I spotted what I believe is an immature Northern Mockingbird at the intersection of Vaquero Rd. & Lobo Ln. in Park County yesterday. I spotted the bird several hours before and noticed the striking white wing bars and white on the tail but my primary purpose was pronghorn hunting and I left the area without getting any photo documentation. However, I returned to the area several hours later and found the bird still to be in essentially the same spot and was able to get photos for my eBird entry (See link here:
https://ebird.org/view/checklist/S48387330): Note that I see on eBird that there is only one documented sighting of a Northern Mockingbird in Park County (Late Aug. of 2006).