Bushtit Range Expansion

[Virginia Holman]

Hi,

Are there any studies/reports re: the Bushtit range expansion in Colorado in the last 10-15 years? 

I'm coming across significant anecdotal information from longtime birders in the region, but nothing formal/science-based yet.

Thanks for any guidance.

Virginia Holman

[David Suddjian]

You might look at the results of the two breeding bird atlas efforts to look for some indication of range expansion, but these will predate the past 10-15 years, mostly. You can also look at eBird species maps for particular years and compare things among years. This is somewhat anecdotal, as it is based on what people happened to have reported (and there has been a marked increase in eBird use over that time period) but might have some useful info. Trend info from the North American Breeding Bird Survey might be of interest, too, looking at results of particular routes or the state as a whole. But these will emphasize roadside results in less developed areas away from some of the more developed regions where Bushtits are perhaps most frequent.

David

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[Eric DeFonso]

Hi Virginia,

The Rocky Mountain Avian Data Center has species reports that you can search, over the years which the IMBCR project has operated (2008-2025). IMBCR is the Integrated Monitoring of Bird Conservation Regions, an ongoing breeding bird study organized by the Bird Conservancy of the Rockies. On the topic of range expansion in the state, the data may be a bit too low resolution to observe the more local "infill" of Bushtits in some areas of interest in the state. But it is easier to note the overall population and density trends statewide since 2008.

Rocky Mountain Avian Data Center:

https://app.bcr.eco/rmadc/

Bird Conservancy of the Rockies (link to IMBCR):

https://www.birdconservancy.org/what-we-do/science/monitoring-and-tracking/

The site takes a little time to figure out how to navigate, but there's a lot of data there, for any of the expected breeding birds in the state, as well as for the other regions that IMBCR studies. I worked seasonally with IMBCR for 10 years, 8 in Colorado.

Eric

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Eric DeFonso
Boulder County, CO

On Thu, Dec 4, 2025 at 2:25 PM Virginia Holman <virginiaho...@gmail.com> wrote:

[Bill Schmoker]

Hi Virginia- I was already digging around a little in the Boulder CBC archives in preparation for this year's count and thought I'd look back to see what our Bushtit situation has been for the last 20 years.  I wonder if a CO-wide CBC picture could also be produced but haven't looked into that.  Anyway, here's what we've had on the count since 2004:

2004: 8 (in 1 territory)

2005: 4 (feeder watcher)

2006: 20 (in 1 territory)

2007: 7 (1)

2008: 19 (4)

2009: 46 (2)

2010: 58 (3)

2011: 67 (2)

2012: 96 (6)

2013: 8 (1)

2014: 86 (7)

2015: 110 (4)

2016: 27 (5)

2017: 163 (9)

2018: 174 (10)

2019: 92 (6)

2020: 135 (8)

2021: 146 (9)

2022: 178 (10)

2023: 142 (11)

2024: 173 (7)

So definitely the expected ups and downs but overall I see a pretty strong increase both in numbers of birds and in numbers of territories reporting Bushtits in our last 20 counts.  I suspect that our earlier counts would show even more Bushtit paucity.

-Bill Schmoker, Longmont

On Thu, Dec 4, 2025 at 2:25 PM Virginia Holman <virginiaho...@gmail.com> wrote:

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[Bill Schmoker]

OK, that got me curious as to what the long-running Boulder CBC data could show so I went back to its start in 1910.  While early counts definitely had more patchy effort, and the count didn't run for many of the early years (no counts in the national database 1912-1919, 1923-1925, 1927-1944, and 1946-1949), Boulder has run every year since 1950 so that's a fun well of data to draw from.

Boulder CBC's first Bushtits were recorded on the 1968 count (18), then again only in these years until 2004:

1973 (4)

1974 (20)

1975 (6)

1978 (17)

1981 (4)

1983 (18)

2000 (14)

Since 2004 we've recorded them annually (details addressed in my previous post.)

In conclusion I'd say that at least in Boulder in the winter Bushtits were absent or quite unusual until after the turn of the millennium, kind of rare through the early 2000s, then growing more abundant and expected especially in the past decade or so.

-Bill Schmoker, Longmont

On Thu, Dec 4, 2025 at 2:25 PM Virginia Holman <virginiaho...@gmail.com> wrote:

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[Eric DeFonso]

The RMADC data for Bushtits that I just briefly looked up is roughly in line with this, showing steady and sometimes significant uptrends in springtime populations. See the attached screenshot that shows the change of Bushtit numbers for three shown strata (regions) measured over 16 years, with year-over-year average increases in density of 7-10% in just the "lowland" parts of Colorado. The site has data for a number of other surveyed regions too, I just wanted to highlight the ones that are where many of us in the Front Range are.

Eric

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Eric DeFonso
Boulder County, CO

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