[Tweeters] Lewis's Woodpecker "invasion"-- further thoughts

Wayne Weber contopus at telus.net
Mon Oct 4 13:26:39 PDT 2010


I would like to continue my friendly discussion with Dennis Paulson about
the reason(s) for unusually high numbers of Lewis's Woodpeckers in western
WA this fall. (Opinions from any other birders are also welcome!!) In a
nutshell (bad pun!), I still believe that this is very unlikely to be a
result of a low acorn crop in Klickitat and Yakima Counties. I would like to
bring up a couple of arguments that I did not mention earlier, and to
suggest some alternatives to the "acorn shortage" theory.

At the same time, I concede that both Dennis's ideas and my own are
speculation, and we don't know the real reason for the sudden spate of
records in western WA.

For a start, I was unaware that there were any "resident" populations of
Lewis's Woodpecker. Unlike the Acorn Woodpecker, which is generally
non-migratory and very sedentary, Lewis's (as I pointed out earlier) is
highly migratory. Are there in fact any areas where Lewis's are found in
good numbers throughout the year?

Although there are good numbers in winter in oak-woodland parts of Klickitat
County, my impression is that they don't breed in those areas. (Somebody
please correct me if I'm wrong!) I know there are good numbers that breed
around Fort Simcoe (Yakima Co.), but I'm not sure if they winter regularly
in that locality. Even if they do, it doesn't mean that there is a "resident
population". The wintering birds there (if any) could be birds from farther
north, and the breeding birds there could winter farther south in OR and CA.
I am not aware of any population of Lewis's Woodpecker anywhere which has
been shown to be "resident" throughout the year by studies of marked

Almost 100% of Lewis's in BC, and I would expect at least 90% of those
breeding in WA, winter in areas south of the Columbia River. Lewis's
Woodpeckers, for the most part, do not eat acorns during the summer, and do
not even breed in areas that have oaks. So how are they going to know that
there is a shortage of acorns before they get to their wintering areas? In
addition, I have read several reports from observers in Oregon's Willamette
Valley (which has much larger areas of oaks than Fort Simcoe or the Columbia
Gorge) that acorn crops there are in very good shape.

The number of Lewis's sightings this fall in western WA may be an all-time
high. However, if one adds to this the number seen in southwestern BC, it is
not quite so impressive. In the Vancouver area, we normally have one to
three Lewis's sightings each fall-- sometimes of single birds, sometimes of
small flocks. (We have one record in the past of 11 Lewis's in a single
flock in fall migration, another of 9 birds). So far this fall, I am aware
of only 4 reports in southwestern BC, for a total of 5 birds. Two of these
were near Vancouver (one in Vancouver and one in Coquitlam), and the other
two were farther up the Fraser Valley (one on the Sumas River near
Abbotsford, and one near Harrison Hot Springs). So for southwestern BC this
fall, the number is only about average, or just slightly above average. As I
said earlier, one can draw misleading conclusions if one looks only at
records from a single state or smaller area.

Another point-why have all the Lewis's sightings in western WA fallen within
the normal migration window for this species (Sept and early Oct.)? Probably
because these birds are migrants, not birds dispersing because of a shortage
of acorns. The story is still unfolding, but if the spate of Lewis's ends by
mid-October, and none stay in western WA during the winter, the "acorn
shortage" theory looks much less likely.

It is also noteworthy that most of the Lewis's reports have been in the
Cascade foothills (locations like Harrison Hot Springs, Deming, and
Marblemount), in areas where there are no oaks, and the birds have
disappeared within a day or two of showing up. If these birds were fleeing
an acorn shortage, one would expect them to show up in the extensive areas
of oaks in the Puget Sound Lowlands (e.g. in Pierce and Thurston Counties,
or around Victoria), and to stay for awhile.

Then why the high number of Lewis's sightings in western WA? I can suggest
two possible reasons other than an acorn shortage:

(1) Weather factors-I have not looked at this in detail, but this could
account for more sightings than usual west of the Cascades this fall.

(2) Increase in Lewis's Woodpecker numbers in BC and northern WA.
Although Lewis's have declined in BC from the numbers that were there 40 or
50 years ago, my impressions are that their numbers have stabilized or
increased in the last 5-10 years. The numbers seen in fall west of the
Cascades always vary somewhat from year to year, but perhaps we can expect a
higher AVERAGE number of sightings per fall in future.

Dennis's "acorn shortage" theory would require that Lewis's move north, or
northwest, from Yakima and Klickitat Counties to reach northwestern WA. If
this is what's happening, it would be the first time in history for such an
event, which would be quite surprising.

I do not question that acorns are a critical food source for Lewis's
Woodpeckers in the winter-- or that the winter distribution of Lewis's can
change WITHIN their normal wintering range from year to year, depending on
the size of acorn crops. However, I am unaware of any previous record (at
least in the Northwest) of good numbers of Lewis's moving OUTSIDE the
species' normal range in response to a food shortage.

Many species of birds are known to stage "irruptions" into areas outside
their normal range in response to food shortages. Examples include Clark's
Nutcracker (which have noteworthy irruptions once or twice per decade), Blue
Jays, and several species of finches (crossbills, redpolls, Evening
Grosbeaks, etc.) However, I am unaware of this having been documented for
Lewis's Woodpeckers.

None of this proves that the Lewis's Woodpeckers in western WA could not
have moved there as a result of acorn shortages in eastern WA-it just makes
it extremely unlikely, in my view. The only way we could be certain would if
there were a number of marked individuals, and we could be certain where
they came from. Maybe it's time for someone to try to capture and band
significant numbers of Lewis's Woodpeckers!

All the best,

Wayne C. Weber

Delta, BC

contopus at telus.net

From: tweeters-bounces at mailman2.u.washington.edu
[mailto:tweeters-bounces at mailman2.u.washington.edu] On Behalf Of Dennis
Sent: October-02-10 8:02 AM
To: TWEETERS tweeters
Cc: DAVE IRONS; OBOL obol; Wayne Weber
Subject: [Tweeters] Re: Above-normal numbers of Lewis's and Acorn

Hello, all.

With all due respect to Wayne, I'm not persuaded by his argument. I do know
that Lewis's Woodpeckers are migratory in BC and most of their WA range, as
also are some OR populations, but there are also many resident populations,
north to Fort Simcoe, as Wayne mentioned.

We have now heard about much reduced acorn crops in Klickitat County from
Joe Higbee and east of Mt. Hood from Dave Catterson.

These are exactly the places where Lewis's Woodpeckers are *resident* in
large tracts of oaks not that far from the Puget Sound lowlands. I will
speculate again that these birds have dispersed away from those sites
because of the scarcity of acorns. Because they are resident, they are just
as likely to disperse north and west as east and south - just as the Acorn
Woodpeckers seem to be doing. On the other hand, I can't come up with a
reason why migratory Lewis's should all of a sudden be much more common than
usual in western Washington.

And the numbers *are* exceptional, as I read the tweeters digest day after
day. I can envision a southbound migrant passing a northbound disperser in
the sky, both thinking ?????


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