Gregg said that, early on, Duane did most of the singing. As things evolved over time, Gregg gradually took on that role. Evidently Duane knew that Gregg was going to be a singer before Gregg did himself and encouraged him in that direction. As his little brother grew more confident in his own voice, Duane continued to improve his singing through his fingers.
~Joe Bell quoted in Skydog: The Duane Allman Story by Randy Poe.I love the metaphor singing through his fingers and it applies to many other guitarists besides Duane Allman. Listen to this climactic portion of In Memory of Elizabeth Reed. Allman finishes his 4 1/2 minute solo with a third and final ululation (technically, an ululation called a trill). Listen to how he doubles the frequency, like an ice dancer pulling in his arms to spin faster (just to mix metaphors for a moment).