At least this discusses the difficulty in arriving at their numbers.
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Date: Thu, 11 Jun 2015 14:52:20 -0500
Subject: [texbirds] Question Black-capped Vireo status and numbers
From: Jane F Tillman <jtillman@xxxxxxxxxx>
Hi, could anyone tell me a best guess estimate of Black-capped Vireos
(BCVI) in the U.S.?
Am visiting El Rancho Cima boy scout ranch tomorrow and they are interested
in learning more about BCVI and Golden-cheeked Warblers.
I have been looking at US Fish and Wildlife Service online materials,
http://www.gpo.gov/fdsys/pkg/FR-2013-09-09/pdf/2013-21809.pdf and it does
not commit to any numbers on the Black-capped.
It appears there was a review conducted to move BCVI from endangered to
threatened, but I guess that did not happen?
After digging for a while, I just decided it was easier to ask you all.
If you have any info on numbers of Golden-cheeks, which I think also occur
at the scout ranch, that would be helpful too.
Thanks so much.