If you've driven under 290 recently, you've probably noticed that the edges of many of the freeway's underpasses are decorated with scores of swallows' nests. On my way to work yesterday, I stopped to take a closer look at some of these.
As you can see, each nest is made of lots of small balls of mud stuck together to form a jug-shaped nest.
While I was looking at the nests, several swallows kept flying in and out of them. I wasn't sure whether the birds were Cliff Swallows or Cave Swallows, because these two species look very similar: Essentially the only difference is that Cliff Swallows have a white forehead while the forehead of Cave Swallows is rusty brown.
Unfortunately, the birds were flying too fast for me to see their heads clearly. So I took a series of photos, hoping some of these would show the swallows' foreheads.
Only a handful of the photos were even partially in focus and most of these didn't help, because they showed only the birds' backs.
However, I finally got one picture that settled the ID issue. The birds were Cliff Swallows.