Tried again tonight, joining about half a dozen other diehard watchers. In the space of just under an hour, we saw two swifts darting about and circling. One finally appeared to dive into the chimney. The other was not seen again. We went home. Will mark our calendars to revisit the scene in September 2017.
P.S. This is an example of what was supposed to happen, what we were trying to see (this from September 2013 in Portland):