This shot was taken on the southern slope of Mount Tamalpais - mostly Redwood/Douglas Fir forest with an elevation of about 2000 feet. This was actually the only one I saw in that area... A few days later I went hiking near Lake Bon Tempe a few miles north of Mount Tam, and these tubes were everywhere. The habitat there was dense forest with a pretty even mix of California Bay, Douglas Fir, Redwood, Oak, and Madrone at an elevation of approximately 1000 feet.

If this is the habitat they prefer, you might try looking on Mount Diablo or the surrounding foothills in similar forests to see if you can find anything. I'm sure that if Marin has them they would live in the East Bay as well. Be sure to post some photos if you go out and find some - maybe you'll be able to actually see the spiders in them. I think it was too warm when I went, and the spiders were deep in their burrows.

These are numerous in Big Basin hiking out of the trails from the HQ on Hwy 236, aka Big Basin Way.

I do public walks there, and everything is of interest. If your interested in Spiders I'm just now submitting a mess of them from Big Basin.
Scott

Thank you for the identification. I thought it belonged to some kind of spider, but at the same time it was difficult for me to imagine such a creature intentionally attaching debris to the tube as camouflage. Because of that, I also thought it could have belonged to some kind of caterpillar.

But I imagine that it is both camouflage, and if a small insect crawls on one of the sticks or pine needles radiating out from the center, it is guided right to the trap.