Recently while talking to neighbors I noticed a squash racket placed on the transom over their front door. I asked whether birds had been trying to nest there: Yes, and succeeded. I took a few steps east, and could see the robin's nest nestled among the objects that they had placed there to keep the robins away.
We had a robin or robins trying to nest on our transom for a while this spring. We put up garden tools and scrap metal, we discarded the bits of nest that we found, and eventually we put bird spikes in place. Perhaps the bird spikes worked, perhaps the robins had simply found a better place to go, to the neighbor's house perhaps. I suspect they succeeded across the street because the family was out of town for a weekend. Considering the amount of nesting material that ended up on our porch, I don't think that a ledge so narrow is the best place to build a nest. Yet it is out of the weather and must be hard for predators to reach.
In the back yard, we have a metal arch next to the garage, over which we have trained a couple of rose bushes. Last fall, when the leaves had fallen, I notice a nest near the top of the arch. It struck me as a good spot, for I couldn't imagine a cat pressing through rose branches to get to it. This spring the nest was in use for a while--a robin in it cried out in indignation and flew off when I tried to take a picture of it. It seems to be disused now; a brood would have had time to grow up and fly off. In the meantime, the nest looks like this: