[texbirds] Re: Bushtits in eastern Edwards Plateau?
- From: Rich Kostecke <rkost73@xxxxxxxxx>
- To: "gcwarbler@xxxxxxxxxxxxx" <gcwarbler@xxxxxxxxxxxxx>, TexBirds TexBirds Posting <texbirds@xxxxxxxxxxxxx>
- Date: Thu, 20 Jun 2013 12:59:58 -0700 (PDT)
Myself and several other TNC folks had some Bushtits along the Devils River
(Val Verde County) this spring. Maybe like a handful of sightings. Those are
not data that would be entered into eBird. One of the draw backs of eBird
being that as widely as it may be used by birders now, not all data are being
input into it (like a lot scientific survey data); so one needs to be careful
about inferences drawn from it. I definitely think there are eBird biases.
So, there may be few or even no reports from places like the western Plateau
and north of I-10 on the Plateau, but how many checklists were actually
submitted for those regions, were they submitted from sites likely to have
Bushtits to start with, and more importantly what is the baseline for Bushtits
in those areas based on past checklists (assuming there are any) submitted to
eBird?
Anyway, not sure if western plateau birds would be more tolerant of drought or
not. It is not a terribly great distance between western and eastern birds
that we are talking about (would eastern Black-capped Vireos be less drought
tolerant than western Black-capped Vireos? Or choose whichever species you want
that has a range across the Plateau and ask the same question). So, I think
things like drought and precipitation gradients could regulate populations in
some ways (e.g., setting carrying capacities, influencing nest success or
survival, etc.), so there may be differences in population sizes or vital rates
between east and west, but I am not sure you could say western birds are
inherently more or less drought tolerant than eastern birds Also, we assume
that the drought impacted or is impacting the species but there are certainly
still a lot of junipers on the western Plateau (even if there were extensive
juniper kills), and there is certainly
still water and food resources . So, the drought may or may not have impacted
the species as much as we think and any impacts could potentially be on a
pretty small scale or localized. And, unlike some other species, no one has
seemingly observed an irruption of Bustits any place that would suggest
displacement by drought. So, I think this drought tolerance question is a
difficult one to answer without better data on the species throughout its
Edwards Plateau range.
I actually probably wouldn't lump the Bushtit and Hutton's Vireo together. I
think there are key habitat differences. I see the vireo as being much more of
an oak woodland or oak-juniper species whereas the Bushtit strikes me as more
of a juniper species. And I think there are other differences that make
comparing the species difficult. The vireo seems to be a colonizing species
that is actively expanding both its winter and breeding range whereas the range
of the Bushtit seems to be fairly static (even if it is nomadic and sporadic
within that range). There are different dynamics happening. I would probably
be more likely to compare the Bushtit with a species like Western Scrub-jay.
Just a few musings,
Rich
Richard Kostecke, Ph.D.
The Nature Conservancy
318 Congress Ave., Austin, Texas 78701
Email: rkost73@xxxxxxxxx or rkostecke@xxxxxxx
________________________________
From: Chuck Sexton <gcwarbler@xxxxxxxxxxxxx>
To: TexBirds TexBirds Posting <texbirds@xxxxxxxxxxxxx>
Sent: Tuesday, June 18, 2013 11:06 AM
Subject: [texbirds] Bushtits in eastern Edwards Plateau?
TexBirders,
As I was compiling my notes for the Spring report for North American
Birds, it occurred to me that I had seen very few Bushtits away from
the Trans-Pecos in the past year or so. I began to wonder if the
species has been especially hard hit by the drought conditions,
particularly in the eastern Edwards Plateau. I began to study eBird
reports and it seems to confirm my impressions.
There is only one report in 2013 of a Bushtit in the region
substantially N and E of Interstate 10: a single report at Fort Hood
(Coryell Co.) in February. There is a small handful of reports in
the San Antonio region and points W and slightly NW. The species is
still being seen (apparently) in the Trans-Pecos, but it's hard to
judge the abundance without a more detailed analysis.
I'm wondering if other TexBirders have noticed a decrease in
Bushtits, particularly in the eastern Edwards Plateau. This can be a
difficult question because the species is so nomadic and sporadic in
occurrence in this region to start with. Is this just a sampling
bias? An eBird bias? Or have Bushtits actually declined in this
region?
Two additional thoughts:
1. The drought has actually been worse in the western part of the
Hill Country, but that is where the species has historically been
more numerous. Are the more westerly Bushtits simply better adapted
to withstand such a long stretch of harsh habitat conditions?
2. The (apparent) Bushtit pattern seems to contrast with the
Hutton's Vireo situation, a species I might think had similar
ecological adaptations and habitat preferences, but which seems to be
continuing its increase in abundance and distribution eastward. Why
the difference in the two?
Chuck Sexton
Austin
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