The White-breasted Nuthatches of Colorado

Transcription

1 IN THE SCOPE The White-breasted Nuthatches of Colorado Steven G. Mlodinow During 2013, the American Ornithologists Union (AOU) evaluated a proposal to split the White-breasted Nuthatch (Sitta carolinensis) into anywhere from two to four species, but decided not to alter the status quo (AOU 2013a). The checklist committee members were, however, nearly unanimous in their desire to revisit the question soon, after more data have been gathered (AOU 2013b). What does this mean for Colorado birders? Potentially, another identification quandary. Background The Basin-and-Range area of western North America formed between 14 and 2.5 million years ago, splitting the forests of the United States and Canada into three distinct regions: eastern (i.e. east of the montane west, which includes much of the continent s northern reaches), interior west (from the Rocky Mountains to the eastern slope of the Sierra Nevada and Cascades) and Pacific (Graham 1999). Photograph from collection at Field Museum of Natural History. The four left hand birds are S.c. nelsoni (sensu strictu) collected during December. Two right hand birds are S.c. carolinensis collected during December as well. This photo nicely demonstrates the differences in tertial pattern. Also, note how minimal the difference in back darkness is. Crown width can not be readily discerned on specimens as many have been on their backs for decades, flattening/ spreading the crown feathers. Photo by Steve Mlodinow These distinctions were reinforced by recent Quaternary Period glacial cycles (VanDevender 1990, Graham 1999) and subsequently led to many of the tripartite groupings of species and subspecies encountered in North America north of Mexico (e.g. Yellowbellied, Red-naped 180 Colorado Birds Spring 2014 Vol. 48 No. 2

2 and Red-breasted Sapsuckers; Blue-headed, Plumbeous and Cassin s Vireos). White-breasted Nuthatches divide nicely along these lines with Eastern (S. c. carolinensis), Interior West (S. c. nelsoni) and Pacific (S. c. aculeata) subspecies groups (Grubb and Pravosudov 2008). Spellman and Klicka (2007) analyzed a single mitochondrial DNA (mtdna) gene and concluded that there were four major groupings (or clades) of White-breasted Nuthatch, splitting the Interior West further into Rocky Mountain and Sierra Nevada/Cascade groupings, though evidence of gene flow between these was not trivial. A further study, looking at one mtdna gene and 19 nuclear DNA genes, found the same four groups (Walstrom et al. 2012), though sampling near contact zones was limited. This flaw was a major reason why the AOU decided not to split White-breasted Nuthatch into multiple species. Furthermore, checklist committee members seemed more inclined to split White-breasted Nuthatch into three (keeping the Interior West group as one) rather than four species (AOU 2013b). Please note that only the Pacific subspecies group of Whitebreasted Nuthatch has a common name (Slender-billed Nuthatch); one of the protests of the AOU checklist committee (AOU 2013b) was that common names were not proposed for the other subspecies groups. Therefore, in lieu of an extant common name, Cordilleran White-breasted Nuthatch will be used in this article for members of the Interior West subspecies group, which includes populations well down into Mexico and on the southern tip of the Baja California Peninsula. Occurrence in Colorado Two of the above mentioned White-breasted Nuthatch taxa are known to occur in Colorado: S. c. carolinensis (hereafter carolinensis or Eastern WBNU) and S. c. nelsoni (hereafter nelsoni or Cordilleran WBNU). The Cordilleran WBNU is predominantly an inhabitant of coniferous woodlands in Colorado, though a few breed in nearby deciduous (largely cottonwood) woodlands of Colorado s plains (e.g. Barr Lake, Adams County), with scattered summer records as far east as Flagler, Kit Carson County (June 2013; pers. obs.). The Eastern WBNU story is more complex as this taxon seems to be undergoing a range expansion in Colorado. Both Bailey and Niedrich (1965) and Phillips (1986) list only nelsoni as present during summer in Colorado, with no White-breasted Nuthatches present in far eastern Colorado during that season. However, Andrews and Righter (1992) list White-breasted Nuthatch as a rare summer resident along the South Platte River drainage near the Nebraska border. Currently the Eastern WBNU is a fairly common breeder Colorado Birds Spring 2014 Vol. 48 No

3 Table. Distinguishing characters of Eastern and Cordilleran WBNU Greater coverts near the Kansas border along the Republican River drainage in Yuma County and along the South Platte River drainage west to Washington County (ebird data, pers. obs.), and it is uncommon farther west along the South Platte into Weld County as far as the Greeley area (ebird data, pers. obs.). Furthermore, an Eastern WBNU was found on 31 May 2012 at Barr Lake (pers. obs.) and at least one bird has been found during summer in the Teller Lakes/White Bluff area of Boulder County from (Ted Floyd, pers. comm.), thus bringing Cordilleran and Eastern WBNU into close proximity during the nesting season. There is little information available as to seasonal movements among various White-breasted Nuthatch taxa, partly because many birders do not attempt to identify them. Using ebird data (accessed 8 January 2014), there appears to be a pulse of Cordilleran WBNUs onto Colorado s plains during September, mostly north of the I-70 corridor. Thereafter, small numbers persist into April on the plains, including in southern Colorado. Additionally, there is a pair of intriguing records from the hilly regions of eastern Las Animas County during May, causing one to wonder if this taxon might breed there. Again using ebird data (accessed 8 January 2014), Eastern Whitebreasted Nuthatch numbers seem to increase in Colorado (and in nearby western Kansas and Nebraska) starting in early September and then drop off again in November, reaching a nadir during the winter months. During March, abundance appears to return to summer levels, but no spring peak is evident. Eastern WBNU have not Eastern WBNU Prominent black centers with contrasting grayish edging Cordilleran WBNU Dusky gray centers with broad grayish edging, little contrast Tertials Black on inner and outer webs Dusky gray on inner webs, limited or no dusky gray on outer webs Crown Broad Narrow Face Black crown edge tends to run straight across face, many birds have small black dash or line extending back from eye Vent Rust extends forward to thighs in about 50 percent of birds from western portion of range Black tends to form arc over face, so much more white over eye; no black mark behind eye Rust extends forward to thighs in about 75 percent of birds Sides/Flanks Pale gray to white, sometimes with buff wash Medium gray, rarely with buff wash anterior to thighs Upperparts Paler gray Darker gray Bill Averages shorter and stouter Averages longer and more slender 182 Colorado Birds Spring 2014 Vol. 48 No. 2

4 yet been found west of the Front Range, with numbers along the Front Range peaking from September into November, then dropping off sharply after November, with a few birds lingering into May. Sightings near the foothills south of the Denver area are scarce, implying that the South Platte River serves as the primary migratory corridor. Identification Commonly, identification criteria for White-breasted Nuthatch taxa emphasize differences in call notes, which is reasonable as these birds typically sound rather different from each other. Cordilleran WBNUs characteristically give a rapid machine-gun-like burst, nyeh-nyeh-nyehnyeh-nyeh, whereas Eastern WBNU usually utter a slower yenk-yenk, reminiscent of the calls of Red-breasted Nuthatch. However, an excited Eastern WBNU will sometimes give a very rapid This White-breasted Nuthatch was photographed at Barr Lake, Adams County, on 30 December Both Eastern and Cordilleran Whitebreasted Nuthatches likely summered at Barr Lake in This bird shows tertial markings that are rather classic for carolinensis, yet the medium gray sides without buff are typical of nelsoni. The crown seems intermediate and there is a tiny black mark behind the eye. The greater coverts are hard to judge as they are partly veiled by the scapulars, but they appear black centered, more like carolinensis. This combination of features argues strongly for this bird being a Eastern x Cordilleran White-breasted Nuthatch hybrid. Importantly, the call was indecipherable to the author. Photo by Steve Mlodinow series of call notes, and occasionally a Cordilleran WBNU will call at a slower rate. In these circumstances the calls of these taxa can be surprisingly confounding, and tape-recorded calls attributed to one taxon have proven to be that of the other via sonographic analysis (Ted Floyd, pers. comm.). Though the standard identification literature has given short shrift to visual identification cues, these are surprisingly useful and the discussion in Alderfer (2006) provides a good starting point. To help clarify the usefulness of various visual characters, I studied approximately 150 specimens each of Eastern and Cordilleran WBNU at the Field Museum of Natural History in Chicago and at the Uni- Colorado Birds Spring 2014 Vol. 48 No

5 Back Cover Photo Key Top: A Cordilleran White-breasted Nuthatch photographed at Conifer, Jefferson County, on 28 December Note the medium/darkish gray underparts, darker than seen on any Eastern White-breasted Nuthatch. The white on face makes long arc over eye, implying a narrow dark crown, and there is no dark line extending backward from the eye. The greater coverts have somewhat dark, but rather narrow, centers.. The bill is intermediate in length, within range of both carolinensis as well as nelsoni. Photo by Steve Mlodinow Middle: An Eastern White-breasted Nuthatch photographed at Wray, Yuma County, on 1 September The black crown closely approaches the eye from which a short black line extends backward. The sides are very pale gray, which would be highly atypical of nelsoni. The bill is rather long for carolinensis, and appears longer than that of the Cordilleran above. Photo by Steve Mlodinow Bottom: An Eastern White-breasted Nuthatch photographed at Ovid, Sedgwick County, on 3 November Note that the flanks are medium gray mixed with buff, but that the sides are rather pale gray with some buff feathers mixed in, a typical pattern for this taxon. The greater coverts that are folded tightly together simply look blue-gray, but the innermost (on this photo, uppermost) are spread more widely showing the typical broad black centers contrasting markedly with the gray edges. The bill appears rather short, perhaps shorter than on any nelsoni. Finally, the black crown of this bird does form an arc over the eye, though perhaps not as high as that of nelsoni. Again, there is a black streak extending back from the eye. Photo by Steve Mlodinow Cordilleran White-breasted Nuthatch (For detailed photo caption, see p. 184) Eastern White-breasted Nuthatch (For detailed photo caption, see p. 184) Eastern White-breasted Nuthatch (For detailed photo caption, see p. 184) In the Scope: The White-breasted Nuthatches of Colorado Colorado Birds Spring 2014 Vol. 48 No. 2 versity of Puget Sound s Slater Museum. Also reviewed were >100 photos online from locations at which only one taxon is likely (e.g. Illinois for Eastern WBNU). I found the most useful characters to be the pattern of the greater secondary wing coverts (hereafter greater coverts), the tertial pattern and the width of the dark crown. The side/flank coloration and the facial pattern, which is in part related to the crown width, are also of some use. Vent pattern, darkness of upperparts and bill size seem only marginally useful. Table 1 summarizes these differences. The pattern of the greater coverts would seem to be a slam-dunk distinction, but feather positioning can muddy this difference. Some184 Colorado Birds Spring 2014 Vol. 48 No. 2

6 times the scapulars or side feathers partly veil the greater coverts, making the dark centers of Eastern WBNU less apparent. Additionally, any dark centers to the greater coverts are less apparent if these feathers are crowded together and more evident when the coverts are spread apart. Some Cordilleran WBNUs have almost blackish centers to their greater coverts, but these are never as dark as those seen on Eastern and, when present, the dark center is narrower. Similarly, feather positioning and lighting can affect the visibility of the inner webs of the tertials and the apparent darkness of the outer webs. Other caveats include 1) The apparent crown width and face pattern can be affected by the bird s posture. 2) There is extensive overlap in bill size, with bill length ranging from 15.4 to 19.5 mm in Eastern WBNU and from in Cordilleran WBNU (Pyle 1997). 3) Hybrids are possible. Conclusion Two taxa of White-breasted Nuthatches occur in Colorado, perhaps representing two species. With a modicum of practice and good views, observers should be able to separate these two: Eastern Whitebreasted Nuthatch (S. c. carolinensis) and Cordilleran White-breasted Nuthatch (S. c. nelsoni). Indeed, whether subspecies or species, identifying White-breasted Nuthatches to taxon is a worthwhile task as such will provide an interesting glimpse into biogeography in action. ACKNOWLEDGMENTS I would like to thank Tony Leukering for his review of an earlier version of this manuscript and for his encouragement, and Ted Floyd, for stimulating discussions on Whitebreasted Nuthatch vocalizations. I owe a debt of gratitude to The Field Museum of Natural History and the Slater Museum (University of Puget Sound) for being so generous in allowing me to ponder their specimens ad nauseum. LITERATURE CITED Alderfer, J Compete Birds of North America. National Geographic Society, Washington, D.C. Andrews, R. and R. Righter Colorado Birds. Denver Museum of Natural History, Denver, CO. A.O.U. 2013a. Fifty-fourth supplement to the American Ornithologists Union Check- List of North American Birds. Auk 130: A.O.U. 2013b. AOU Committee on Classification and Nomenclature (North & Middle America) Votes on Proposals 2013-C. proposals/2013_c_votes_web.php#2013-c-2. Accessed online January Bailey, A. M and R. J. Neidrach Birds of Colorado. Denver Museum of Natural History, Denver, CO. Colorado Birds Spring 2014 Vol. 48 No

7 Graham, A Late Cretaceous and Early Cenozoic History of North American Vegetation. Oxford University Press, New York, NY. Grubb, Jr., T. C. and V. V. Pravosudov White-breasted Nuthatch (Sitta carolinensis), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: cornell.edu/bna/species/054 doi: /bna.54 Phillips, A. R The Known Birds of North and Middle America, part 1. A.R. Phillips, Denver, CO. Pyle, P Identification Guide to North American Birds, part 1. Slate Creek Press, Bolinas, CA. Spellman, G. M. and J. Klicka Phylogeography of the White-breasted Nuthatch (Sitta carolinensis): diversification in North American Pine and Oak Woodlands. Molecular Ecology 16: Van Devender, T. R Late Quaternary vegetation and climate of the Sonoran Desert, United States and Mexico. Pages in J. Betancourt, T. R. Van Devender, and P. S. Martin, editors. Packrat middens: the last 40,000 years of biotic change. The University of Arizona Press, Tucson, Arizona, USA. Walstrom, J. W., J. Klicka, and G. M. Spellman Speciation in the White-breasted Nuthatch (Sitta carolinensis): a multi-locus perspective. Molecular Ecology 21: Steven G. Mlodinow, sgmlod@aol.com 186 Colorado Birds Spring 2014 Vol. 48 No. 2