A hat-trick from the in-form Jimmy-Jay Morgan and an excellent Kamari Doyle free kick continued Southampton B team’s perfect start in the Premier League Cup.
Morgan netted twice before half-time as Saints could have scored more in the opening half.
Doyle added the third shortly after the break, before Morgan completed his hat-trick in the second half.
Saints make it two wins from two to start their Premier League Cup group.
Dave Horseman made just one change from the team that kickstarted their cup campaign with a win over Everton, as Thierry Small returned from his loan spell in place of Josh Lett at left-back.
The rest of Saints’ side had started to click into gear as a group in recent weeks, and they showed their brimming confidence right from the start against their Midlands opponents.
Dom Ballard was presented with the game’s first chance after just five minutes; he was played in down the right-hand side after a surging run from Jack Turner, but the shot from Saints’ forward was deflected over the crossbar.
Minutes later, Lewis Payne was also found in plenty of space on the right with time to deliver a dangerous cross that was only cleared at the second time of asking.
On the other touchline, Small was enjoying his individual battle and delivered a dangerous cross that dropped to Ballard, but the curling shot was fired over.
Ballard looked to turn provider shortly after as he raced into the area and pulled the ball back for Kamari Doyle, but the latter’s effort was deflected over.
The seemingly inevitable opener did come in the 15th minute, and it was the in-form Morgan who was alive to tap home from close range after Turner’s powerful drive from range was spilled by the goalkeeper.
Morgan almost had his second before the half hour mark, but his header from a corner dropped onto the post.
In the 28th minute, the visitors had their first opportunity of the game as Rico Patterson raced beyond the Saints defence but straight into the path of Ollie Wright who made a vital tackle.
Sam Bellis had an effort that flashed just wide from a tight angle and Ballard had a header also drop wide of the target before the lead was increased just before half time.
Kept relatively quiet after his opener, Morgan fired home his third brace in three games with a powerful effort from distance after being played in via a perfect through ball from Payne.
It took less than ten second half minutes for Saints to extend their lead, this time from an inch-perfect set piece.
After Payne was fouled during a driving run, Doyle stepped up from 25 yards out and found the top corner beyond a despairing dive.
Birmingham did have a chance to pull a goal back through Patterson, but his shot just after the hour mark was wide from a tight angle.
Saints remained composed in the second half and Morgan capped off another clinical display with his hat-trick in the 73rd minute, pouncing on a defensive slip to slot home from inside the area.
The game was beyond all doubt and, in truth, the scoreline could have been larger but Saints picked up a comfortable victory to continue their perfect to start Group F in the Premier League Cup.