Likewise, Christmas Bird Count data, especially aggregated across large
areas (e.g. a state) for common species, can tell a story.
I haven't looked at the statewide data for WA, but there has been some
analysis in the Central Valley of California.
I explored the nationwide trends and plotted the Central Valley records
when there were still only 24 records.
Hampton, S. (2006). The expansion of the Eurasian Collared-Dove into the
Central Valley of California.
<
https://www.cvbirds.org/wp-content/themes/cvbirds/files/V.9no.1/V.9no.1pp7-14..pdf>
Then Ed Pandolfino used CBC and BBS data to show they had peaked in 2017.
Pandolfino, E. (2020). The Eurasian Collared-Dove invasion in California:
Has it peaked?
<
https://www.cvbirds.org/wp-content/uploads/2022/03/1-Pandolfino-The-Eurasian-vol23-no3.pdf>
I expect WA is somewhere along a similar path.
On Sat, Nov 9, 2024 at 2:08 PM Bill Tweit via Tweeters <
tweeters at u.washington.edu> wrote:
>
A couple of days ago, Hans Feddern noted on Tweeters that he had observed
>
a decline of Eurasian Collared-Doves in areas he has birded, and asked
>
about other evidence of a decline.
>
>
This is exactly the sort of question that eBird data are quite useful for
>
answering. But before I point to some eBird data sources, I'll just note
>
that it would be highly surprising if they were NOT declining at this
>
point. The basic ecological dynamic that is operative here is that newly
>
established species often show very rapid population growth until natural
>
checks, such as predation, disease and competition, begin to function
>
effectively. Typically, the development of population controls lags
>
population growth, which leads to population declines as the newly arrived
>
species begins to transition from a growth phase to an equilibrium phase.
>
>
In this particular case, the growth phase of Eurasian Collared-Dove
>
expansion into western North America was explosive, and may well be one of
>
the most impressive vertebrate range expansions in our experience. One
>
would expect that explosive growth would then result in noticeable
>
declines, simply because the population grew so rapidly that it noticeably
>
outstripped its 'equilibrium' population size when the inevitable
>
population checks are fully operative. How explosive? Look at the eBird
>
species map for collared-dove in Washington in 2005 (
>
https://ebird.org/map/eucdov?neg=true&env.minX=-134.18099999999998&env.minY=41.418329269627435&env.maxX=-106.05599999999998&env.maxY=50.04329885125577&zh=true&gp=false&ev=Z&excludeExX=false&excludeExAll=false&mr=1-12&bmo=1&emo=12&yr=range&byr=2005&eyr=2005)
>
with the 2015 map (
>
https://ebird.org/map/eucdov?neg=true&env.minX=-127.14974999999998&env.minY=43.69995417791555&env.maxX=-113.08724999999998&env.maxY=48.01225544880609&zh=true&gp=false&ev=Z&excludeExX=false&excludeExAll=false&mr=1-12&bmo=1&emo=12&yr=range&byr=2015&eyr=2015).
>
In a mere decade, their distribution in the state transitions from a few
>
scattered areas to uniformly present in all of the non-mountainous areas or
>
heavily forested areas. Another data source for looking at the increase
>
is the Washington Bird Record Committee records: the first year with
>
multiple reports was 2005, and three years later the WBRC removed it from
>
the review list as it exceeded the criteria for inclusion in the review
>
list. In three years! Obviously, WBRC records don't provide any
>
information on subsequent population fluctuations.
>
>
Turning to eBird for evidence of decline. My data analysis skills are
>
surface level, so there is plenty of room for more detailed dives into
>
eBird data. But, take a look at the annual line chart of the eBird
>
statistic for frequency of collared-dove statewide for the five years
>
2017-2021 (
>
https://ebird.org/barchart?byr=2017&eyr=2021&bmo=1&emo=12&r=US&spp=eucdov&separateYears=true).
>
It very clearly shows declines each year from the 2017 to 2021, in fact the
>
2021 line never reaches the 2017 line at any point in the year. The eBird
>
statistic for abundance shows a very similar pattern. These charts can
>
only show five consecutive years, a quick look at the 2019-2023 chart shows
>
some continued decline but a lot more overlap between years, so the
>
population levels now are more similar to the 2021 levels, and remain well
>
below the high point of around a decade ago. eBird provides data that
>
allow us to understand how the expected decline has actually unfolded.
>
>
It would be interesting to look at Cooper's Hawk numbers over the same
>
time period, which I have not done yet, as they are clearly one of the
>
primary predators of collared-dove. For instance, the first recorded
>
breeding of Cooper's Hawks in the Salton Sea basin in southern California
>
was coincident with the dramatic increase of collared-doves in that region,
>
and they remain regular breeders at several locations in the region. I
>
expect that eBird would show some amount of increase, maybe not as obvious
>
as the population dynamics of the collared-dove.
>
>
Bottom line is that birders do not have to rely on anecdotal information
>
for answering questions such as the one Hans posed. eBird is available to
>
anyone, and is easy to access.
>
>
Bill Tweit
>
_______________________________________________
>
Tweeters mailing list
>
Tweeters at u.washington.edu
>
http://mailman11.u.washington.edu/mailman/listinfo/tweeters
>
--
Steve Hampton
Port Townsend, WA (qatáy)