The Black Cats arrived at St Andrew's on Saturday having taken four points from the last six possible - at Manchester United and at home to Liverpool.
Hopes were therefore high as they headed south. But that optimism was blown away inside 48 minutes as Birmingham, playing in front of new owner Carson Yeung for the first time, served up a 2-1 home victory.
Strikes from Liam Ridgewell and James McFadden secured the points, although Scott Dann's late own goal prompted a frenetic finish during which the visitors very nearly snatched a point they did not deserve.
It was all too familiar for Sunderland, who had earlier lost at both Stoke and Burnley after good home wins.
Cana said: "It's very disappointing for us, but it's not the first time this season after some good results.
"Especially when we have won at home sometimes, we can't keep our level of play - and we must turn that around in future games.
"Everybody knows the potential and quality of our team. But it's a hard league, and you have to be 100 per cent concentrated all the time and start well in games.
"We must learn from that and change our mentality to keep our level over two or three games.
"We have a lot of work to do."
Sunderland simply did not turn up until the closing stages of the game.
They may count themselves slightly unfortunate to have gone behind when Ridgewell bundled Sebastian Larsson's free-kick past Craig Gordon from a suspiciously advanced position, but no flag came to their rescue.
It was no more than City deserved after front two Christian Benitez and Cameron Jerome had comfortably overshadowed the efforts of the much-vaunted Darren Bent-Kenwyne Jones partnership.
They increased their lead within three minutes of the restart - the first time they have scored twice in a league game this season - and this time, Bruce's men could have no complaints.
Jerome cleverly back-heeled Larsson's cross into the path of McFadden, who stepped inside full-back Phil Bardsley before calmly passing the ball past Gordon and behind the covering Michael Turner into the bottom corner.
Bruce made a triple substitution with 22 minutes remaining - sending on George McCartney, Bolo Zenden and Fraizer Campbell - and his side rallied.
Dann could only help Turner's 82nd-minute header into his own net, with goalkeeper Joe Hart all at sea.
But Campbell headed wastefully over with six minutes remaining, and Bent sent a low volley skidding off the far post with just seconds left.
"How often have we seen it, after the Lord Mayor's Show?" Bruce asked.
"For an hour in particular, we were nowhere near the standards we have set."
For opposite number Alex McLeish, who denied former China international defender Fan Zhiyi has been offered a coaching role at the club, the home performance was as important as the Blues' third league win of the campaign - with Ecuadorian striker Benitez catching the eye.
He said: "Benitez, the new kid on the block, knows where to find space. We found him a lot from the midfield area, from full-back areas.
"He got himself into areas where it's hard for defenders to pick him up. He's a natural in terms of that.
"Judging on that performance, he looks as if he is going to be a star.
"Not only does he create opportunities for himself, he creates for others - and he certainly looks as if he will take a bit of watching by the centre-halves he is going to face."
Sunderland, meanwhile, will turn their attention to Tuesday's Carling Cup fourth-round clash with Aston Villa - knowing they will have to be much, much better.
Cana said: "The cup is a short way to European competition. But it is going to be a tough game, because Villa are a very good team."