[CT Birds] Wood Thrushes
elphick at sbcglobal.net
Tue Jun 20 17:26:58 EDT 2017
As most people on ctbirds probably know, wood thrushes have been undergoing a long-term population decline in the state. That said, I would caution anyone from reading too much into the apparent changes at any given location, or even in any one year.
This link goes to an animated graphic that Chris Field (postdoc in my lab) made a few years ago using the Breeding Bird Survey (BBS) data for the state:http://www.ctbirdtrends.org/woodthrush.html
The graph on the right shows how the average wood thrush population has declined over the past 40 years. (Note, if your internet is as slow as mine, you might need to wait a minute or two for the animation to load and for the graphs to show all the information they contain.) The graph on the left shows the actual data, with each bar representing the total count for a given BBS route. The grey horizontal line shows the average population size in 1966 when the survey began, and the shifting blue line shows how that average changes from year to year (corresponding with the points in the graph on the right).
If you look at how the bars for individual BBS routes bounce around in the left-hand graph you'll see dramatic shifts from year to year. Averaged over all sites, however, the overall trend does go up and down some, but it follows a fairly steady (and far less dramatic) decline that at any individual site. None of that up and down at individual sites is especially unusual for a songbird population, yet it is often what is most apparent to us during day-to-day birding, frequently exaggerating our impressions of how things are changing overall.
It is also my impression that we generally take more note of local absences of expected species than their reappearance .. this is certainly true for me ... I'll notice if my local Carolina wren or mockingbird disappears for a year, but I often don't pay nearly as much attention to their return a couple of years later. Also, I think people are far more likely to post a message about an absence of local birds, than about their continued persistence (or return) - again possibly creating a misleading impression of how bad things are overall.
Having said all that, I should emphasize that I do not mean to minimize the loss of species, even locally. Serious regional/long-term declines always start with a series of local/short-term declines. I just wanted to point out that fluctuating populations are normal, especially at a local scale, and that we really need to look through all that noise and find out what the bigger/broader signals are, so that we can focus our attention on those.
Chris Elphick @ssts
elphick at sbcglobal.net
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