As weird as it may sound, there's more than one kind of
Brown-headed Cowbird. The "Nevada" cowbird (_Molothrus
ater artemisiae_) which is the native subspecies found
east of the Cascades and (though not officially proven)
the likely one that is seen in winter flocks.

There is very good evidence (see references posted yesterday)
that the birds found breeding on the west side are the more
migratory "dwarf" cowbird (_M.a.obscurus_).

At any rate, it's very possible that there is a "cowbird gap"
representing the period when _artemisiae_ leaves, but before
_obscurus_ arrives. This is all purely speculative, mind you.

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

Half-a-bee, philosophically must ipso-facto half not-be.
But half the bee, has got to bee Vis-a-vis its entity...
d'you see?
But can a bee be said to be or not to be an entire bee
When half the bee is not a bee due to some ancient injury?
-Monty Python

http://www.pacifier.com/~mpatters/bird/bird.html