Keith Urban performed at a private 40th birthday celebration in L.A. last week for Oscar-Winning actress Reese Witherspoon. He teamed up with Taylor Swift on her hit “Shake It Off,” and later accompanied Reese and the rest of the party-goers on a rendition of Lynard Skynyrd’s “Sweet Home Alabama.” Other guests included Jennifer Aniston and her husband, Kate Hudson, Courtney Cox, Robert Downey Jr. and Keith’s wife, Nicole Kidman.