City suffered a sixth straight defeat as Birmingham ran out comfortable winners at St Andrew’s.
Chris Burke opened the scoring just before the hour mark after the hosts had squandered a series of chances, before Marlon King put the result beyond doubt with a well-taken second.
Blues manager Lee Clark will feel the margin of victory should have been greater, not least after an excellent penalty save from Dean Gerken low to his right to deny Peter Lovenkrands when substitute Martyn Woolford brought down Burke in stoppage time.
It was a night to forget for Derek McInnes, who also saw Cole Skuse and Stephen McManus leave the field with concerning injuries.
The City boss named an unchanged line-up for the first time this season despite defeat at Huddersfield last time out, and it was Skuse who fired the first warning shot early on only to see his speculative volley sail wide from a half-cleared corner.
But Birmingham soon responded with the first attempt on target, as Wade Elliott stung the palms of Dean Gerken who partially spilled the ball and must have been relieved to see it bobble just beyond the far post.
And moments later King had Gerken scrambling low to his left, but this time the former Colchester United stopper was on hand to comfortably smother the danger.
The hosts have been accused of starting slowly in recent weeks, but another Elliott burst from midfield conjured Blues’ third sight of goal inside the first 13 minutes, although this time Gerken was untroubled by a wayward finish.
City were struggling to cope, and ever-dangerous winger Burke was next to exploit the space afforded to a blue shirt, cutting inside and seeing his left-footed shot blocked on the line by Liam Fontaine before Gerken picked up the pieces.
A first City raid of note threatened to break the deadlock against the run of play on 20 minutes, as Albert Adomah
and Sam Baldock exchanged passes before the winger shot straight down the throat of England keeper Jack Butland with Stephen Pearson well placed but unseen by the Ghanaian.
Suddenly, having finally given the hosts something to think about defensively, the first half descended into a lull – City having seemingly weathered the early storm. And seven minutes before the interval, Fontaine’s powerful header required an athletic tip over from Butland following Skuse’s corner.
But at the other end, King cut inside on to his right foot and was only denied by Gerken’s trailing leg to give City another let off.
There was a spring back in the step of Lee Clark’s men, and Elliott should have done better either side of the break, first firing over after exchanging passes with King, before the ex-Burnley midfielder latched on to Ravel Morrison’s through ball only to clear the crossbar with a free run on goal.
City were coming under pressure and were not helped when McManus’ crucial slide tackle to dispossess Morrison on the edge of the penalty area had repercussions for Skuse, who stayed down in considerable pain and eventually hobbled off to be replaced by Neil Kilkenny.
And it was the Birmingham old boy who immediately instigated City’s best chance of the game to date, setting Adomah free down the right flank, who found Pearson wide open 18 yards out only for the Scot to blaze over with time to take aim.
Having squandered the opportunity to test Butland, City were punished on the counter-attack as Burke exchanged passes with King, surged into the box and rifled home from 15 yards.
In truth, the home side were good value for their lead and only a gilt-edged miss from Nikola Zigic kept the score at 1-0. Fed by strike partner King, the towering Serb only had to hit the target from six yards but somehow headed over with the goal at his mercy.
But the same combination were on target to double the trouble as far as City were concerned 17 minutes from time. A seemingly aimless punt downfield was nodded down by Zigic for King to do the honours 12 yards out with a calm sidefooted finish low to Gerken’s left.
City looked a beaten side, with McInnes’ woes only compounded by the late loss of McManus, who was stretchered off after coming off worse in an aerial challenge with Nikola Zigic, leaving the Robins with ten men having made all three substitutions.
There was still time for Redmond and Lovenkrands to be denied by Gerken – the latter from the spot midway through eight minutes of stoppage time – but City’s fate was already confirmed.