A fine headed brace from defender Curtis Davies saw off Watford and propelled Birmingham into the play-off places in the Championship.
Davies now has four headed goals in his last seven games for the Blues, who started slowly but improved.
He opened the scoring when he thumped a Keith Fahey cross into the net, and then doubled the lead from a corner.
Chris Burke smashed in a rebound to wrap up a win which extended Birmingham's recent good run.
DID YOU KNOW?
Curtis Davies only scored three goals during his three-and-a-half years at Aston Villa, but has scored four in his last seven games for Birmingham this campaign.
Building on their midweek FA Cup success against Premier League opponents Wolves , and last weekend's 6-0 demolition of Millwall in the league , Chris Hughton's Birmingham started this game slowly.
It was visitors Watford who made the early running, with Hornets' top scorer Marvin Sordell played through on goal.
But Blues goalkeeper Boaz Myhill did well, forcing the attacker wide before seeing Sordell's pull-back cleared.
That would prove Watford's best chance in the 90 minutes as slowly Birmingham began to make an impression on the game.
Burke began to make some headway down the wing and the home supporters had something to cheer when a hanging Fahey cross was powered home by athletic centre-back Davies.
Former Aston Villa man Davies then converted a corner on the hour to effectively put the game beyond Watford's reach.
Burke made it 3-0 when he finished coolly from the edge of the area after Hornets' keeper Scott Loach could only parry a Jordon Mutch effort.
Victory makes it eight games unbeaten in all competitions for Birmingham, who find themselves well placed in sixth.
Sean Dyche's Watford side, however, now find themselves in 18th position - just six points clear of the relegation zone.
Live Text Commentary
Full Time The referee ends the match.90:00 +2:19
Free kick awarded for an unfair challenge on Troy Deeney by Nathan Redmond. Free kick taken by Nyron Nosworthy.90:00 +1:24
Unfair challenge on Troy Deeney by David Murphy results in a free kick. Direct free kick taken by Scott Loach.90:00 +0:16
A cross is delivered by Nathan Redmond, Header from deep inside the area by Wade Elliott goes harmlessly over the crossbar.89:00
Wade Elliott gives away a free kick for an unfair challenge on Adrian Mariappa. Direct free kick taken by Scott Loach.88:13
Keith Fahey takes a shot. Blocked by Troy Deeney.87:41
Outswinging corner taken from the right by-line by Jordon Mutch.86:58
Free kick taken by Chris Burke.86:58 Booking
Booking Carl Dickinson receives a yellow card.86:53
Carl Dickinson gives away a free kick for an unfair challenge on Chris Burke.84:39
A cross is delivered by Chris Burke, Adrian Mariappa manages to make a clearance.82:52 Substitution
Substitution Ross Jenkins on for Jonathan Hogg.82:52 Substitution
Substitution Jean Beausejour leaves the field to be replaced by Nathan Redmond.82:52 Substitution
Substitution Wade Elliott joins the action as a substitute, replacing Nikola Zigic.80:58 Goal scored
Goal! - Chris Burke - Birmingham 3 - 0 Watford Chris Burke finds the net with a goal from inside the box to the bottom left corner of the goal. Birmingham 3-0 Watford.80:17
Corner taken by Jordon Mutch, save made by Scott Loach.78:39
Centre by Jean Beausejour, Jordon Mutch takes a shot. Scott Loach makes a save.77:36
Free kick awarded for an unfair challenge on John Eustace by Keith Fahey. Mark Yeates crosses the ball from the free kick right-footed from right wing, Steven Caldwell makes a clearance. John Eustace sends in a cross.77:01
Free kick awarded for a foul by Adam Rooney on Marvin Sordell. Lee Hodson takes the direct free kick.76:12
Unfair challenge on Jean Beausejour by Troy Deeney results in a free kick. Jordon Mutch crosses the ball in from the free kick.
Jonathan Spector fouled by John Eustace, the ref awards a free kick. Direct free kick taken by Boaz Myhill.71:04
Unfair challenge on Chris Burke by Mark Yeates results in a free kick. Free kick taken by Boaz Myhill.70:34 Substitution
Substitution Chris Iwelumo replaces Prince Buaben.68:52
Jordon Mutch concedes a free kick for a foul on Troy Deeney. Direct free kick taken by Lee Hodson.67:56
The ball is crossed by Marvin Sordell, Curtis Davies manages to make a clearance.64:24
Free kick awarded for an unfair challenge on Steven Caldwell by Troy Deeney. Free kick crossed right-footed by Jordon Mutch from left channel, Scott Loach makes a save.63:27
The ball is swung over by Jonathan Spector, Lee Hodson manages to make a clearance.62:35
Direct free kick taken by Nyron Nosworthy.62:35 Booking
Booking Nikola Zigic goes into the book.62:26
Nikola Zigic gives away a free kick for an unfair challenge on Jonathan Hogg.61:25
A cross is delivered by Chris Burke, clearance by John Eustace.60:48
The ball is swung over by Keith Fahey, Nyron Nosworthy manages to make a clearance. Corner taken right-footed by Jordon Mutch from the right by-line, clearance made by Marvin Sordell.59:44
Jordon Mutch provided the assist for the goal.59:44 Goal scored
Goal! - Curtis Davies - Birmingham 2 - 0 Watford Curtis Davies grabs a headed goal from close range. Birmingham 2-0 Watford.59:10
Chris Burke takes a shot. Blocked by Jonathan Hogg. Jordon Mutch takes the outswinging corner,51:49
Shot by Marvin Sordell. Boaz Myhill makes a save.50:35 Substitution
Substitution Marlon King goes off and Adam Rooney comes on.48:57
Unfair challenge on Marlon King by Jonathan Hogg results in a free kick. Steven Caldwell takes the free kick.48:51
The ball is swung over by Lee Hodson, clearance by Curtis Davies.45:01
The match restarts for the second half.45:00 +1:00 Half time
Half Time The ref blows to signal half-time.45:00 +0:21
Unfair challenge on Lee Hodson by Nikola Zigic results in a free kick. Mark Yeates crosses the ball from the free kick right-footed from right wing, Jordon Mutch manages to make a clearance.44:11
The referee blows for offside against Marvin Sordell. Steven Caldwell takes the indirect free kick.41:26
Nikola Zigic concedes a free kick for a foul on Carl Dickinson. Mark Yeates delivers the ball from the free kick right-footed from left wing, save by Boaz Myhill.34:59
Keith Fahey provided the assist for the goal.34:59 Goal scored
Goal! - Curtis Davies - Birmingham 1 - 0 Watford Curtis Davies finds the back of the net with a headed goal from inside the box. Birmingham 1-0 Watford.34:59
The ball is delivered by Keith Fahey,33:56
The ball is sent over by Chris Burke, Jordon Mutch takes a shot. Nyron Nosworthy gets a block in. Inswinging corner taken right-footed by Jordon Mutch from the left by-line, clearance by Marvin Sordell.33:06
The ball is delivered by Jonathan Spector.31:58
Jonathan Hogg concedes a free kick for a foul on Nikola Zigic. Boaz Myhill takes the direct free kick.30:09
Jean Beausejour sends in a cross, Carl Dickinson makes a clearance. Outswinging corner taken by Jordon Mutch.29:07
Shot by Jordon Mutch. Comfortable save by Scott Loach.28:45
Adrian Mariappa sends in a cross, David Murphy makes a clearance.27:00
The assistant referee signals for offside against Marvin Sordell. Boaz Myhill takes the free kick.26:17
The ball is delivered by Mark Yeates, Steven Caldwell manages to make a clearance.25:14
The ball is crossed by Keith Fahey, Adrian Mariappa makes a clearance.24:30
John Eustace has shot on goal from just outside the penalty box which goes wide of the right-hand upright.24:15
Free kick awarded for a foul by Nikola Zigic on Jonathan Hogg. Direct free kick taken by Carl Dickinson.23:04
Mark Yeates has a shot on goal from outside the area which misses right.22:28
Jordon Mutch is penalised for handball and concedes a free kick. Free kick taken by Carl Dickinson.20:29
Unfair challenge on Nyron Nosworthy by Nikola Zigic results in a free kick. Scott Loach restarts play with the free kick.18:44
Marlon King takes a shot. Blocked by John Eustace. Corner taken right-footed by Jordon Mutch from the left by-line, clearance made by Prince Buaben.18:37
The ball is swung over by David Murphy, clearance made by Adrian Mariappa.17:51
Centre by Troy Deeney, save by Boaz Myhill.16:43
Chris Burke delivers the ball, save by Scott Loach.16:36
Troy Deeney delivers the ball, clearance made by Jonathan Spector.16:04
Free kick awarded for an unfair challenge on Jonathan Hogg by Nikola Zigic. Adrian Mariappa takes the direct free kick.15:37
A cross is delivered by Troy Deeney, save by Boaz Myhill.15:01
Free kick awarded for an unfair challenge on Jonathan Hogg by Jordon Mutch. Direct free kick taken by Nyron Nosworthy.14:27
The referee blows for offside against Troy Deeney. Indirect free kick taken by Boaz Myhill.12:51
Free kick awarded for a foul by Marlon King on Nyron Nosworthy. Scott Loach restarts play with the free kick.11:06
Keith Fahey fouled by Prince Buaben, the ref awards a free kick. Direct free kick taken by Jordon Mutch.10:05
Steven Caldwell fouled by Marvin Sordell, the ref awards a free kick. Boaz Myhill takes the free kick.8:39
The ball is crossed by Lee Hodson.8:29
Marvin Sordell sends in a cross, David Murphy makes a clearance.7:13
Foul by Carl Dickinson on Chris Burke, free kick awarded. David Murphy crosses the ball in from the free kick, Prince Buaben makes a clearance.3:24
Centre by Carl Dickinson, clearance made by Steven Caldwell.0:39
Jordon Mutch challenges Troy Deeney unfairly and gives away a free kick. Direct free kick taken by Nyron Nosworthy.0:00
The game kicks-off.
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