Kentucky Warbler (Geothlypis formosa). This was a new sighting for me, so it was well worth my time.
The yellow underside without any spots or stripes, the black mask that extends down the neck like side burns, and the yellow stripe that wraps around the eye were the distinctive characteristics that helped me identify the bird. After reading that these birds nest in bushes near the ground I realized the bird was trying to defend a nest (which I never saw). This also explains the mohawk hair style the bird was sporting. I never heard the bird's normal call, since it was so irritated with the foot traffic along the path.
I did finally make over to the roses, but by then it was hot and there isn't a lot of shade in that area of the garden for comfortable sketching. The roses sketching would have to wait for another day.