Here's what the entire flock looked like from the road. Because the birds are relatively small, it's probably hard to count. If you'd like to try, remember that you can click on the picture to view a slightly larger version. I'll reveal the actual count below the image.
There are 50 (exactly 50!) birds perched on the wires. From top to bottom: 2 on the top wire, then 10, 27, and 11.
The next image will help identify the species:
This was a large flock of Violet-green Swallows (Tachycineta thalassina). Here's an even closer view:
The Birds of North America account says that not much is known about migratory behavior in Violet-green Swallows, but mentions that there is "some flocking prior to migration." The timing seems right for this to be a pre-migratory flock. (Although some Violet-green Swallows overwinter in California, most will spend the winter somewhere between Mexico and Guatemala/El Salvador/Honduras.)
I wish I had an even closer view of this beautiful swallow, but there was a fence in my way. This is the best I could do for now.
Many of these swallows were actively preening, but when I was reviewing the pictures, it seemed as though some of them were just basking or taking in the last rays of the setting sun (eyes closed, facing west).
I checked the behavior section of the Birds of North America account and found this — "Reported sunbathing in large numbers on telephone wires (Bent 1942)." I love when there's a direct match between a current observation and a description in Bent's Life Histories of North American Birds!