All:
Since one of my favorite birds has come up as a topic in this venue, I find it
impossible to not jump in. In my experience, White-winged Junco's primary
winter range is the Ponderosa Pine belt on the east side of the Front Range and
Wet Mountains. It is nowhere numerous and is nearly always greatly outnumbered
by other junco taxa. This is not surprising considering that the breeding
range of White-winged Junco is so small and its winter range is so much larger.
So, while the taxon is fairly readily findable in winter at the
foothill-plains interface (particularly at a few favored locations, such as Red
Rocks Park), White-winged Junco is typically found there only in tiny numbers.
Hiking uphill is the way to find them, such as illustrated by Ted's checklist.
Below, I provide links to the eBird distribution maps of White-winged Junco for
select winters:
Current winter
2014-2015
2013-2014
2012-2013
As can be seen on the above maps, overall distribution in CO across recent
winters does not differ in any appreciable way. As comparison to a decade ago,
note the eBird map for winter 2005-2006. HOWEVER, many, many fewer birders
used eBird ten years ago, so old data like these cannot be considered to
accurately represent actual occurrence patterns. That leaves us with the
Christmas Bird Count (CBC) as the only long-term data set available for looking
at such questions. Unfortunately, there is a huge gob of methodological
problems with the CBC (both CBC policy and individual-CBC mechanics) that make
analyzing the data an exercise best left for a Ph.D in statistics. Though I
will not delve into those problems here, a very obvious one is the effect of
deep-sixing the participation fee within the past ten years ago and the
resultant great increase of participation (both in observers/circle and in
number of circles), which has to have had a large impact on data and their
analysis. However, as a very rough indication of patterns for taxa found in
sufficient numbers, it does a reasonable job; White-winged Junco is NOT such a
taxon.
For the below discussion, I queried the online National Audubon Society's CBC
database and obtained the previous 20-year history for White-winged Junco in
Colorado and for five select CBCs. The span of CBCs queried was from the
2005-2006 season through the 2014-2015 season (the 2015-2016 data set is not
complete, nor available for query). I chose five Colorado CBCs as the best
representative counts to analyze Colorado White-winged Junco abundance in order
to cover as much of the spread of occurrence in the state as possible while
also maintaining sufficient sample size for useful analysis. The five count
circles, all with extensive acreage of suitable habitat for the taxon are, from
north to south: Boulder, Evergreen-Idaho Springs, Black Forest, Pikes Peak,
and Penrose. The graphs use number of individuals/party-hour in order to
normalize the data for, at times, highly variable number of observers across
years. I made screen captures of the graphs and placed them on my Flickr site.
In Colorado as a whole, the last half of the 20-year period seems to show a
decline (but see caveat above). However, the most striking feature of the
graph is the great variability in abundance, particularly in the first ten
years, with a fairly regular cycle of boom and bust. While weather might also
explain it, my experience with that variable is that winter weather regimes are
not so precisely cyclical as to explain this result. Other passerine species
are known to exhibit similar boom-and-bust cycles, with Common Redpoll and Pine
Siskin being two of the best examples, and these two species also match the
short temporal pattern of the apparent White-winged Junco cycle. The two finch
species are thought to be responding to specific food sources; the Pine Siskin
to the availability of pine seeds. Considering that White-winged Junco has a
high preference for pine forest, perhaps it, too, is responding to localized
food resources, perhaps with larger proportions of the taxon staying closer to
the breeding grounds in some years, and having to travel farther afield in
others. Inter-annual variation in breeding success might also be a causative
factor behind the cycle. Documentation of age structure in winter in Colorado
and in the Black Hills might provide useful data at getting at the cause of the
cycle.
Regardless of the cause(s) of the apparent population cycle, the system seems
to have broken down in the last ten years or so, with four consecutive years of
decline, from the 2008-2009 season to the 2011-2012, with a return to something
of a cycle in the last four years, albeit at a considerably lower abundance
value.
Looking at individual CBC results (see below), the five selected CBCs throw a
huge monkey wrench into the analysis, one that might be explainable with a
more-in-depth analysis, but which lies outside the scope of this post. In the
below graphs, we can see that no two of the five CBC circles present results
similar to each other, nor to the general Colorado results!
Boulder
Evergreen-Idaho Springs
Black Forest
Pikes Peak
Penrose
Table 1 illustrates a general north-to-south trend of decreasing reliability of
detection, suggesting that there is a geographic correlate of abundance as it
affects detection probability, thus of frequency, and that southern Colorado is
closer to the edge of the winter range than is northern Colorado. That
suggestion seems borne out by the overall eBird winter-distribution map.
Somewhat contrastingly, both maximum abundance and average abundance reached
their highest values in central Colorado. However, the three central counts
all have a higher percentage of their circles occupied by suitable White-winged
Junco habitat than does either the Boulder CBC, a large portion of which is
occupied by urbanized plains habitats, or Penrose, a large portion of which is
occupied by low-elevation habitats. Table 1 suggests that the southern part of
the Front Range and the associated Palmer Divide is the epicenter of Colorado
White-winged Junco abundance, with the Black Forest CBC circle being especially
so. However, the small sample size of Colorado CBCs incorporating extensive
suitable habitat (n=5) makes that assumption somewhat problematic.
Table 1. Averages of White-winged Junco abundance on five
Colorado Christmas Bird Counts, 1995-1996 to 2014-2015.
=======================================================
# years Abundance
Avg.
CBC detected Min Max
abundance*
-------------------------------------------------------------------------------------------------------------
Boulder 20 0.0122 0.3447
0.1309
Evergreen-Idaho Springs 20 0.0898 0.6612 0.3848
Black Forest 16 0 4.4615
0.6287
Pikes Peak 11 0 1.1195
0.1918
Penrose 11 0 0.6387
0.0826
-------------------------------------------------------------------------------------------------------------
*Abundance is generated by summing annual values of
birds/party-hour and then dividing by 20.
Table 2 presents the range of variation of participation for each of the five
CBCs. Note that the Black Forest and Pikes Peak CBCs have been subject to much
higher variability in participation that, depending upon deployment of
observers, might impact the results across years. More and more-consistent
participation would be beneficial for these two counts (particularly Pikes
Peak).
Table 2. Range of the number of participants for five Colorado
Christmas Bird Counts, 1995-1996 to 2014-2015.
============================================
CBC Min* Max % difference
----------------------------------------------------------------------------------------
Boulder 81 137 69.1
Evergreen-Idaho Springs 42 70 70.0
Black Forest 14 39 179.9
Pikes Peak 9 21 133.3
Penrose 19 30 58.9
----------------------------------------------------------------------------------------
*Number of participants for individual CBCs on which zero
White-winged Juncos were detected are not readily
available without more effort, so values presented may not
represent actual minima and maxima for those counts.
Sincerely,
Tony
Tony Leukering
Largo, FL
http://cobirds.org/CFO/Resources/Columns.aspx?id=2
--
You received this message because you are subscribed to the Google Groups
"Colorado Birds" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To post to this group, send email to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/cobirds/152b3346e93-4786-e317%40webstg-a05.mail.aol.com.
For more options, visit https://groups.google.com/d/optout.
