Where? Missoula Montana, United States of America. Specifically, Greenough Park, on the riverbank (as mentioned earlier)
When? today, May 28, 2019
Behavior? They were hard to approach and scuttled away fairly speedily whenever I approached.
Population? They were everywhere, everywhere I looked they seemed to be there, occasionally, I found them in pairs, but never in groups of three.
Additional Info: they were passive until approached, moving rarely if observed from a distance, but when they did move it seemed that their movements were somewhat sporadic, with quick bursts of speed and then stopping and resuming (if they found that necessary)