I picked up on their call and found a group of 3 or so among saplings and low bushes. The area small trees covered an area passing over a creek connecting two of the ponds in Stiles & Hart. I stopped and watched them for probably a half hour and even saw a Northern Waterthrush walking along and pecking at the edges of the creek but unfortunately didn't get a picture.
It was very difficult photographing them between the emerging leaves, but here are some of the best pictures. I will also point out some tips for male yellow-rumped warbler identification.
First, I notice the black inverted "V" on its breast:
Here are some other great pictures I think help give different perspectives of the bird: