell-Ryce (Harrold 61), McCormack (Francis 60), Maher, Gower, Eastwood (Hooper 60), Bradbury.
Subs Not Used: Welch, Barrett.
Birmingham: Maik Taylor, Kelly, Jaidi, Upson, Sadler, Johnson, Nafti, Clemence (Muamba 82), McSheffrey (Danns 73), Campbell, Bendtner (Kilkenny 78).
Subs Not Used: Doyle, Martin Taylor.
Booked: Sadler, Bendtner.
Goals: Campbell 8, Clemence 38, McSheffrey 54, Jaidi 84.
Att: 9,781
Ref: M Thorpe (Suffolk).