The bull charged me again

The warning sign Jess and I had printed over a year ago.  I based the design on one posted at the Tallgrass National Preserve in Kansas.  The picture is hard to misinterpret.  When we first received the printed signs in the mail we decided to hold off on posting them.  At that time it felt somewhat excessive, but I now intend to post them.
As I've mentioned before in my previous blog post, our recently acquired bison bull has become more aggressive towards me over the last few weeks.  The spring mating season (not as common as the late summer mating season) is in full swing here and this afternoon he gave me his most threatening display of aggression to date.
The bull (the one with the largest horns on the left) attempting the divert the herd away from me. 
I was walking around outside the pasture fence with my camera, when the herd (at a distance of about 150 yards) observed me and began to walk in my direction.  The bull immediately trotted in front of the others in an apparent attempt to head some of the females off and move the herd away from me. 
The bull after his attempt to divert the herd away from me failed.  As you can see in this picture his tail is partially raised, which, in this context, probably indicates annoyance.
This attempt on his part failed, however, and they simply walked around him and continued in my direction until they were about 20 yards away.  At this point they stared at me for several minutes while I took photographs.  Then, one by one, they lost interest and returned to their grazing.  As the herd moved away from me, the bull suddenly turned and charged towards me at a dead run from 50 yards.  
The bull shortly before charging me.
Though this was probably not the most frightening moment of my life, I do recall that time seemed to slow down as the distance closed between us.  I recall at 40 yards thinking "he's not going to charge through the fence," and, seconds later, at 20 yards thinking "should I run or stand my ground?"  After realizing there was nowhere safe to run to, I decided to stand my ground and, fortunately, he stopped at 10 yards.  He then proceeded to stare me down for several long minutes before returning to the herd.

Based upon this and the previous experiences, I've decided to increase my distance from the bison (while on foot) to 250 yards during breeding periods.  I hope this will reduce the risk of being gored.

On a completely different note, it appears that a certain species of bumble bee has chosen today to emerge from hibernation and reproduce.  Everywhere I went on the ranch today it seemed there were bumble bees buzzing around.
Bumble bees in our driveway. 
At this same time I've also noticed the presence of many small bumble-bee-sized holes in the ground.  I speculate that these holes were where the bumble bee spent the winter hibernating, but I don't really know for sure.