Well, I just read on the web the responses to my post on flocking flickers
and further remarks about wintering Barn Swallows. Interesting comments.

On flickers, I agree with Steve Mlodinov that flocking may well be
correlated with migration, but I don't think that explains *why* they flock.
Lots of other birds migrate without flocking, and of course Y-b and R-n
sapsuckers are woodpeckers that migrate without flocking. Steve, were you
implying they migrate in flocks, actually flying over the countryside in a
loose assemblage of birds that stay together over time? That would be quite
interesting, something I haven't seen, not living on a flicker flyway - or
at least not a flocking flicker flyway.

Birds flock for several reasons, usually concerned with food acquisition,
predator avoidance, or enhancement of social interaction, and I wonder which
of these might be significant for flickers.

I'll have to cogitate over Jerry Tangren's remarks about the difference
between warblers and swallows, as I don't see the difference so clearly.
Both groups are under the same selective pressures, and both groups
presumably have similar post-glacial migratory histories, spreading
northward into higher latitudes as the glaciers retreated. As there are
many tropical swallows, a higher diversity than temperate ones, I don't see
that family as clearly of temperate origin. Just like warblers, tropical
swallows would benefit by migrating to higher latitudes, where predation and
competition are reduced, to breed. And just like swallows, warblers must
retreat from high latitudes in the winter because their prey become
unavailable. However, I speculate that the Barn Swallow (the species
Hirundo rustica) originated in Africa, where it has lots of resident
relatives, then moved into the New World during an interglacial period, so
that species at least probably did do as Jerry wrote.

One difference between swallows and warblers (other than their shorter
bills) is that swallows migrate farther and presumably faster because of
their superior flight abilities. They can afford to move farther into
unsuitable conditions because they can retreat faster from them. Tree
Swallows are well known to do this on the Atlantic coast.

The farthest north that Barn Swallows winter regularly is central Mexico
(including, some years, southern Baja California). Have these southerly
winds been blowing from that far away? If so, I could see these birds being
Pineapple Expressed (PinExed) up here. But that's a very long distance to
fly, even for a swallow, and I'm surprised they would keep going if they ran
into cooler weather and reduced insects. As they migrate by day, they can
easily keep track of local conditions. They would certainly not be
stimulated to leave their wintering grounds by the changes in day length
that have taken place so far since the solstice, so something else got them
moving, if indeed these are birds from the normal winter range. As Wayne
Weber wrote, it would be very interesting to try to understand this, just as
interesting from an ornithological standpoint as the landfall of all those
Bristle-thighed Curlews a few years ago.

Dennis
--
Netta Smith and Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115