Date: 4/13/18 11:52 am
From: <douglitas...>
Subject: [cobirds] What CBC data can tell us about House Sparrows in Colorado Springs

Hi all,

First off, hi! I'm brand new to this list. As dumb luck would have it, the
first post I ever received was yesterday from Diana Beatty. She had the
wonderful idea to do a linear regression on Christmas Bird Count (CBC) data
from Colorado Springs to see if there has been a decline in House Sparrow
population size since 1950. Due to my burgeoning interest in House Sparrows
as a graduate student, I asked her if she had any more details. In
response, she sent the raw data for me to have a look at. A big thanks to
her for sending that along!

Diana's analysis was of course correct: that is, that when looking from
1950-2017, there has been no overall trend toward decline. However, my eyes
wouldn't stop perceiving little peaks and dips in the cloud of data points.
So I split the data up and found that there have been 3 cycles of
statistically significant growth and decline since 1950. We're currently in
the middle of a decline that began in 2001.

It's no surprise that there have been fluctuations in the 67 years of CBC
data that we have. All wild populations fluctuate. The interesting part is
telling a story as to why they fluctuate. Often, growth and decline cycles
have something to do with climatic patterns, possibly interacting with
things like competition and selection. Unfortunately, I'm not sure that I
have the expertise to attempt any associations with climate or other
factors right now. But it's likely there's something of interest going on,
even if we don't know what it is!

I'm attaching a visual representation of the CBC data to this post. I
color-coded each of the cycles. The x-axis shows passage of time with the
far left side being 1950 and the far right being 2017. On the y-axis is the
CBC count data, with lower values on the bottom and higher counts up
higher. Note that the red dots, spanning the years 1950-1984, represent the
longest and slowest decline. The last 2 declines (the second of which we're
currently in right now) occurred on much smaller time scales, from
1985-2000 (black dots) and from 2001-present (blue dots). The lowest ever
count in the entire data set was in 2016 with only 177 House Sparrows

While it's likely that a population ecologist could point out several ways
I've poorly described these patterns, I think it's cool that Diana began
all this with an analysis of publicly-accessible data and shared it on a
bird listserv. Thanks a lot to all of you for reading this, and I'd love to
continue the conversation if anyone is interested!

Good birding,
Doug Eddy, Laramie, WY


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