[Tweeters] Eurasian Collared-Dove decline

[Steve Hampton via Tweeters]

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

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>



-- 
​Steve Hampton​
Port Townsend, WA  (qatáy)
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