Over the past decade, Andrew Bird has released six albums that trace a remarkable artistic progression. His first two solo albums have their way with American jazz and blues, Caribbean melting-pot styles, and European cabaret music from the early 20th century, while his third album, The Swimming Hour, marked a leap into alchemy, tying together threads from across popular music. But his last three albums have sketched out what is essentially Bird's own genre, full of looping violins, otherwordly whistling, and wordplay that flips through a library's worth of science, pop psychology, politics, and mortal fear.

Beneath all this, Bird has also produced a sort of shadow discography in the form of his limited-release Fingerlings volumes. They're full of experiments, sketches, things that got crowded off of albums, and remarkable live performances, and they illustrate parts of his development that we don't get to hear on his albums. Soldier On, a long, eight-track EP sold at shows on his European tour, doesn't bear the Fingerlings name, but it serves much the same purpose, gathering one demo, one remix and a trove of other artifacts from his past couple years of recording. Some come from various artist compilations, some are completely new, and at least one seems like it might be a glimpse of where he's headed.

The remix of "Plasticities" is a nice example of how pulling apart the elements of Bird's tracks can create a sort of exploded diagram of the song, giving each little detail a new airing, while the early version of "Heretics" is a very basic acoustic guitar-and-voice demo. They're nice, but they're for fans, as is the almost ambient instrumental "Sectionate City". "How You Gonna Keep 'Em Down on the Farm" and "The Water Jet Cilice" are slow, sparse, and mournful tracks that hit on a very specific type of melancholy that Bird seems to be able to reach with minimal effort. Oddly, "The Water Jet Cilice" is not the same piece of music as the instrumental of the same title on Fingerlings 3-- this is a vocal song that shares nothing in common with the other song.

The most remarkable track on the EP is the one that kicks it off, though. "The Trees Were Mistaken" is something new for Bird, and really quite good. It takes the looping technique he's perfected over the years to a new level, throwing blankets of sound over a programmed beat, with dabs of pizzicato violin and splashes of electric guitar splattered across the surface. Bird joins the layers of texture with his voice, riding the song's swelling pulse into a thorny den of oscillating violins and eerie whistling. It's a track that makes this compilation more notable than your average side release-in fact, Soldier On is exceptionally good for a quick collection of rarities, a must-have for fans and a genuine expansion of his range.