At the KC Stadium, Hull thrashed Blues to climb back into the automatic promotion places.
George Boyd and Mohamed Gedo both scored twice for the Tigers with Robert Koren also weighing in with a goal for Steve Bruce's side as they bounced back from last weekend's 4-1 defeat by Bolton in style.
Hull were two goals to the good after just 11 minutes with Boyd scoring after just 46 seconds and Gedo firing home from 12 yards in a blistering start.
Boyd added his second just before the half hour mark as Hull secured a three-goal cushion before Koren struck after the break with Gedo beating Jack Butland again in the 76th minute.
Peter Lovenkrands and Christopher Burke scored late goals for Birmingham but it was never going to be enough, as Hull made the most of Watford's 1-1 draw against Wolves on Friday night to leapfrog them and move back into second place.
Leaders Cardiff saw their margin at the top cut to five points following Hull's win and their 2-1 defeat by Middlesbrough.
Kieron Dyer's first league goal since 2007 helped Boro spring a surprise on Malky Mackay's side when he put the hosts ahead before debutant Sammy Ameobi doubled their advantage.
Aron Gunnarsson's seventh goal of the campaign reduced the deficit after the break but there was no way back for Cardiff.
Brighton continued their own promotion push with a comfortable 4-1 win over Huddersfield.
Leonardo Ulloa grabbed a sparkling hat-trick before David Lopez wrapped up Brighton's third straight win from the penalty spot.
James Vaughan had levelled the score at half-time but Brighton were too strong, with their Argentinian striker seemingly unstoppable.
Blackburn were blown away by a first-half Dwight Gayle hat-trick before Peterborough withstood a second-half fightback to climb off the bottom of the table with a 3-2 win at Ewood Park.
Gayle completed his hat-trick in the 27th minute before Jordan Rhodes and on-loan Wigan midfielder David Jones fired a stoppage-time score.
Leeds completed a 1-0 win over Millwall at Elland Road thanks to Stephen Warnock's penalty after Ross McCormack saw his earlier shot saved from the spot.
The win leaves Neil Warnock's side six points adrift of Brighton in the last play-off position.
Meanwhile, Craig Dawson struck the decisive goal for Bolton in the 3-2 win at Barnsley as they stepped up their play-off push by extending their unbeaten run to six matches.
David Ngog and Jay Spearing scored for Bolton before Tomasz Cywka and Chris O'Grady levelled the scores. Dawson then fired the winner in the 59th minute.
Leicester slipped up in their promotion bid as they lost 1-0 at Ipswich. Substitute David McGoldrick claimed an 85th-minute winner for Mick McCarthy's side to leave the Foxes four points adrift of fourth-placed Crystal Palace.
In the late kick-off, Nottingham Forest's unbeaten start to manager Billy Davies' second reign stretched to four games after an impressive 1-0 win at Sheffield Wednesday.
Forest were in control for much of the game and Radoslaw Majewski's 27th-minute goal did the damage, with the midfielder applying the simple finish after a fine run by Simon Cox.
Elsewhere, Burnley won 1-0 at Charlton thanks to Charlie Austin's first-half goal, while Blackpool drew 0-0 against Bristol City.