I've just returned to Tweeters after a few weeks in SE Alaska and the central
coast of British Columbia (some good birds, big salmon and bigger whales).
Perhaps I can add a bit to Dennis's response to Eric Greenwood.

1. A few pairs of White-throated Swifts are nesting on the bluffs just to
the east of Princeton (above the highway), which is about 80 km (50 mi) west
of the Okanagan valley, where, as Eric points out, this species also nests.
Although Princeton is still east of the Coast (Cascade) Mountains, it is
conceivable that one or two may have found their way through the mountains to
the Chilliwack/Cultus Lake area. However, as Dennis points out,
identification is the first priority, and I know that I have been fooled a
number of times by what appears to be white on the bellies (or flanks) of
swift Black Swifts, especially with the sun a low angles. Neverthelesss,
keep looking.

2. The Vancouver Island population of Purple Martins has grown remarkably in
the past decade, starting from one pair known to be nesting on the Cowichan
River Estuary at Duncan in 1984. I placed the first 24 nest boxes there in
1985 to 87 and others (Calvor Palmateer, Darren Copley, etc.) have since
erected many more boxes along the SE coast of Vancouver Island. I don't
have current estimates of nesting pairs or numbers fledged this year, but
very encouraging numbers are now found at Cowichan Bay, Victoria West,
Esquimalt Harbour, Crofton, Chemainus, Nanaimo and possibly even as far north
as Campbell River.

It occurs to me that some of these birds may be island-hopping from Vancouver
Island to the Greater Vancouver (BC) and Bellingham area, rather than
crossing south over Juan de Fuca Strait to the Olympic Peninsula. They are
currently being reported some distances from their known nest sites here on
Vancouver Island as they disperse, so a pre-migration flight to the lower
Fraser River valley (and Cultus Lake) may not be out of the question.

3. One other point comes to mind, however. On 3 or 4 occasions in August I
have seen Common Nighthawks feeding actively with Black Swifts, especially
during early evening "hatches" of termites (or flying ants?) from beach logs
and lake shores. The two species have usually been in large aggregations.
Therefore, since Eric saw the Purple Martin while watching Common Nighthawks,
I must ask if there is any chance that identification is in question here,
too?

Perhaps others have some observations.

Bryan Gates, Victoria bgates at assessment.env.gov.bc.ca