Warm-Colored Mountain Bluebirds: an Unappreciated Identification Pitfall

IN THE SCOPE

Warm-colored Mountain Bluebirds: An unappreciated identification pitfall

Tony Leukering Despite housing just three species, the genus Sialia still causes con- sternation for birders. Confusion often occurs when birders inexpe- rienced with all three species mistake male Western Bluebirds, some of which sport little rust on the back, with Mountain Bluebirds. A second challenge involves separating males and females of Western and Eastern bluebirds. However, one of the most common pitfalls that I have seen trap birders is created by the small percentage of female Mountain Bluebirds that exhibit obvious rufescent coloration on the chest. This essay is intended to be read with reference to the picture on the back cover of this issue.

Bluebird molt strategy (this is relevant, I promise!) Bluebirds, like most thrush species, conduct just one molt per year after their first year (Pyle 1997). Bluebirds in their first plumage cycle obtain their typical spotted juvenal plumage by conducting their first prebasic molt (first basic plumage = juvenal plumage) in the nest; they leave the nest in this plumage. Some time after fledging, they conduct their second molt – the preformative molt – of their short lives, replacing all of their juvenal body plumage and some of their inner greater coverts. Once this molt is finished (as late as October in more-northern and/or later-season broods), these young birds are quite difficult to discern as such in the field, as correct ageing requires discerning the difference between old and new greater coverts and/ or the presence of contrasting older primary coverts or flight feath- ers. In contrast, adult bluebirds conduct a complete prebasic molt, that is, they replace every feather. All bluebird molting is done on or

Back Cover Photo

This female Mountain Bluebird caused the precise identification quandary about which this essay was written. Fortunately, the excel- lent photographs (one of which is produced here) provided a superb example of warm-colored females of the species, something that I have tried to capture photographically for some time in order to write this very essay. Photograph by David Waltman, 1 December 2012

Colorado Birds October 2013 Vol. 47 No. 4 297 near the breeding grounds. With the completion of the preformative (immature) and prebasic (adult) molts, bluebirds are in the freshest plumage they will wear until conducting their next molt the follow- ing summer/fall.

Warm-colored female Mountain Bluebirds The typical female Mountain Bluebird plumage, and the only one illustrated in most field guides, is of nearly unrelieved grayish body plumage (blue-gray when freshest), with variably bright blue or gray- blue wings and tail. Most females sport at least a bit of orangish cast to the chest when fresh, though this may not be discernible unless viewed at close range. Variety is inherent in biology, and this brighter color is quite variable in extent and brightness, with a minority of females (probably less than 10%) having obvious rusty coloration on the chest. This color can even extend a bit down the sides into the gray flanks and, very rarely, up into the lower part of the throat plum- age (pers. obs.). Sibley (2000) is the only major ABA-area field guide to illustrate this plumage well (though the plumage is mentioned in text in Leukering [2006]). The presentation in Sibley (2000; at least the first printings) exaggerates the orange tones. Since many birders do not expect to see orange on female Moun- tain Bluebird chests, such birds can create identification quandaries. Typically, these birds are identified as either Western or Eastern blue- birds, but they have also been suggested as possible hybrid progeny of Mountain Bluebird and one of the orange-breasted species. However, a more complete examination of such birds should reveal the longer, paler-blue wings and tail and the thinner bill lacking pale tones typi- cal of Mountain Bluebird.

Relevance of molt timing Bird plumage begins to wear and fade as soon as it is grown, with feathers becoming ragged and colors duller. Thus, female Mountain Bluebirds with only slightly rufescent tones to the chest will likely lose such appearance as wear and fading progress. In fact, even the brightest females’ plumage will fade, making them stand out less from the crowd. Because of this facet of plumage, we birders are much more likely to notice such birds in fall and early winter, when the plumage is still fresh.

Literature Cited Leukering, T. 2006. Thrushes. In NGS Complete Birds of North America 2006

298 Colorado Birds October 2013 Vol. 47 No. 4 Pyle, P. 1997. Identification Guide to North American Birds, part I. Slate Creek Press, Bolinas, CA. Sibley, D. A. 2000. The Sibley Guide to Birds. Alfred A. Knopf, New York.

Tony Leukering, 1 Pindo Palm St. W, Largo, FL 33770 ([email protected])

Colorado Birds October 2013 Vol. 47 No. 4 299 Female Mountain Bluebird, city of Longmont, Boulder County, 1 December 2012. Photograph by David Waltman

In the Scope: Warm-colored Mountain Bluebirds . . . 297

308 Colorado Birds October 2013 Vol. 47 No. 4