[Tweeters] Eurasian Collared-Dove decline

[Bill Tweit via Tweeters]

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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