Friday, December 12, 2008

Tricky ID #3 - Bendire's & Curve-billed Thrashers

I was surprised that Bendire's Thrasher was even on the cumulative list for this Christmas Bird Count. In my experience, in Southeastern Arizona there are a few places where Bendire's Thrasher occurs. They are in those locations all year, and I have never bumped into the species in any other location that does not closely match the same habitat. This indicates to me that the species is resident and does not wander in this part of the state. Populations to the north of us are apparently migratory, but it's not clear where they go. Places where I've seen them include northern Avra Valley north into the Santa Cruz Flats, the Whitewater drainage of the Sulphur Springs Valley, and the San Simon Valley east of the Chiricahuas.

The species appears to prefer very flat topography with sandy or loose soils and patches or rows of mesquite with a grassy, weedy understory. An absence of cholla cactus seems to be a unifying characteristic too, and my guess is that this keeps out Curve-billed Thrasher (which usually builds its nest in cholla). Curve-billed is a large, aggressive species that would displace Bendire's.

The best field mark is the smaller bill on Bendire's. With a good profile view (close range or through a spotting scope), the lower edge of the bill appears straight, while both the upper and lower mandible in a Curve-billed are curved. Supporting field marks are the more clearly defined spots across the breast on Bendire's (somewhat arrow-shaped with very good view), a pale base to the bill, paler eyes, and a vaguely warmer color to the lower face. Earlier in the year, immature Curve-billed Thrashers have all of these exact same field marks and are very frequently misidentified as Bendire's in urban Tucson and Phoenix.

Bendire's is usually silent, sometimes giving a slight "chuck," while Curve-billed has the very familiar upslurred double whistle.

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