The common poorwill (Phalaenoptilus nuttallii) is found in various habitats across western North America, including areas on Mount Diablo in California. Mount Diablo is known for its diverse ecosystem, including chaparral, oak woodlands, and rocky outcrops, which provide suitable habitat for common poorwills. Common poorwills are nocturnal birds, meaning they are most active during the night. They have a unique habit of hibernating during the winter months, which is uncommon among birds. Their diet consists mainly of insects, particularly moths, beetles, and other flying insects. They hunt for prey during their nocturnal flights. Common poorwills have cryptic plumage that helps them blend into their surroundings, making them difficult to spot during the day. They have large heads, short bills, and wide mouths adapted for catching insects in flight. They typically breed in the spring and summer months. The female lays two eggs in a shallow depression on the ground, usually hidden among vegetation or rocks. Both parents take turns incubating the eggs and caring for the chicks.

On Mount Diablo, common poorwills inhabit areas with sparse vegetation, rocky terrain, and open woodlands where they can roost during the day and forage for insects at night. Birdwatchers and wildlife enthusiasts visiting Mount Diablo may have the opportunity to spot common poorwills, especially during the evening and nighttime hours when these nocturnal birds are most active. They are often heard before they are seen, with their distinctive "poor-will" calls echoing through the night air.