A couple of dozen of these attractive swallows were sweeping at the south end of Lake Merced. The species is an uncommon breeder in San Francisco and these birds were likely late migrants.
This is a close relative of the Tree Swallow from which it differs in having more white on flanks and face. The male (top) shows contrast between the iridescent violet nape and the green back. The female (below) is very similar to the female Tree Swallow, but the white eye markings are a good clue to its identity as well as the short tail.
Digiscoped with Panasonic DMC-LZ5 | Nikon FieldScope 3 | 30X WA | hand-held (no adapter)