Bruce noticed this pile and pointed out both the hollow - which he believes is part of the design and should be called a "thunderbird nest" - and also pointed out the unusual rock at on the side of the hollow. It was a piece of gray knapp-able rhyollite (sometimes called "felsite").Correction: Bruce email'ed to say: "...used the term [Thunderbird Nest] loosely with you but never assumed you thought I was really serious about this. This is an offhand name I use for piles with a hollowed pit. I have no idea what their intended use was and also know you think they're just from pot hunters.".
I would add that it is possible these are architectural features but sometimes they seem like clear examples of post contruction vandalism.