Originally Posted By: zmjezhd
I would treat a flock of sheep as plural, but a herd of bison as singular: e.g., "A flock of sheep were cropping my lawn", but "a herd of bison was blocking the road".


In this case it's the individual sheep that were cropping your lawn, not the flock. On the other hand, it's really the herd that's blocking the road, not any of the individual bisons.

I would say that the flock of sheep was blocking the road and the herd of bison were cropping my lawn.