In the summer, I could see lots of baby acorns on our Burr Oak tree. By autumn, they had mostly disappeared. I found this one acorn shell; proof positive of at least one.
I had been photographing one set of Burr Oak acorns every week, sort of a time-lapse thing. Then one day, those acorns were gone. In fact, most of the acorns had dropped off (during a storm, I think). Fortunately, some remain.
I found only three acorns on our tree.