Saturday, December 12, 2009
Warbler Guy, have wood-warblers shifted their winter distribution as warmer winter temperatures have become more common in recent years?
(Above, thanks to Martin Meyers for submitting a Common Yellowthroat male photo (taken in Nevada) )
That’s a fine question, Howie (in Minneapolis).
Turns out the answer is “yes,” if you agree with a recent technical report titled “Birds and Climate Technical Report (Niven, Bucher, and Bancroft, 2009; birdsandclimate.Audubon.org)
This report tracked the locations of 305 species by using Audubon Christmas Bird Count (CBC) data collected in the past 40 years throughout the lower 48 states (and s. Alaska and s. Canada).
Among these species, researchers suggest 58% of them shifted farther north in the past 40 years. They include many non-warblers such as Fox Sparrow, Pileated Woodpecker, Pine Siskin, Purple Finch, Ring-Billed Gull, and Fox Sparrow, among others.
Which wood-warbler species has shifted father northward during winter? It’s the Common Yellowthroat. Seeing this moisture-centric species on CBCs is not uncommon during the breeding season, especially because they are found throughout all of the lower 48 continental states. In addition, even seeing them throughout the winter in southern states is regular occurrence.
More eyebrow-raising: Seeing them increase their presence in northern USA states when the December/January CBC surveys occur. By this time, insect resources should have ebbed. Resources are few and far between. The pantry is empty for most insect-dependent bird species, such as Yellowthroats.
In other words, a sighting of a Common Yellowthroat on an Illinois or Indiana (or Wisconsin) CBC used to be much more rare. But the current report suggests the shift northward has made a winter-time Yellowthroat observation less rare.
The authors of the report cite four lines of evidence as verification of warming climate being a major factor in why bird species’ (including the Yellowthroat’s) have shifted north during the last 40 winters.
Time and space doesn’t allow me to discuss these four areas. But it’s worth noting that the birds’ movement north is consistent with computer model predictions based on a hypothesis of global warming/climate-change effects, according to the authors’ contention.
To read more about this report, see: www.birdsandclimate.audubon.org