I'm forwarding another copy of Dennis' posting (reformatted for easier
reading.) --Dan

---------- Forwarded message ----------
Date: Mon, 16 May 94 14:41:08 -0700
From: Dennis Paulson <dpaulson at ups.edu>
To: Multiple recipients of list <tweeters at u.washington.edu>
Subject: choice of migrant traps

Rob's statements are well-taken, but there seems no or little chance of
our putting observers on Tatoosh or Smith islands through a migration
season to test his hypothesis. In addition, both Tatoosh and Smith are
rather close to mainland forests in terms of a migrating bird's visual
ability, and Destruction Island, well off the coast, would really be the
place to look (surely those Brown Shrikes and Dusky Warblers found in
California stopped at Destruction Island for a day or so). What a shame
access is so difficult to all these places. However, in terms of rarities
a single Magnolia Warbler was found on Protection Island by a birder who
lived there a year or more and was in the field almost every day, so--like
it or not--there's no way PI could be called a migrant trap. Because its
forest could be checked in a matter of hours, it still seems a good place
to check, but Smith and Tatoosh, with really limited trees, would be
better. Who volunteers to live on one of them through a migration? (I'd
love to, but I'm not footloose and fancy-free.)

I still think latitude plays a part, and I don't think the West and East
are exactly comparable. The great mass of migration across much of=
Canada and even parts of Alaska flows from Northwest to Southeast in
fall, as most of the birds breeding all across the boreal forest are of
eastern origin and go back to their "eastern" Neotropical wintering
grounds. If the "mirror-image orientation" hypothesis--that is, some birds
in fall migrate 90=B0 away from their normal direction--has any validity
to explain eastern vagrants in the west, then a tiny proportion of
migrating birds all across Canada will deviate each fall at right angles
from the normal NW-SE (or even WNW-ESE) path. It makes sense for many more
birds to hit the California coast than the Washington coast, as when I
look at a map of North America and superimpose on it the ranges of all the
"eastern" species of birds, it looks to me that a simple explanation could
be that there would be many more birds to the NE (or ENE) of California
than of Washington.

Another way of looking at this is that, all other things being equal, fall
vagrancy must increase with decreased latitude, as the farther along in
migration a bird going the wrong direction would be, the farther away it
would be from its "true" destination. This seems incontrovertible. Thus why
not fewer fall vagrants in WA than OR? Now spring vagrancy is another
story, because it should increase with *increased* latitude for the same
reason, and we know how many spring vagrants Malheur Refuge gets compared
with anywhere in WA. There, Rob's explanation of fewer and more isolated
oases does have a ring of truth about it. It would be interesting to
compare WA and OR with regard to proportion of vagrants in spring vs. fall;
maybe someone out there will do it. Phil Mattocks has a complete list of WA
vagrant records, and many of those from Oregon up to a certain date are
published in "Rare Birds of Oregon."

Another fruitful comparison that would be testing the mirror-image
orientation hypothesis would be to compare records of eastern birds West
and western birds East. In fall, eastern birds tend to migrate toward the S
and SE, but western birds, which typically winter in western Mexico, also
tend to fly S or slightly SE, with only a small fraction probably on a SW
heading. Thus there should be far fewer vagrant Western Tanagers in the
East (mirror image of a SW heading) than Scarlet Tanagers in the West
(mirror image of a SE heading); however, I don't think that's the case.
There are mitigating factors, the biggest of which is presumably prevailing
wind directions. There's a lot of food for thought in the analysis of
vagrant patterns, and I don't know if anyone has written much on this in
recent years. Dave DeSante, who proposed the mirror-image orientation
hypothesis, may have done some of these analyses in his PhD dissertation,
which I haven't seen.

May your vagrant traps be full each time you check them!

Dennis Paulson
dpaulson at ups.edu