Hello Wayne and Jim,
On the other hand, many passerines do begin southward migration in late
July, and some even earlier. Last night while camping on Lake Kapowsin in
Pierce County I heard 6 or 7 warbler notes, a couple of sparrows, and a
Swainson's Thrush, all in nocturnal migration (presumably, as I've never
heard of a claimed case of "nocturnal post-breeding dispersal").

I would somewhat disagree with Wayne and say that in many cases,
particularly with passerines, the distinction between migration and
post-breeding dispersal is poorly defined and not critically important. It
is all part of the relatively drawn-out process of the birds, having wrapped
up (or abandoned) their reproductive efforts, getting themselves down to the
wintering grounds in good condition. Birds in fall migration don't exhibit
the same fiery tenacity to move that birds do in the spring, because they
generally have less to lose by taking their time. The later push of mostly
hatch-year birds further emphasizes the length of fall migration.

On a general birding note, also at Lake Kapowsin yesterday and today were a
Red-eyed Vireo, an adult Northern Goshawk, and at least 8 Purple Martins,
which I saw here last summer as well. The martins are almost certainly
nesting in snags bordering the lake, as they were all there right up until
dark, and "singing" loudly in the morning. It looked like there were a few
juveniles in the mix.

Cheers
Charlie Wright
Bonney Lake, Washington