SouthamptonÂ’s James Beattie scored twice to take his tally to 11 in his last eight matches to push his claims for an England place still more firmly.
Beattie put away a penalty in the 58th minute and then scored from ten yards with eight minutes remaining.
Southampton, on the back of his two goals, moved into seventh position to maintain their unbeaten home record.
Birmingham badly missed the suspended Robbie Savage and created few opportunities of their own.
They were indebted instead to their goalkeeper Nico Vaesen who made two outstanding first-half saves with his legs from Anders Svensson and Brett Ormerod.
Clinton Morrison forced Paul Jones into a sharp first-half save and dragged another effort across the face of the goal but they were rare moments of Birmingham aggression.
Blues manager Steve Bruce believed referee Jeff Winter should have given his team a penalty when a shot from Bryan Hughes appeared to be deflected, but Saints were always in control.
They had to wait until the 58th minute when defender Darren Purse handled under no pressure to allow Beattie to put away the spot-kick with his customary efficiency.
Only then did Birmingham mount a serious challenge and they were caught on the break with seven minutes remaining when substitute Jo Tessem got away down the right and his low centre was touched on by Ormerod for Beattie to finish off from ten yards.
Southampton had Rory Delap, outstanding in midfield, booked for a first-half challenge and Mr Winter showed yellow cards to Birmingham's Damien Johnson and Morrison for dissent after the penalty appeal had been rejected.
Southampton might have won by a bigger margin, but Anders Svensson bent one shot wide in the first half and then with only the goalkeeper to beat, sent in a tame shot when a goal looked certain.