I thought for sure I found a new-to-me bird. Turns out I was watching immature gulls of some kind. My books disagree on which gulls live in Wyoming; however, the pictures I found and the online website that lists Wyoming birds suggests they are immature Hermann Gulls.
I was able to get good photos because I sat still for awhile. Usually I walk and stop just long enough to get a photo. While sitting, birds came in closer and/or I could photograph them in different positions.