I was driving past the Buffalo Ranch today on my way to meet up with Brad and Caroline for an afternoon hike. To the North across from Coyote or Dorothy's (I always get these two mixed up) a small herd of bison grazed in a hollow. I slowed down to check for calves. And there were! One above and three down below. About then, I noticed a black shape moving in from the northeast. A wolf!
The un-collared frosted black wolf approached the lone bison and calf. Stopping a few yards short, it nosed around at a bare spot on the ground. Afterbirth? A few minutes and it started moving in on the bison again. Suddenly the cow charged and the the wolf sprinted away. But then there it came again sneaking in from a different direction. Again the cow charged and the wolf ran, only to work in close again. This continued a few more times when the wolf gave up and moved onto the small herd below. But they were ready for her and marched forward as a group to keep the wolf at bay.
I watched a bit longer, but it appeared the action was over. I packed up and drove on.
I've watched wolves, sometimes whole packs try for bison calves in the past. I've yet to see them be successful.