For ID Purposes Only (ID Needed)

Sometimes when I am photographing out in the field or in my photoblind I will spot an animal or a bird that is unfamiliar to me.Rather than trying to remember all the details of it I will just shoot a quick documentary image of it. Then I'll to look up later in one of my field guides or try and ID it online. This image is one of those images.I spotted this dark Chickadee coming in to feed with the Chestnut Backed Chickadees at my backyard feeders. It kept landing on the end of a sawn branch before it went to feed. Not a very photogenic perch but at least it would give me some way to ID it. I have checked all of my field guides and numerous places online and so far I have no ID on it. Any thoughts out there ornithologists? Is this a Dark Phase Chestnut Backed Chickadee or something else? Habitat was Mixed Coniferous Forest 2,500 ft. Elev. Coastal Range. Mendocino County, Northern California. God's love and blessings, chris