Friday, September 10, 2010

Disappearing Act

What happened to Bay-breasted Warblers during this past spring migration? Sightings by birders for this species were dramatically down from previous years in many mid-western states. In fact, I recorded only a single individual at Pheasant Branch Conservancy during May. Looking at the eBird frequency graph for this warbler species in Wisconsin, you can clearly see the drop in 2010 compared to 2007, 2008, and 2009:

What was interesting was that the US graph showed little overall change:

Bay-breasted Warbler range map:

What explains this? This got me wondering if some states had increases in order to offset decreases. Using the same combined years, I plotted graphs for eastern states (South Carolina had no data) and colored those with 2010 frequency decreases in yellow, relatively unchanged frequencies are orange, and increases for 2010 are in red:

It appears Bay-breasted Warblers might have taken a more easterly migration route during spring 2010. Why? Pure conjecture, but I wonder if these particular trans-gulf migrants flew around smoke from the Deepwater Horizon fire. Bay-breasted Warblers began arriving on the gulf coast the 2nd week of April and the explosion was April 22th. While flying, did they detect smoke and buzz over to Florida instead? Perhaps the 2010 differential was an anomaly or weather system related. We know that warblers tend to migrate in mixed-flocks, so why weren't other species affected the same way? There are a lot of explanations that might fit the data, but probably no way to know the real explanation or cause. Whatever the case may be, I doubt midwest birders will forget the spring they saw so few or no Bay-breasted Warblers.

Bay-breasted Warbler © 2010 Mike McDowell


  1. Excellent use of eBird data to explore a birding mystery. I love it! May I ask where you got the USA map and what tools you used to color in states? I've been looking for something like that for my own citizen science using eBird. You can e-mail me at robertmortensen at cableone dot net.

    Nice work!

  2. Mike-

    Nice post. Most people I know complained about the lack of warblers in general, particularly the Bay-breasted, all spring. Strangely, I had a record number of species, but only one individual of nearly all of them! I missed Connecticut, Worm-eating, Black-throated Blue, Kirtland's, Prairie, and Yellow-breasted Chat, all of which I've never had before, as well as Orange-crowned (which I hardly ever get in Spring) and, of course, BAY-BREASTED, a shame to, because it is one of my favorites. Hoping to see one tomorrow, I like your theory of the oil-spill.


  3. Idaho Birder,


    It was just a random map I found using google image search. I colored the states with Adobe Photoshop.