"Byrd, what a game he had to get us back in the game," manager Bruce Bochy said.
Byrd got the party started in the first inning when he slapped a two-run double down the right-field line that snuck inside the foul pole to bring the Giants within a run.
In his third plate appearance with the Giants down by four, the right fielder nearly mimicked his first hit, punching a first-pitch fastball off the right-field wall to drive in Brandon Belt.
"I got enough carry, enough backspin to get the ball off the fence twice," Byrd said. "It was just one of those days where the swing's feeling good."
Byrd tied a bow on his phenomenal day at the dish by smoking an RBI triple to right-center to cut the Cardinals' lead to 7-5 in the eighth.
The newest member of the Giants could have had a perfect Sunday, if not for a spectacular diving catch by the Cardinals' Brandon Moss in the third inning.
With San Francisco trailing, 4-2, Byrd lined a ball that appeared it would drop in front of Moss, but the left fielder laid out to rob him of another RBI hit.
"If that gets in there, it changes the game a little bit," Bochy said.
With the Giants missing Hunter Pence (oblique), Byrd's acquisition has helped patch up the middle of Bochy's batting order.
In nine games wearing black and orange, Byrd has seven extra-base hits (including three home runs) and has hit safely in six of those contests. He's also driven in 14 runs during that span.
"He's definitely been big," Matt Duffy said. "He kept us in the ballgame today and gave us a chance to win. It's nice to have another bat like that, another veteran bat."
The birthday boy, however, was not satisfied with his impressive performance.
"Yes," Byrd said with a smile when asked if there could be a better way to spend his birthday. "With a 'W.'"