IN THE SCOPE

Ravens in : An Impossible Field Identification?

Tony Leukering Colorado hosts two species of , the holarctic Common ( corax), grist for many an authorial mill (includ- ing that of Poe), and the limited-range (Cor- vus cryptoleucus), which has probably seen little, if any, poetry or prose written about it. 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 , possibly returning to pre-slaughter range (Aiken and Warren 1914). The first Colorado Breeding 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 in both plumage and soft-parts coloration? Aye, there’s the rub.

Colorado 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 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). In general, Chihuahuan Ravens have rounded tails and Common Ravens wedge-shaped tails. However, tail shape seems to be a factor associated with sex, with females of both species possibly being rounder-tailed than are males. Thus, as with overall size, male Chihuahuans and female Commons

Table 1. Chihuahuan and Common Raven measurements1. Measurements are in mm; sample sizes (where provided in source literature) are in parentheses.

Chihuahuan Raven Common Raven (sinuatus) Character Female Male Female Male Wing chord 321-361 (100) 332-380 (100) 390-425 (19) 412-440 (17) Tail length 179-205 (20) 181-214 (20) 218-242 (19) 225-250 (17)

All All Wing chord 321-380 (200) 390-440 (36) Tail length (A) 179-214 (40) 218-250 (36) r1-r62 (B) 25-45 34-633 Ratio of B:A4 0.14-0.21 0.16-0.25 Length of longest 29-42 39-56 throat feathers Bill length5 (C) 49.5-59 63-71 (14) Bill depth6 (D) 20-22.5 21.7-26.1 Ratio of C:D4 2.48-2.62 2.72-2.90 1 All values from Pyle (1997) except those of Bill length of Common Raven are from Oberholser (1918) 2 The differences in length between innermost (r1) and outermost (r6) rectrices, a measure of "wedged-ness" 3 Pyle (1997) provides this measure for the species as a whole, not for sinuatus, which is a medium-sized subspecies. 4 Range obtained by comparing minimum value of both variables and maximum value of both variables 5 Exposed culmen measurement; that is from base of bill to tip 6 Measured at nares

Colorado Birds Fall 2015 Vol. 49 No. 4 259 may very well overlap in tail shape. Yes, those nearly diamond- shaped tails of large, long-tailed male Commons probably provide an excellent identification clue, but round tails do not do much for us. Unfortunately, Pyle (1997) did not provide a range of values of “wedged-ness” (r1-r6; the higher the value, the more wedge-shaped) for the subspecies found in Colorado, so the range provided in Table 1 includes all four North American subspecies. Because sinuatus is a medium-sized subspecies, the larger and smaller values of that range are probably not found in Colorado. Also, it is probably worth men- tioning that factors including how spread the tail is as well as the degree of molt and/or wear of the tail can impact one’s impression of tail shape.

Back-cover Photo, Part II Are you ready to tackle the identification of this raven using the above criteria? Size: With nothing in the picture other than raven and blue sky, we cannot determine the bird’s overall size. Bill shape: With the bird’s bill being open, we cannot adequately assess the depth of the bill, thus the ratio of length to depth cannot be determined. (There do seem to be some odd aspects of this bird’s bill, which are discussed below.) Nasal-bristle length: Despite the low angle of the photo, we can see the extent of the nasal bristles, which appear to lay on about half the length of the top of the bill, so that character is not useful to identify this individual. Color of the base of neck and breast feathers: There seem to be some gray bits in the bird’s neck/breast plumage, which might suggest Common Raven as the identification. However, this appearance is not due to ruffled feathers and exposure of the feather bases to the light (more on this below). Throat shagginess: The throat is shadowed in the picture and no shagginess is evident (more on this below). Wing length and shape: The wings seem fairly short and wide, at least to me, and the wingtip seems an odd combination of pointed [long distance between the tip of the outermost (p10) and longest primaries) and squared-off (p6-p8 being of the same length). Perhaps this feature points toward Chihuahuan (more on this below). Tail shape: The tip of the tail is smoothly rounded, with no sug- gestion of a wedge shape, though the absolute tail length seems a bit long to my eye. This character may be a wash, or could lean toward Chihuahuan.

260 Colorado Birds Fall 2015 Vol. 49 No. 4 Ageing Ravens I know…why would you want to? Well, as with gulls and many other bird species, determining the bird’s age can instruct on the identification process. At least with ravens, we have to deal with only two age classes, adult and juvenile/immature (birds in their first plumage cycle; Leukering 2010). The pictured bird is readily aged as an immature by at least three features. 1) The gape and the roof of the mouth (the latter of which we can see well thanks to the open bill) are pale and contrast with the black of most of the outside of the bill. Adults lack such an obvious gape (and what they show is black) and have the roof of the mouth black. 2) The body shows a mix of feather generations, with scattered clumps of gray-brown neck, breast and belly feathers that are remnants of juvenile plumage. 3) The color of the primaries and secondaries are a grayish-brown that produces a strong contrast with the black formative (Leukering 2010) feathering of the wing linings. Knowing that the picture was taken in July, and knowing that there has been extensive replacement of juvenile body plumage, but no replacement of flight feathers, we can determine that this bird was hatched this year and is, at best, a few months old. Additionally, because the bird is still wearing its juvenile flight feathers, we might want to consider any wing-shape characters to be suspect as wing shape in juveniles/immatures of many bird species differs from that of adults of the same species (e. g., many or most eagle species, most or all Buteo species, Little Gull). Taking into account all of the above, I find that there are no strong clues to allow identification of this raven. Well, there is one, but un- less you looked ahead, you wouldn’t know that the photo was taken in northwestern Montana, about ten miles from the U.S. border with Canada. The location should nicely rule out Chihuahuan Raven. While range-based identification may not be very satisfying, bird- ers use that criterion all the time. A silent wood-pewee seen in Utah is a Western, but seen in Virginia it’s Eastern. Yet as birders state all the time, “birds have wings” and the ability to show up where they are not “supposed” to be, so why is the pictured bird not documenta- tion for Montana’s first record of Chihuahuan Raven? Despite the aforementioned poorly supported scattering of eBird reports of Chihuahuan Raven nearly the length and width of Colo- rado, the vast majority of reports of the species come from that part of the state where they are “supposed to be.” Colorado birding has the belief that, at least in the breeding season, Chihuahuan Ravens occupy the prairie, Common Ravens the foothills and mountains, with a bit of overlap on the prairies within a few miles of “suitable”

Colorado Birds Fall 2015 Vol. 49 No. 4 261 Common Raven habitat. This creates the situation in which ravens on the plains in summer are identified as Chihuahuans and those in the mountains and foothills as Commons, a circular-reasoning situ- ation if ever I saw one. Raise your hand if you have ever identified a distant summer raven on the plains as a Chihuahuan, despite the fact that you discerned no useful field characters. Did your hand join mine, which was wildly waving in the air? Why is it that we are will- ing to use range maps as the sole feature to identify individuals of some difficult groups, but not of other groups? Consider this essay a plea for more thought and care in raven identification in the state, which should lead to more extensive use of the “raven sp.” category in field notebooks and in eBird.

Acknowledgments I greatly appreciate discussion of raven bill morphology with and unpublished data on such from Peter Pyle. I also thank Steve Mlodinow for a thoughtful review of a previous draft of this essay.

Literature Cited Aiken, C. and E. Warren. 1914. Birds of El Paso County. Colorado College Publ. General Series 74, Science Series No. 13. Colorado Springs, CO. Henshaw, H. W. 1875. Report on the ornithological collections made in por- tions of Nevada, Utah, California, Colorado, , and , during the years 1871, 1872, 1873, and 1874 in report upon geographical and geological explorations and surveys west of the one hundredth merid- ian, vol. 5. U. S. Government Printing Office, Washington, DC. Leukering, T. 2010. Molt and plumage: A primer. Colorado Birds 44:135- 142. Nelson, D. L. 1998. Chihuahuan Raven. In Colorado Breeding Bird Atlas (H. E. Kingery, ed.), Colorado Bird Atlas Partnership and Colorado Divi- sion of Wildlife, Denver, CO. Pieplow, N. 2014. Common vs. Chihuahuan Ravens. Earbirding.com (http:// earbirding.com/blog/archives/4736). [Accessed 4 August 2015] 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. Sibley D. A. 2014. The Sibley Guide, 2nd ed. Alfred A. Knopf, New York.

Tony Leukering, [email protected]

Fig. 1 (back cover). Immature Common Raven, Toole Co., MT, 30 July 2015. Photo by Tony Leukering

262 Colorado Birds Fall 2015 Vol. 49 No. 4 In the Scope Ravens in Colorado: An Impossible Field Identification? . . . 255