Hi Chris, We looked online to confirm their diet as well. Up until now, we always thought that they were herbivores with the occasional opportunistic supplement such as bird eggs.
A few years ago we travelled the Red Rock Highway in Waterton NP where we saw the highest number of Columbian Ground Squirrel roadkill. It seemed there was one every few metres but than never observed this scene there or anywhere else in our travels. I have to say it was quite a surprise to come across. After taking the photo, Tony moved the carcass off the road so that the diner wouldn't become roadkill himself!