March 2000 Mystery Birds

Thanks again to everybody for some very informative discussion. I learned a lot. The birds are a Bendire's Thrasher and a Western Bluebird.


The thrasher was photographed 10 November 1990 at Lodi, San Joaquin County. This individual wintered in the area for several consecutive years and was widely seen. Curve-billed Thrasher has never been recorded in Northern California, and unequivocal records come primarily from the Imperial Valley in winter where their favorite habitat seems to be old rusty machinery.

Identification of Bendire's vs. Curve-billed has been the subject of several papers. I particularly like the nice set of photographs in Kaufman and Bowers (Am. Birds 44:359-362, 1990). Key features are the shape of the lower edge of the mandible and the shape of the breast spots. The edge of the lower mandible is nearly straight on the mystery bird. Its mandible may appear more curved because of the curvature of its upper edge, but this is more illusion than reality. The breast pattern is pure Bendire's with nice arrow shaped spots. Those of Curve-billed should be more blurry and oval-shaped. The pale base to the lower mandible is barely discernible in the photo, but this character is unreliable in my opinion. Most Curve-billed Thrasher photos I have seen also show a pale base to the bill. Eye color is also unreliable, although a bird with deep red eyes is probably a Curve-billed.


The bluebird was photographed along Panoche Road, San Benito County, 26 March 1986. This area is famous for Mountain Bluebirds in migration and in winter, but the species has never been known to nest there. If it were a Mountain Bluebird, it would be a highly significant breeding record. But Western Bluebird is a common breeder in the area and the photographer identified the bird as that species.

When reviewing bluebird identification in my ornithology class, I used this slide in a quiz. The students identified it as Mountain Bluebird and I agreed at the time. It was not until I removed the slide from the tray that I noticed it was actually labeled "Western Bluebird." So I am not surprised that people found it particularly confusing. I was also fooled by the long wings and pale blue coloration.

One could argue, that there would be no problem in the field, and it is just an artifact of the photograph that makes it difficult. However, a similar looking female bluebird found on the Palo Alto Christmas Bird Count this last season caused similar confusion. Originally it was identified as a Western Bluebird, later changed to Mountain Bluebird, and finally established as a Western Bluebird after all. It also showed very long primary projection and pale blue coloration as the mystery bird. It was studied critically by numerous experienced observers and demonstrated that this can be a real problem in the field and not just an academic exercise.

I was fascinated by the discussion centering around primary projection. Everybody agreed that the primary projection was very long, but there was disagreement about whether it was diagnostic for any particular species. Kaufman (Am. Birds 46:159-162, 1992) wrote:

"Mountain Bluebird has, on average, distinctly longer wings than Eastern Bluebird. It averages slightly longer-winged than Western Bluebird, but the difference is not apparent on perched birds. (Contrary to some published statements, the wingtips of Mountain Bluebird do not extend 'to or beyond the tip of the tail when the bird is at rest.) The wingtip-to-tail ratio looks about the same on perched Western and Mountain bluebirds. Western may even seem relatively longer-winged then, perhaps because its body is shorter and more compact."

The light blue coloration of the mystery bird looks good for Mountain, but again Kaufman (op. cit.) warns:

"Females always show some blue in the wings and tail (more in adults, less in first-winter females) but the slight differences in shade of blue are not helpful for separating the species."

The color of the belly should be a good clue. It is white on Mountain and grayish on Western. The mystery bird seems to best fit Western, but the contrast in the image makes it difficult to be sure. More important is that the brownish-pink wash on the back contrasts with the nape. Mountain Bluebird has a grayish wash on the back, not brownish and it never shows contrast in the nape area. Also I feel the rusty-pinkish patch on the flanks is well outside the range of variation shown by Mountain Bluebird and best fits Western Bluebird.

The original images are below. To view public comments or join the fray by adding your opinion, click here. Have fun!

Photos © Nancy Conzett and Peter LaTourrette.
All rights reserved.

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