Birmingham City appear to be on course to extend their unbeaten run to six games.
Shinnie broke the deadlock at Dean Court on three minutes with his first goal in English league football.
Oliver Lee crossed low from the right and the one-time Scotland international slammed the ball into the roof of the net from close range.
Birmingham, who have been much the better side, doubled their lead just after the half-hour mark.
Lee won possession and passed to Jesse Lingard, who slid the ball through to Zigic for the Serbian's fourth goal of the season.