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The Short Answer: Lee, I’m hoping you can figure out a way to exploit this behavior to get some landscaping done around your yard!
I wrote to several experts on the behavior of the American badger (Taxidea taxus), and a couple of them said they’d never seen or heard of hole filling behavior. But I did come across a clue in a paper on interactions between badgers and black tailed prairie dogs (Cynomys ludovicianus). Badgers are predators whose primary prey is rodents. One of their methods of capturing a rodent such as a ground squirrel is to trap it underground and rapidly dig down to it. Many rodents counter that technique by having multiple exits to the surface so they can escape from a digging badger. The authors observed a badger that was attempting to dig out a prairie dog. The badger used excavated dirt to fill escape holes in, presumably in an effort to block escape. The paper also referenced older published reports of badgers using dirt or clods of dirt and even blocks of wood to block exit holes.
After reading that, I wondered if maybe your badger friend was hunting, and thought your postholes were exit holes, so it filled them in. I posed that guess to Dr. David Eads, a scientist at the United States Geological Survey’s Fort Collins Science Center, lead authors of the study on badgers and their prey. He said he had not heard of badgers filling in human dug holes, although he had observed prairie dogs doing exactly that. He also said that “During our observations, a badger usually found an opening to an active rodent burrow and presumably detected the scent of prey first, and then plugged nearby openings. We think they refrain from plugging openings unless they sense prey is available.”
Dr. Eads wondered if there were signs of rodents living in the soil nearby where you were installing your new fence. If so, it’s possible the badger had located a nearby rodent tunnel and then assumed that your posthole was an escape hole. That seems plausible.
It’s also possible that badgers just like to move dirt and as with the prairie dogs Dr. Eads has observed, they responded to a strange hole in their territory by filling it in. Or that they are doing it to discourage other badgers from moving into the neighborhood. Unlike prairie dogs and other rodents, which are often out during the day, badgers are primarily nocturnal and therefore not as easily observed. It’s possible there are people all over North America wondering who keeps filling in their postholes overnight. They just haven’t caught the badger hanging around the scene of the crime the way yours was.
In any event, all the badger people I wrote to expressed great interest in this behavior. If anyone ever observes this, please let me know. If you can get it on video, that would be terrific.
Sources: Eads, D.A., D.E. Biggins, S.M. Grassel, T.M. Livieri, and D.S. Licht. 2016. Interactions among American badgers, black-footed ferrets, and prairie dogs in the grasslands of western North America. Pages 193-218 in G. Proulx and E. Do Linh San, editors. Badgers: systematics, biology, conservation and research techniques. Alpha Wildlife Publications, Sherwood Park, Alberta, Canada.
Proulx, Gilbert, and Neil MacKenzie. 2012. Observations of nocturnal hunting behaviour of American Badgers, Taxidea taxus, in southwestern Saskatchewan. Canadian Field-Naturalist 126(3): 226–230.