Date: 11/10/24 6:37 am From: Steve Hampton via Tweeters <tweeters...> Subject: Re: [Tweeters] Eurasian Collared-Dove decline
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.
On Sat, Nov 9, 2024 at 2:08 PM Bill Tweit via Tweeters <
<tweeters...> 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...>&<env.minY...>&<env.maxX...>&<env.maxY...>&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...>&<env.minY...>&<env.maxX...>&<env.maxY...>&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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