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Ravens in Colorado: an Impossible Field Identification?

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Ravens in Colorado: an Impossible Field Identification? IN THE SCOPE Ravens in Colorado: An Impossible Field Identification? Tony Leukering Colorado hosts two species of ravens, the holarctic Common Raven (Corvus corax), grist for many an authorial mill (includ- ing that of Poe), and the limited-range Chihuahuan Raven (Cor- vus cryptoleucus), which has probably seen little, if any, poetry or prose written about it. Common Raven is represented in Colorado by the subspecies sinuatus. Its range is largely restricted to West Slope and montane habitats in Colorado, including on the Mesa de Maya, which extends from Las Animas County into Baca, Bent and Otero counties. Common Ravens are also regularly seen on the northwest plains adjacent to the foothills, typically west of I-25 though with some exceptions [especially westernmost Arapahoe County and the vicinity of Rocky Mountain Arsenal N. W. R. in Adams County (eBird 2015)] and on the plains in the Arkansas River drainage. The Chihuahuan Raven’s range is something of an enigma. Henshaw (1875) reported the species as widespread on the Colorado plains during the period of American Bison slaugh- ter (mid- to late 1800s), but its range contracted greatly following the near extinction of wild bison in the United States, possibly returning to pre-slaughter range (Aiken and Warren 1914). The first Colorado Breeding Bird Atlas presented the breeding range of Chihuahuan Raven as restricted to 11 southeastern counties: south from southern Pueblo County east to southern Prowers County, with scattered records north through Crowley and Kiowa counties and single records from the southern borders of El Paso and Chey- enne counties (Nelson 1998). With the great increase and intensity of birding efforts in the state that began in the early 1990s (pers. obs.), sightings of Chihua- huan Ravens outside of the range mapped by Andrews and Righter (1992) and Nelson (1998) began to accumulate and seems to be accelerating (pers. obs.). There are eBird (eBird 2015) reports from 38 of the state’s 64 counties, including those as far north as Routt, Larimer and Weld and as far west as Mesa and Montezuma. The vast majority of these far-flung reports is supported by little or no defini- tive details, and therefore is not included in the public eBird data. But, what constitutes definitive details for two species so similar, that are entirely black in both plumage and soft-parts coloration? Aye, there’s the rub. Colorado Birds Fall 2015 Vol. 49 No. 4 255 Back-cover Photo, Part I The photograph on the back cover of this issue was taken in July 2015 and serves as an excellent illustrative piece for this essay. Clear- ly it’s a raven, but which species? Perhaps the following will assist in the bird’s identification. Raven Identification “Distinguishing Chihuahuan Raven from small Common indi- viduals can be extremely difficult.” (Sibley 2000) As indicated above, raven identification is at times, “extremely difficult.” In fact, some birders (including yours truly) consider this the single most-difficult identification quandary in the ABA area. Yes, there are other very difficult groups, but how many are of such large birds that are often seen so well? My personal experience with raven identification in southeastern Colorado followed an interest- ing course. I first considered them quite difficult, then fairly straight- forward as I got more experience with them. However, as additional experience piled up, I’ve forced myself to keep an open mind and have now reverted, considering them to be at the very least quite dif- ficult to separate, if they’re definitively and reliably separable at all. Due to the near-complete lack of plumage characters permitting sepa- ration (both are all black), birders in the New World area of sympatry or nearby have had to resort to shape and vocal clues to attempt raven iden- tification, with the various individual characters discussed below. Pieplow (2014) discussed the vocal aspects of raven identification, and given the difficulty of discussing vocalizations in a print medium, I highly recom- mend reading (and listening to) that post, including the comments. Before tackling the individual characters that are typically used to attempt raven identification, please note the following caveats with regard to size. While Common Raven averages considerably larger than Chihuahuan Raven, there is enough variation in overall size in both species as to make an in-the-field distinction between a large Chihuahuan Raven and a small Common Raven problematic. Ad- ditionally, some of that variation in size is due to sex, with males in both species being larger than females in nearly all characters. In the treatment of individual characters below, all statements are of rela- tive size. You may also refer to the data presented in Table 1. As we all know, correctly assessing size in the field without some comparison, direct or indirect, is fraught with uncertainty. A direct comparison of size with some other bird species may be useful, but only with a suit- able yardstick: An American Crow mobbing a raven would provide a useful comparison, but a much smaller bird, such as a Western King- bird, would not, due to the significant size disparity. 256 Colorado Birds Fall 2015 Vol. 49 No. 4 Fig. 2. Two each Chihuahuan (left) and Common (right) Ravens (specimens at Mu- seum of Vertebrate Zoology, CA). Photo by Peter Pyle Bill Shape Common Raven bills are longer and, generally, deeper than are those of Chihuahuan Raven. With the overlap in bill depth between the two species, but the absolute greater length of bill in Common Raven, Common Raven generally appears to have a longer bill rela- tive to depth than that of Chihuahuan [midpoints of extreme ratios are length being 2.55x greater than depth in Chihuahuan, 2.81x in Common (Ratio of C:D); Fig. 1]. However, without direct compari- son, this character is probably nearly useless in field situations. Nasal-bristle Length The nasal bristles are the feathers that lie atop the basal part of the bill in all members of the genus Corvus, and much is made of this character’s usefulness in raven identification by some birders. The ex- tent of the bristles on Chihuahuan is generally more (often distinctly more) than half the bill length, while that of Common is generally less than half the bill length. This would mean that individuals with nasal-bristle length of about half the bill length are not identifiable using just this character. Additionally, there has been suspicion raised Colorado Birds Fall 2015 Vol. 49 No. 4 257 about the true usefulness of the character in Colorado (B. Maynard pers. comm., S. Mlodinow pers. comm.), as most ravens assumed to be Chihuahuans in Colorado seem to lack the excessive nasal bristles typical of the species in the core of its range (Fig. 1). Whether this is due to misidentification of ravens on our part, variability in this character on the part of Colorado Chihuahuan Ravens or, egad!, a result of hybridization of ravens in Colorado is not determinable at this time. Suffice it to say that this character may not be particularly useful in the field, at least not in Colorado. Color of the Base of Neck and Breast Feathers Chihuahuan Raven used to be known by the moniker White- necked Raven due to the fact that the base of the neck and breast feathers is white versus the gray of the same feather bases in Common Raven. However, given that looks at these feather bases good enough to provide certainly of the color just about requires having the bird in question in the hand, that means that this character’s usefulness is akin to, but probably less useful than, that of the brown neck ring in male Ring-necked Ducks. At least with the duck, if you can see the bird well enough to note the brown neck ring, you certainly have a sufficient view to note all of the other much-more-obvious identifi- cation features! In order for the white feather bases to be seen, the feathers usually need to be ruffled by the wind. Pyle (1997) presents this caution: “Beware that the throat and breast plumage differences can be subtle, particularly without direct comparison.” And that in a publication aimed at bird banders with birds in the hand! Some observers report seeing single Common Ravens in very windy con- ditions exhibiting apparent white bases to these feathers in direct comparison with gray feather bases on other Common Ravens in the same flock (S. Mlodinow pers. comm.). Throat Shagginess This character is created by the long feathers of the throat, with Chihuahuans having, generally, shorter such feathers than do Com- mons. However, given the overlap in the range of values in the two species, and the difficulty in correctly assessing the feature in the field [the non-overlapped part of the Common range is only 14 mm (0.55 in.)], this is probably another character with limited field use- fulness. Wing Length and Shape Common Raven may have longer wings, relative to overall size, than does Chihuahuan [at least Sibley (2000) seems to present the 258 Colorado Birds Fall 2015 Vol. 49 No. 4 species that way], but if so, any difference is probably not reliably detectable in the field. Sibley (2000) also presents a different wing- tip shape in the two species, with Chihuahuan wings more square- or round-tipped versus more pointed Common Raven wings, but this difference is much less apparent in Sibley (2014). Although Pyle (1997) presents a wingtip metric for Chihuahuan (the relative lengths of primaries 5 and 9), no such measurement is given for Com- mon. Tail Shape With the exception of voice, tail shape is probably the single char- acter used most often by birders to identify ravens to species in Colo- rado (and elsewhere in the New World).
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