Early in his career, Michael McDonald sang backup for the slinky soft rockers Steely Dan, adding soul to “Peg” and “Kid Charlemagne.” While his work with The Doobie Brothers and on his own cemented him as a frontline star, his collaborative spirit hasn't subsided: He's added harmonic heft to smashes by Christopher Cross and Toto, and he reunited with his “What a Fool Believes” cowriter Kenny Loggins on Thundercat's “Show You the Way.”