Do you have any areas where you don't mow and the old acorns sit ?
There are a lot of insects that nest in the decaying acorns...in particular there are ants that nest in them after a particular moth has laid it's egg in it...the larvae then eats the meat insde the nut leaving a hollow nut...and a perfect entry / exit hole...and there is a particular species that is known as a "slavemaker" ant...it's a social parasite that makes "slaves" out of several smaller host species.
If you find some, I'd be curious to know.