As we walked farther down the trail we came to the overpass where there were several rock pigeons nesting under the overpass. But we also saw one lone swallow's nest. I think it's a cliff swallow. My confusion though was that I thought cliff swallows always nested in colonies, and not by themselves. If you look closely (and you can enlarge the photo by clicking on it), I think you can see the swallow's tail sticking out of the upper left side of the nest.
There were several thistles along the trail, and many were in bloom. Some of them were already finished blooming for the season. But this one looked like it had just bloomed in the past couple days.