The tiny Oak Titmouse is one of the species that mates for life. It’s also among the plainest of all birds, grey all over, with no contrasting accent. Males and females look the same. Its only sport is the crest that it sometimes displays, but not always. In the photo here its head feathers lie smooth.
According to the Cornell bird lab, monogamy in this species has a hostile edge. While most birds mate in their first adult season, some don’t. Those have problems. Mated pairs stake out and defend territories. Bachelor birds are not welcome there and have to live in marginal spaces until they get with the program.
I saw only this one titmouse at this spot in the park. Was this one of the ostracized singletons, or was its mate otherwise occupied for the moment? This bird moved so quickly and appeared so briefly that I couldn’t tell.