After the Jacoby Ellsbury injury, the Red Sox were in a pretty bad way when it came to their outfield alignment. They were mixing and matching Darnell McDonald, Cody Ross, Jason Repko, and Ryan Sweeney. The Cubs had an extra outfielder in Marlon Byrd, which led to this from WEEI:
The Red Sox acquired outfielder Marlon Byrd from the Cubs in exchange for right-hander Michael Bowden and a player to be named, with Chicago taking on nearly all of Byrd’s salary, according to an industry source.
Byrd has been productive in his two seasons with the Cubs, though he started the 2012 season just 3-for-43. I'm sure the calm, rational sports-talk radio in Boston won't focus on that.
Michael Bowden is a 25-year-old right-hander who has appeared in 39 games in the majors, all but two in relief. He was a starting pitcher until last year, when he was converted to relief in Pawtucket. He has a career rate of eight strikeouts per nine innings pitched.