Monday, July 04, 2011

Downhome Breeding Birds Plus 5 Years

Having completed last year my little project of running BBS-style surveys on nearly every public road in the county, this year it was time to start my 5-years-later resample. I reran the first three routes, run previously in 2006. Putting the data side-by-side, they suggest some substantial changes over that time; but three routes of 50 stops each is not a big sample. Still, it is the minimum necessary to do a paired t-test on them and look for any statistically significant changes. So I did just that.

The particulars -- square-root transformed data, N = 3, DF = 2, total species examined = 86. Note that at a significance level of p=.05 I would expect 4 or 5 spurious hits; at p=.01 I would expect about 1 spurious hit.

A total of 5 species showed a significant change (p=.05) from 2006 to 2011. Those marked with ** are also significant at the p=.01 level (2 species)

Red-tailed Hawk
Yellow-billed Cuckoo
Blue Jay**
American Crow
Scarlet Tanager**

The power of this test is very low; many species showed large differences between the two years that were not statistically significant. This is not a whole lot more than what would be expected from random data, except for one thing: ALL of these changes are increases. No species showed a significant decline.

The biggest (statistically) change was the increase in Scarlet Tanagers. This is consistent with my general impression that this bird has substantially increased in middle Tennessee since I arrived in 2002. It does not seem to have been at the expense of Summer Tanagers, however: their total for these three routes were exactly the same in 2006 and 2011.

A lot of species showed pretty large changes that were not significant using this (low power) test. Many of these changes were consistent with patterns I feel like I have been seeing in general -- for example, increases in all woodpeckers except Northern Flicker, which is decreasing. When I get the chance I hope to pull up the statewaide long-term BBS data for Tennessee and see what it shows for the larger trends.


At 4:59 PM, Blogger Anna said...

I don't know how long you've been birding (just stumbled across your blog and am catching up), but have you considered the possibility that you're just a better birder than you were five years ago, thus all of the increases? (Love the data!)

At 5:16 PM, Blogger Bill Pulliam said...

I've been birding for 37 years and just turned 50. If anything, one might worry that my bird detection skills would be waning with my hearing and eyesight, not improving! So far they (skills, hearing, eyesight) seem to be holding steady.

At 7:40 PM, Blogger Jim R said...

Of course there's always a bad-news interpretation: the species you see becoming more common in your neck of the woods is being squeezed in other areas, and you're getting "refugees".


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