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 Cornell bird lab website has these “Cool Facts” about the Lincoln’s Sparrow:
“Sometimes, singing a beautiful song might not be enough to win over a female. In a laboratory study, female Lincoln’s Sparrows were more attracted to males that sang during colder mornings more than those singing during warmer mornings. This may be because males singing in the cold showed off more than just their song; singing in the cold requires more energy and could mean that males singing in the cold would make better mates.
John James Audubon named the Lincoln’s Sparrow after his travel companion Thomas Lincoln, who accompanied him on an expedition to the coast of Labrador. The expedition found the sparrow in a valley in Natashquan, Quebec, and Mr. Lincoln was the only person who managed to bring back a specimen for study.
The Lincoln’s Sparrow shows less geographical variation in song than any other species in its genus, perhaps a result of high dispersal rates among juveniles.
The oldest recorded Lincoln’s Sparrow was a male, and at least 7 years, 11 months old when he was recaptured and released during banding operations in Colorado on 2002. He had been banded in the same state in 1995.”