Cordylanthus nidularius is a rare species of flowering plant in the broomrape family known by the common name Mt. Diablo bird's beak. It is endemic to Mount Diablo in the San Francisco Bay Area in California, where it grows in chaparral on serpentine soils. This small annual herb is red-tinted gray-green in color and coated with glandular hairs and woolly fibers. The flowers are each surrounded by 2 or 3 bracts divided into three narrow lobes up to 1.5 centimeters long.