Aphids on milkweed watched over by a ladybird larvae.
I was on the west side of the levee, and was alerted by two other birders. After spotting it, I rode my bike to the east side for closer observation. The bird was exhibiting typical flycatcher behavior, fluttering up and back to a perch on the river slope of the levee, and then a perch on a bare shrub right at the water's edge. This was immediately across San Lorenzo Blvd from Riverside Park.