They are adding to that total this fall and bringing along a few friends – like Bonnie Raitt, Jackson Browne, David Crosby, Dar Williams, Shawn Mullins, Indigenous and special guest Joan Baez.

The lineup for each show will vary, with Baez appearing through October 3 and Raitt through October 6, after which Browne takes the stage.

The outing launches September 30 at The Shrine in Billings, Mont., – the first of a handful of shows in Montana – and travels through the West and Midwest, including stops at some Native American reservations along the way.

The tour winds down October 19 at the Menominee Reservation in Menominee, Wisc.

Honor the Earth – which counts buffalo protection and restoration among its prime projects – reports that state and federal officials have killed more than 2,500 buffalo outside Yellowstone National Park over the last four years.

The Indigo Girls have raised more than $465,000 over the years for the group, which has distributed grants to 65 Native American groups for environmental defense, sacred land protections and restoration of sustainable communities.

“As non-Indians, we cannot sit by silently and watch the continued destruction of Native communities,” the Indigo Girls said in an open letter. “The basic issues of human rights, justice and our own survival are ultimately at stake.”

In addition to the Honor the Earth benefits, the Indigo Girls will stay on the road through the end of October, stopping at a number of colleges. The tour coincides with the October 3 release of a 16-track Indigo Girls collection titled A Retrospective.