One big factor is the Gulf. There is no single barrier quite so
formidible in the west.

Another is diversity. Sit down with your Sibley and count how
many warbler and thrush species occur east of the Rockies and
compare that to neotrops west of the Rockies. The west got all
the hummngbirds, but the east got most of the warblers.

Of course, fallouts do occur around here. The trick is figuring
out where. Malheur NWR can have some pretty spectacular fallouts.
I've seen some fallouts on Coxcomb Hill that may not have had
20 species of warblers, but they had 100+ Townsend's Warblers,
with Black-throated Gray's, Hermits and others mixed in. In some
ways this question is a matter of perspective.

Subject: Why are east coast migration fall outs more spectacular?
From: "Ed Swan" <edswan AT centurytel.net>
Date: Thu, 13 Oct 2005 13:56:52 -0700

Can someone remind me why the passerine migration on the east coast has such
more intense and numerous fallout occurrences?
Ed Swan


--
Mike Patterson
Astoria, OR
celata at pacifier.com

And now for something completely different... Salamanders
http://www.surfbirds.com/blogs/mbalame/archives/002899.html