This shrub in the “Nature Area” in the northern part of the park was host to two different birds within minutes as I watched. The first gave itself away by its bright yellow tail patch and soft yellow flank: a Yellow-rumped Warbler. Sadly it didn’t pose for me in a classic full broadside position so that we could see its eyes. It only showed its back and tail.
Moments later — so quickly that at first I thought it might be the same bird — another individual appeared, also obviously feeding off something on the shrub, I couldn’t tell what. This bird was slender, almost dainty, without a single yellow feather visible. After consulting the online authorities I’m leaning to say that it’s a Lincoln’s Sparrow. An advanced birder a few weeks ago had pointed out one of these birds not far from my spot today, and I had got some partial photos. Today’s photo of this bird was way better.
Update 11/15/2020: Expert review now identifies this bird as a Song Sparrow, not a Lincoln’s Sparrow. The Lincoln’s Sparrow continues to elude me.