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Sunday, December 28, 2014

Irruption


Large numbers of Varied Thrushes (Ixoreus naevius) have been reported in California this year.  They're known for having "irruption years" when more individuals than expected move to areas beyond their normal wintering range.  Irruptions occur in irregular cycles (i.e., not every year) and are unpredictable.  Although I've probably missed them in the past, this is the first time I can recall seeing Varied Thrushes on Bodega Head since moving here in 2005.

[Varied Thrushes normally winter from Alaska to southern California (Ventura County).  But during irruption years, larger numbers are seen in California, and some may appear in areas where they're not typically found, including further south.  Some individuals may stray quite far from their normal range, e.g., to the Midwest or to the East Coast.]

There were at least four birds under the cypress trees across from Gaffney Point on 25 December 2014.  The male below had soil on its bill, making me wonder what the thrushes had been eating.  The Birds of North America account says that along with fruits and berries, Varied Thrushes will eat litter-dwelling arthropods.


Rich Stallcup wrote an article about Varied Thrush invasions for the Point Reyes Bird Observatory newsletter during the winter of 1994-1995.  Interestingly, he mentions that "...Varied Thrush invasions to California are followed here by record-making rains in winter."  We'll see if it holds true this year.

I happened to take a quick photo of two Varied Thrushes in flight over Owl Canyon on 12 October 2014perhaps at the onset of this irruption.  Although it could be better, it's a valuable photo to learn from, as Varied Thrushes have an unusual appearance in flight.  Look for the short tail and two wing bars:

 
 
If you'd like to read Rich's article about Varied Thrush irruptions (or invasions), click here

And click here to read a BirdCast article demonstrating how eBird (an online bird checklist program) data can be used to document an irruption, using Varied Thrushes in 2014 as an example.

This is a good year to keep an eye out for these beautiful thrushes!

1 comment:

Alice Chan said...

One of the best things that happens when there's a Varied Thrush irruption is walking out in the morning to hear their wonderful eerie calls coming from all around. It's magical!