Thursday, October 23, 2008

Location and Seasonal Bias?

One bird species it seems like I'm seeing less this fall is White-throated Sparrow. I'm not the only birder who has noted an apparent drop in certain species this migration, so I became curious and decided to check eBird data to see if it at least might help explain this perceived decline. I began by looking at fall migration abundance graphs for White-throated Sparrow for the entire state of Wisconsin.

According to eBird's glossary, “abundance” is:

“The average number of birds reported on all checklists within a specified date range and region. These data tell us what we might to expect when going out birding on an average day. The checklists used in this calculation include those that didn't report the species, providing a measure of relative abundance or how commonly the bird is reported compared to all other species in the region.”

For Wisconsin as a region, White-throated Sparrow migration this fall doesn't seem to be all that different from previous years (going back to 2005):

Next, I restricted the data for Dane County only:

And finally, just Pheasant Branch Conservancy:

In considering these graphs, it seems like I may have biased myself with an exceptional 2007 fall season for White-throated Sparrow at the conservancy. Apparently, just one year later I'm unable to recall a more typical seasonal abundance. I haven't drastically altered my frequency of birding outings over the years. Last fall, why didn't I mention seeing more White-throated Sparrows over 2006? It might be a positive perception versus a negative one – is this also a type of bias? Is there a tendency to remark on the comparatively poor seasons versus good ones? Considering that 2008 has been relatively normal on a county and state level, it would seem that Pheasant Branch Conservancy hosted a disproportionately higher population of White-throated Sparrows in 2007. Now that's interesting.

© 2008 Mike McDowell

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