Eastern Wood-Pewee Contopus virens

Identification challenge: Eastern vs. Western Wood Pewee

These two species are so closely-related and so similar that they remain one of the most challenging field identification problems in North America. Voice (specifically song) is the only sure way to identify the species, although there are several average differences in plumage and minor differences in structure that continue to tantalize keen observers with the possibility of identification by sight.

Voice:

• Song a clear slurred whistle (vs. a rough, burry whistle)

Full territorial songs are very distinctive, with no overlap, and since these songs are primarily innate (not learned) the species can be identified with complete confidence based on song. Unfortunately full song is heard only on or near the breeding grounds in May-July. Pitfall: Eastern Wood-Pewee can give some short calls that sound rough and buzzy, similar to Western song, and Western often gives short clear whistles, similar to Eastern. In order to use voice succesfully for identification you have to have a good sense of the repertoire of each species and know what type of call you are hearing.

• Non-breeding call a high, strongly upslurred whistle (vs slightly lower-pitched and nearly monotone)

Migrants and wintering birds of both species often give clear whistled notes. Eastern consistently gives a strongly upslurred tu-eee while Western gives a slightly lower and flat teee, often ending with a more or less obvious burry quality.

Structure:

Nearly identical. No differences in head shape or head/bill proportions have ever been reported. Only a very subtle difference in wing/tail proportions as described below.

• Eastern averages slightly longer-tailed and shorter-winged than Western.

Pyle (1997) reports that a formula using tail projection (tip of tail to tip of longest uppertail covert) minus primary projection (tip of longest primary to tip of longest secondary) identifies about 97.5% of all specimens regardless of age, sex, or season. I have long held the vague impression that Western has a slightly more Empidonax-like overall shape than Eastern, which may be related to the shorter tail. Lee et al. (2008) suggest that these proportions can be seen in the field by comparing the primary projection to the tail projection (beyond the primary tips). Eastern has the tail projection slightly longer than the primary projection and in Western the two are about equal. This deserves more study.

• Eastern may hold its tail angled down when perched (vs. Western holding tail more in line with body)

This difference was proposed by Lee et al. (2008) but this is another very small and subjective difference that (if it works at all) would require lots of experience and probably varies quite a bit, particularly if the bird in question is out of range or out of season and is not sitting normally. Further field testing would be helpful, but I think it’s unlikely this difference will hold up.

Plumage:

Overall Eastern Wood-Pewee may give the impression of being paler and “cleaner” with distinct green and gray tones above and a narrow, broken breastband, while Western is darker and “sooty” with brownish tones above and a broad, unbroken breastband. Many of the plumage features touted to distinguish the species simply focus on one particular facet of this overall darkness, but variation in the birds, extensive overlap in color, and the difficulty of judging subtle differences in color, all combine to make these features nearly useless in the field. An impression of overall color might be used as an indicator to pick out an interesting bird, but of all the plumage features, only the difference in wingbars holds the potential to move the identification beyond “possible”.

• wingbars paler and more contrasting, especially the upper wingbar (tips of the median coverts), which is nearly as bright as the lower (vs. Western has upper wingbar drab gray-brown, noticeably darker than the lower)

This seems to be the most reliable plumage difference between the species (which is not saying much). There is some age-related variation, and some overlap in this feature. Further field testing is needed.

• upperparts tinged olive and gray (vs. brownish)

a minor difference in color that may be apparent when viewing a large series of specimens under the same lighting conditions, but is too subtle and too subjective to be useful in the field.

• crown darker than back (vs. concolor)

a minor difference that may be apparent when comparing specimens, but too subtle and too variable to be useful in the field or in photographs.

• edging of underwing coverts and bend of wing pale yellow (vs dusky or brownish yellow)

This difference is subtle, difficult to see in the field, and many birds overlap.

• breastband narrow, paler, often broken by longitudinal pale streak in center of breast (vs. broader, darker, unbroken)

This contributes to the impression that Western is darker, sooty, or “smoky” gray overall. Unfortunately there is quite a bit of variation in overall color, and some Easterns can have relatively dark and solid breastbands, while some Westerns have relatively pale and broken breastbands. Not to mention the difficulty of judging these subtle differences in the field in often-changing lighting conditions.

• undertail coverts with smaller and paler gray centers

another difference related to the overall darkness of Western Wood-Pewee, but there is considerable overlap, and this feature is difficult to see in the field

• lower mandible usually mostly orange (vs. often mostly dark)

Often stressed as a distinguishing feature between these species, but even though most Easterns have orange lower mandibles, it is not unusual to find one with a mostly dark lower mandible. Similarly, most Westerns have a dark lower mandible, but it is fairly common to see individuals with a mostly orange lower mandible.

References

Lee, C., A. Birch, and T. L. Eubanks. 2008. Field Identification of Western and Eastern Wood-Pewees. Birding 40:34-40. pdf here: http://www.aba.org/birding/v40n5p34.pdf

Pyle, P. 1997. Identification Guide to North American Birds: Part I. Slate Creek Press.