Another visit at the end of January failed to find either one. (See accounts of my searches for Crissal Thrasher and Le Conte's Thrasher) (Also see the birding site guide to the Mesquite Bosque and Borrego Springs waste treatment ponds).
In December 2015 there was a California Thrasher at the waste treatment ponds. While this is a common San Diego County bird, it is quite rare out in the desert. However, it seems that this bird has remained to April, perhaps joined by a mate. I found this bird right away on my most-recent visit, singing away.
But also, others had been reporting a Crissal Thrasher there, so both were possible. They are quite similar, but the California Thrasher has a dark eye and pale butterscotch color under the tail; the Crissal Thrasher has a pale eye and dark rusty under tail coverts. Songs are disjointed whistles, the California Thrasher with harsh notes.
I found it!
|Crissal Thrasher. Borrego Springs, California. April 24, 2016. Greg Gillson.|
Mesquite are quite brushy trees with long hooked thorns. Eventually they were too thick to get any closer to the singer. So I present the photo above from maybe 150 feet away--too far for tack-sharp "magazine cover" photos, but sufficient for ID purposes. You can see the pale iris, right? (You can click on the photo for a larger (and fuzzier) view.)