Purple martin (Progne subis arboricola), a California Species of Special Concern, historically nested in existing wood power poles at Mt Thayer in Santa Clara County. By 2008, only 100-220 pairs were estimated in the southern coastal area of California from Marin to Santa Barbara Counties and are suggested to be close to local extirpation, making the small Mt Thayer population regionally significant. For several years the Midpeninsula Regional Open Space District has monitored martin breeding activity at Mt Thayer and provided a variety of nesting structures ranging from traditional nest boxes to experimental hollow cavity structures that emulate natural habitat, to support state goals of reaching >2,000 martin pairs and to support nesting in cavities rather than bird boxes or houses. In 2024, martins selected experimental cavity structures over traditional boxes, and successfully fledged a record high of 13 young at the site. Cavity structures can be used as an effective tool to support nesting martins and to facilitate preference for natural cavities. The District has formalized the cavity structure design to share with others for implementation at suitable sites within their western range.