This morning, Tim and I drove down to see what we could see. We heard them before we saw them — a kind of repetitive clicking noise.
I got a crick in my neck looking straight up into the trees. All the nests seemed to be in two groups of trees in Commodore Park, located on the Magnolia side of the Ballard Locks. Considering the size of the nests, it's a little amazing that they are so high up in the trees, where the branches are thinner.
It was a little hard to see the herons through all the leaves, but we were able to spot a few. They mostly seemed to be hanging out and grooming themselves. Occasionally, one would fly away.
This blog has some great photos, and here is a video taken in late March when there were fewer leaves on the trees.