The bassist, who has two children with the Boardwalk Empire star, has penned a track called Moving, which is about the couple's long-distance relationship, and frontman Fran Healy insists the song is a beautiful tribute to MACdonald.
He tells British newspaper the Daily Record, "Dougie wrote the song Moving. It's a very personal song about his life with Kelly.
"It's about them always moving around and coming to the resolution that 'It doesn't matter where we live, so long as I'm with you. I live where you are'. It's an amazing and beautiful statement, there's a real clarity to it."