Birdsongs of the Mesozoic have survived the threat of extinction several times during their two-decade career. First, there was the 1987 departure of founding keyboardist Roger Miller, a profoundly creative force within the avant-rock-outfit-turned-new-music-ensemble. Then his replacement, saxist Steve Adams, left after just a year, once again putting Birdsongs on fawn’s legs. But the lowest ebb probably came in the early ’90s, when the band still did get-in-the-van club tours.

“We were on a 20-day road trip and a lot of the dates were in the South,” recounts guitarist Michael Bierylo. “A few days before the last stop, which was at the Knitting Factory in New York, we had to drive all through the night to get to New Haven. We played this place there about the size of Axis with a huge sound system, and there were, like, three people there. We had a live radio broadcast the next morning in East Orange, New Jersey, so we drove there right after the gig. We got in at four in the morning, slept on the DJ’s floor, dragged ourselves up, had breakfast and did the radio broadcast. Then we had the day off before the Knitting Factory show. Ken [Field, the band’s reed player] took the van with the gear into the city, and Rick [Scott, keyboardist and co-founder], Erik [Lindgren, pianist and co-founder], and I decided to take the train to visit various friends.

“And I remember the three of us sitting silently at the platform; by this point we couldn’t look at each other. And Erik was especially dejected — because he booked the whole thing, and he takes everything very much to heart. Plus he was going through a rough time personally. He was sitting there looking straight down at the ground. I remember kind of thinking, ‘Oh-oh, this is it.’ We actually had a very good show at the Knitting Factory, which helped, but when we got back we went our separate ways for about a month or so.”

Three albums and another decade later, Birdsongs of the Mesozoic continue to survive and thrive. They’ve progressed from headlining clubs like the late-but-beloved Rat, once the heart of the local-music scene in Kenmore Square and the site of their first gig, to conducting residencies and playing concerts at institutions ranging from Emory University and the University of North Carolina to the Salvador Dalí Museum and the Disney Institute. Reduced mileage, like absence, seems to make bandmates’ hearts grow fonder.

This Saturday, Birdsongs will mark their 20-year history with an 8 p.m. concert at the Somerville Theatre. The program will embrace material from the band’s entire history — which has yielded eight albums, an EP, numerous compilation cuts, and several soundtracks. Some of Birdsongs’ formative contributors will return to the fold for the night. Roger Miller will play keyboards, and he’ll bring along his current Alloy Orchestra bandmates Ken Winokur and Terry Donahue, percussionists who have contributed to Birdsongs recordings, including the new Petrophonics (Cuneiform). DJ Flack will also perform, on turntables, and he’ll have a visual counterpart in live video shredder Walter Wright. Derek VanBeever will contribute acoustic bass, and Bill Scheniman, producer of the Birdsongs’ spoken-word offshoot project 1001 Real Apes, will read from its texts.

The gig also marks the release of Petrophonics, which represents both a sonic and a compositional breakthrough for the group. It is the first Birdsongs album to consist entirely of fully written pieces — no improvised works, though the ensemble are world-class free-players.

“Since Roger left the band, as we’ve evolved we’ve become less improvisational,” says Lindgren. “We tend to build in pockets for improvisation. For example, in my composition ‘Nevergreen’ there’s a 16-bar section so Michael can play a solo there. It’s not unlike a Beethoven cadenza, where he could improvise for the coda. Most of our stuff today is very tightly composed. We’re more aligned with a Bartók string quartet than a Pink Floyd track off Meddle.”

Field refers to Birdsongs as a “composers’ collective,” and indeed, though Lindgren has the most writer’s credits on Petrophonics, each member has at least one track. Nonetheless, the CD is cohesive. Somehow their diverse backgrounds — Lindgren and Scott are classically trained, Field and Bierylo came up in the trenches of rock and jazz and funk — and similar tastes for raw, passionate music from Iggy Pop to Ornette Coleman jell into something beautiful. “One of the greatest things about this group is how our friendship and mutual respect for each other has grown over the years,” Scott notes.

Petrophonics is Birdsongs’ most consistently melodic album, even as compositions like Field’s “One Hundred Cycles” toy with dissonance and shuffle the lead instruments’ voices with sleight-of-hand grace. On these 14 compositions the band expand their palette with acoustic bass, klezmer-derived sax lines, and a percussive bedrock that bubbles like brewing lava; yet tunefulness rules. Often the melodies are treated like sonic batons, being passed from piano to flute to guitar and back. But even on Lindgren’s “Ptoccata II,” where a multi-timbral bed of pulses shifts constantly, those melodies dominate. The album’s most demanding piece may be Scott’s transporting “Study of Unintended Consequences,” where a kind of sonic tectonics seems to be the guiding principle. Blocks of sound collide or intersect harmoniously, then slide along as the next intersection occurs — sometimes gently oozing away, sometimes thumping off like a malcontent cartoon sledgehammer. The piece reveals the bones of Scott’s contributions as a sound designer, and they make up a sort of skeleton for Birdsongs’ current studio work.

Absence also played a role in the recording of Petrophonics. Although most Birdsongs of the Mesozoic recordings have found the members convening in a studio, the new disc’s tracks were recorded either alone or in clusters of a few members and contributors at a time. “Part of that is a scheduling issue,” says Field, who — like his fellow prehistoric practitioners — has other projects, teaching, and performances pulling at his time. “Another factor is that we’ve all beefed up our homes studios,” says Scott. “ProTools and other sound design tools allowed us to make the record at our own pace, and to do a lot of looping and sampling. But I think we’ve done a thorough enough job so that no one can hear the technology — just the music.”

Birdsongs of the Mesozoic and guests play their 20th Anniversary Concert this Saturday, May 5, at the Somerville Theatre at 8 p.m. Tickets are $20 and available at the box office or by calling World Music at (617) 876-4275 or TicketMaster at (617) 931-2787.

UN-CORKED. After spending the day working radio — regaling WAAF and WZLX listeners with stories of the plaster casters, theories on Hendrix’s demise, and other ’60s flashbacks — former Jimi Hendrix Experience bassist Noel Redding and Mountain drummer Corky Lang dropped into Faneuil Hall poolroom-cum-meet-market the Rack, of all places, a couple Wednesdays ago to debut material from the forthcoming second album by their band Cork. The snag: guitarist/singer Eric Schenkman of the Spin Doctors, the third member, was bottled up in New York City dealing with his infant’s health issues. So, as Lang warned, what they’d do instead was jam with substitute guitarist Charlie Karp, who’s got a band called Slo Leak with guitarist/producer/X-Pensive Wino Danny Kortchmar. It was fair warning, but we’ll get to that later.

Lang was robust and chipper, a disheveled ball of energy before the set when we talked at the lobby of the Bostonian Hotel, the posh oasis across the street where the band had their “green room.” Asked what expectations he has for Cork’s latest, Under the Radar (King Biscuit), he replied, “None. I think that’s the enjoyment of it. I think of this as almost like jazz, because we’re never going to get paid for playing it.”

The project sprang up in 1995, when Lang was in New York working on a Mountain reunion with band guitarist Leslie West, in conjunction with a Sony reissue CD. Redding, whom Lang had known since 1968 when Mountain and the Experience shared a bill, was also working in a nearby studio. And the two met Schenkman, who was recording a cut for a Hendrix compilation at the time. Thus entwined, they threw down together.

A three-song sampler from Under the Radar finds Cork — named after the county in Ireland where Redding has his ’60s-pop-star-appropriate castle — biting hard into meat-and-potatoes rock. Schenkman sings and brandishes a slide to good, gritty effect on “She Stands Alone” and churns up wah-cranked riffs elsewhere. He’s grown as a musician since the Spin Doctors’ heyday. Lang drives with his customary power, and Redding — whom Lang describes as a “guest” member — applies his trademark Marshall-fuzzed bass.

But the Rack set was nasty. They played mediocre tunes save for “Purple Haze,” “Red House,” and “Mississippi Queen,” and the latter two were mauled. Especially “Red House,” which Karp was leading capably until the wizened Redding — who looked and played like a blissed-out version of your slightly dotty grandfather — came in on the wrong key. The song spilled out like a trashcan as the band struggled to pull him in, finally fudging their way into another blues for a few minutes until the tune could be resurrected. And neither Redding nor Lang nor Karp sang well. Lang, who played hard, was the jam’s sole virtue, living up to his claim that “today, I don’t waste a beat. For 15 years I was shooting from the hip. Now I’ve become more efficient.”

So why the Rack? Paul Barclay, its owner, was executive producer of Under the Radar. The band plan to follow the album’s June release with a return to Boston. Let’s hope it’s with Schenkman.