Do loud noises 'spook' bison?

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Summer thunderstorms, which occur frequently during the hottest weather.
One question that I've been asked about bison has been if loud noises 'spook' them with the implication being that they might stampede.  I have not had enough experience with enough bison to definitively answer this question.  Consequently, the most I can do is relate what I have witnessed thus far.

In our remote part of southern Virginia the pristine rural silence is usually only shattered by one of five things: thunder, military aircraft, artillery, my tractor, and guns.

As indicated by the above photograph, lightning and thunder are not uncommon on our ranch during the summer.  They do not, however, appear to influence the bison.  Even when a deafening bolt of lightning struck the trees within 50 yards (which nearly made me wet my pants), the bison didn't care.

An F 15 Eagle engaged in low level flight training.
Our ranch lies squarely in the center of both a military operations area (MOA) and a military training route (MTR).  This mean that we will occasionally witness either low flying F 15 as they screech overhead during the day or the even lower flying helicopters at night that fly with only navigation lights on (see below). 

 While both the F 15's and helicopters often fly low enough to startle most people, the bison, again don't seem to care.
A bison sleeping as I drive by with the tractor. 
In addition to the MOA and MTR, our ranch is also not too far from an artillery range, which every few months engages in training exercises.  These rounds, which are slightly louder and have a deeper tone than distant thunder (deep enough that you can sometimes feel them as well as hear them), do not appear to bother the bison at all.

The same is true regarding their response to my tractor, though they don't seem to like it when I drive the tractor at night with its headlights turned on.

Finally, the last and only loud noise I've ever seen alarm the bison is gunfire.  The moment they hear a gun firing within 300 yards of their location they become very alarmed.  Their tails raise up, they run to the highest nearby hill, and stand together tight group.  These are all signs of upset bison.  Even a small .22 caliber rifle fired 200 yards away can startle them.

It's hard to say for sure what would cause them to disregard so many other loud sounds and react so negatively to gunfire. Perhaps they have had a negative experience with guns prior to coming to our ranch or perhaps it's instinctive.  Either way it's something we need to keep in mind whenever we consider discharging a firearm on the property.