[Tweeters] Eurasian Collared-Dove decline further thoughts

[Bill Tweit via Tweeters]

Really great input from Steve H and Bob O. on this subject.

Bob suggests also looking at the Status and Trends tool in eBird, and that
it may present a different picture of their population trends.   I agree
and disagree.   The Status and Trends tool is an amazing tool to have at
your fingertips, for looking at several of the trends that he lists.
 Where I disagree is in the interpretation for EUCD status.   Exactly as
Steve H. notes, a single index of a ten year time span can disguise the
actual population dynamic.  In this case, the Status and Trend tool
currently looks at 2011-2021, and essentially evaluates whether the
population has increased or decreased over that period.   Steve H. points
out that it can miss a peak and subsequent decline.   In this case, the
peak happened late in the period, probably in 2017, so seven of the eleven
years in the index were increasing and only four were decreasing.   And, as
I've already noted, the increase was really impressive, while any decrease
is clearly more subtle.   So, essentially the Status and Trends index is
telling us that the 2021 population was larger than the 2011 population,
without showing the trajectory.   In stock market terms, if you had bought
EUCD stock in 2011 and sold in 2021 you would have made money, but if you
had bought in 2017 and sold in 2021, you would have lost money.

Bob's examples of other species are really thought provoking.   The Cattle
Egret population explosion may well have been as impressive as the
collared-dove expansion, and the Crested Mynah increase followed by their
disappearance is a great reminder that not all 'promising' starts turn into
positive outcomes in the long-term (Spotted Dove in Southern CA and
northern Baja shows a very similar trend; they appear to be on the road to
vanishing).  And, while it is tempting to speculate that the Anna's and
Rufous population trends are related, please note that Bob did not suggest
that displacement was the operative factor, he just suggested that it
should be examined.

His closing reminder of the value of eBird for providing definitive,
data-driven (is that redundant?) answers to questions about range shift
versus population decline
is worth re-reading.   eBird is an amazing resource, collecting birders
observations and generating useful information for scientists, resource
managers, conservation organizations, planners and birders.

Bill Tweit
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