I would also be amazed if a crow could lift a big enough rock to damage a panel. I have a pile of panels in use; they're pretty damn tough to break that glass. Only time I've seen one broken was the wind tore it off it's mount and smashed it into the top of a fence post. I measure the production afterwards and it was still 90% despite the glass being crazed across the entire surface.

"there was a high possibility of actual damage occurring, such as the cover glass being broken by falling rocks or the panel surface being soiled with droppings" from the Japanese article via google translate.

Our crows here are larger than the US ("grackles" in Texas may be close, but I think they're still smaller), but I still wonder if they could drop rocks heavy enough (or repeatedly enough) to do anything.