Cryptic
Species
The White-breasted
Nuthatches
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In Brief
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D
uring 2013, the American Ornithologists’ Union
(AOU) evaluated a proposal to “split” the Whitebreasted Nuthatch (Sitta carolinensis) into any-
where from two to four species. The proposal did not
pass (AOU 2013a). Members of the AOU’s Committee on Classification and Nomenclature of North and
Middle American Birds 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 North American birders? Certainly more
identification challenges, but also an opportunity to contribute to our understanding of the current and changing
ranges of these birds.
Background
A key geological event was the formation, 14–2½ million years ago, of the “Basin and Range” district of western North America. This event promoted the splitting
of forests of the U.S. and Canada into three distinct regions: Eastern (east of the Mountain West, and extending north to include much of the continent’s northern
reaches); Interior West (from the Rocky Mountains west
to the eastern slopes of the Sierra Nevada and the Cascades, south to the mountains of northern Mexico, and
including the Davis and Chisos mountains of Texas);
and Pacific (from the crests of the Sierra Nevada and the
Cascades west to the Pacific Coast). A more complete
summary is given by Graham (1999). These distinctions
were reinforced by “recent” Quaternary Period glacial
cycles (Van Devender 1990, Graham 1999) and sub-
This article presents an overview, based on recent scientific literature and the author’s field and museum
work, of our present knowledge of the identification and geographic distribution of the White-breasted
Nuthatches of North America. The bird currently classified as the White-breasted Nuthatch exhibits
considerable geographic variation, and may well comprise multiple species. Three distinctive populations—the “Carolina,” “Rocky Mountain,” and “Slender-billed” nuthatches—occur in the ABA Area.
Field identification is problematic where the ranges of these populations overlap (Carolina and
Rocky Mountain, mid-continent; Rocky Mountain and Slender-billed, farther west). Additional challenges involve vagrancy, hybrids, and natural variation within each group. Fortunately, careful study in
the field often permits a credible identification, especially if a nuthatch is both seen and heard; photos
and audio can be very helpful too.
26
# * 3 % * / (t.":+ 6 / &
sequently led to many of the tripartite groupings
of species and subspecies encountered in North
America north of Mexico (Yellow-bellied, Rednaped, and Red-breasted sapsuckers; Blue-headed,
Plumbeous, and Cassin’s vireos; etc.).
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)
represented. Spellman and Klicka (2007) analyzed
a single mitochondrial DNA (mtDNA) gene and
concluded that there are four major groupings (or
“clades”) of White-breasted Nuthatches; they split
the Interior West group further into Rocky Mountain and Sierra Nevada/Cascade groupings, S. c.
nelsoni and S. c. tenuissima, respectively. Gene flow
between these two Interior West groups is not trivial. Another study, looking at one mtDNA gene and
19 nuclear DNA genes, found the same four groups
(Walstrom et al. 2012), although
sampling near contact zones was
limited. Issues with gene flow and
sample size were major reasons why
the AOU decided not to split the White-breasted Nuthatch into multiple species. Furthermore,
checklist committee members seemed inclined to
split the White-breasted Nuthatch into three species (keeping the Interior West group as one) rather
than four (AOU 2013b).
No common names were proposed for these possible new species. Therefore, I follow the AOU’s
(1910) nomenclature, using Carolina Nuthatch for
the carolinensis group, Rocky Mountain Nuthatch
for the nelsoni group, and Slender-billed Nuthatch
for the aculeata group. Given current knowledge,
the Rocky Mountain (nelsoni, in the strict sense)
and Sierra Nevada/Cascade (tenuissima) taxa are not
reliably separable in the field, so I will discuss them
as a single entity, the Rocky Mountain Nuthatch
(see Grubb and Pravosudov 2008).
Distribution of Proposed Species
Carolina Nuthatch
This nuthatch is largely resident from the Peace
River region of northeastern British Columbia east
through central Alberta, Saskatchewan, and Manitoba to Prince Edward Island and south to eastern
Texas, northern Louisiana, and Georgia, plus a
small area of the Florida Panhandle (Dunn and Al-
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Crow Valley Campground, Weld County, Colorado; 21 September 2012. Photo by © Steven G. Mlodinow. Fig. 2c. Slender-billed Nuthatch. Ridgefield, Clark County, Washington; 12 January 2009. Photo by © Scott Carpenter.
derfer 2011, eBird data accessed 26 March 2014). The Carolina
Nuthatch is at least partially migratory, with wintering birds
westward to the Front Range of Colorado (Mlodinow 2014),
southwest to central Texas (Phillips 1986, Lockwood and
Freeman 2014), and south to the Gulf Coast from Louisiana
to Florida (Stevenson and Anderson 1992, Turcotte and Watts
1999, eBird data accessed 26 March 2014). Additionally, large
numbers have been noted migrating past Cape May Point (T.
Leukering, personal communication).
A range shift appears to have occurred over the past century,
with Carolina Nuthatches establishing themselves as breeders
along the South Platte River in northeastern Colorado, starting
approximately in the 1980s (Mlodinow 2014), and retreating
as breeders from central and most of northern Florida since at
least the 1950s (Stevenson and Anderson 1992).
Vagrants have been recorded on Sable Island, Nova Scotia
(Phillips 1986), in easternmost New Mexico (Baumann et al.
2013), and in southern Florida (Stevenson and Anderson 1992).
Rocky Mountain Nuthatch
This nuthatch is largely resident from south-central British Columbia and the Washington Cascades south into Nevada and
easternmost southern California and southeastern Arizona.
The range extends east to southernmost Alberta, the southwestern half of Montana, Wyoming, and westernmost Texas,
28
then into the cordilleras of Mexico south to Oaxaca. The Rocky
Mountain Nuthatch also occurs east from the Rocky Mountains
along the southernmost edge of Colorado east to westernmost
Oklahoma, plus there are isolated populations in the Sierra de
la Laguna of Baja California Sur, the Black Hills of South Dakota (Spellman and Klicka 2007, Grubb and Pravosudov 2008,
Dunn and Alderfer 2011), and the Pine Ridge of Nebraska (R.
Wright, personal communication).
The Rocky Mountain Nuthatch is at least a partial migrant,
with a few occurring regularly to the eastern border of Colorado (Mlodinow 2014), into the Nebraska panhandle (eBird
data accessed 26 April 2013), and in the lowland areas of New
Mexico (Dunn and Alderfer 2011). Vagrants have occurred in
southwestern Kansas (Phillips 1986) and San Diego County,
California (Unitt 2004).
Slender-billed Nuthatch
This nuthatch is largely resident from southwestern Washington
through western Oregon, most of California, and into northwestern Baja California; there also is an isolated population in
Sierra de San Pedro Mártir of Baja California (Spellman and
Klicka 2007, Grubb and Pravosudov 2008, Dunn and Alderfer 2011). An isolated population in the southern Puget Sound
area of Washington has been extirpated since at least the 1990s
(Wahl et al. 2005). Vagrants have occurred in central Arizona
# * 3 % * / (t.":+ 6 / &
and on Vancouver Island, British Columbia (Phillips 1986).
Identification
Call notes
Identification criteria for White-breasted Nuthatch taxa typically
emphasize the differences in the call notes, which is reasonable,
as these birds’ calls are generally distinct from one another. The
most distinctive is the Rocky Mountain Nuthatch, which usually utters a rapid, stuttering, polysyllabic di-di-di-di-di burst that
carries with it a sense of urgency. Carolina and Slender-billed
nuthatches both give calls that recall the Red-breasted Nuthatch,
being somewhat nasal and usually emitted at a more leisurely
pace than the calls of the Rocky Mountain Nuthatch: a throaty,
emphatic yarnk...yarnk... for the Carolina, and a shriller, more
wavering yurrr...yurrr... for the Slender-billed. With both Carolina and Slender-billed nuthatches, these notes are typically given
one at a time, monosyllabically.
Excited Carolina and Slender-billed nuthatches are prone to
speeding up the rate of their calling, thus approaching the Rocky
Mountain Nuthatch, and occasional Rocky Mountain Nuthatches
will call in a more leisurely fashion, thus sounding more like the
other two taxa. Indeed, recorded calls attributed to one taxon in the
field have proven to be of another via subsequent analysis of sound
spectrograms of the calls (T. Floyd, personal communication).
The songs of White-breasted Nuthatches—soft, whistled sequences, sounding like a distant Northern Flicker—are less variable among the three main groups, and are not discussed here.
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Conifer, Jefferson
County, Colorado; 28 December 2012. Photo by © Steven G. Mlodinow.
Fig. 3b. Carolina Nuthatch. Wray, Yuma County, Colorado; 1 September 2013. Photo by © Steven G. Mlodinow. Fig. 3c. Carolina Nuthatch.
Ovid, Sedgwick County, Colorado; 3 November 2013. Photo by © Steven
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Photo by © Scott Carpenter.
ABA.ORG/BIRDING
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3b
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W H I T E - B R E A S T E D N U T H ATC H E S
Table 1%JTUJOHVJTIJOH$IBSBDUFSTPG8IJUFCSFBTUFE/VUIBUDI(SPVQT
Carolina
Rocky Mountain
Slender-billed
Greater Coverts Pattern
Prominent black or blackish centers with
contrasting grayish edging
Dusky to gray centers with broad grayish edging;
little or no contrast
Typically gray centers, sometimes dusky, with little
to no contrast; plainest of all three
Outermost Tertial Pattern
Black on inner and outer webs
Black to dusky on inner webs, dusky on outer webs
Black to dusky on inner webs, dusky on outer
webs; averages paler than Rocky Mountain
Upperparts Coloration
Paler gray
Darker gray
Medium gray
Dark Crown Width
Broad
Narrow
Broad
Facial Pattern
Black of crown approaches eye more closely than
Rocky Mountain; many birds have small black dash
or line extending back from eye
Black of crown farther from eye, typically forming arc
over face; no black mark behind eye
Black of crown approaches eye fairly closely;
occasionally small black dash back from eye
Side/Flanks Coloration
Pale gray to white, sometimes suffused with buff
Medium blue-gray, rarely with buff wash
Pale gray to white, sometimes suffused with buff
Bill Length
Averages shorter and stouter
Averages longer and more slender
Averages shorter and more slender
Also, all White-breasted Nuthatches give weak, mumbling, nasal notes when foraging: plink, pleek, etc. Audio recordings and
sound spectrograms of typical calling sequences are available
on the Birding magazine website: aba.org/birding
Visual characters
The standard identification literature has given short shrift to
visual identification cues, but I have found that these are surprisingly useful. The discussion in Wood (2006) provides a
good starting point. To help clarify the usefulness of various visual characters, I studied approximately 150 specimens each of
Carolina and Rocky Mountain nuthatches and 50 Slender-billed
Nuthatch at The Field Museum of Natural History in Chicago
and at the University of Puget Sound’s Slater Museum of Natural History. Also reviewed were more than 100 photos of each
taxon online from locations at which only one is likely—for example, Illinois for Carolina Nuthatch. I found the three most useful characters to be the following: (1) the pattern of the greater
secondary coverts (hereafter, greater coverts), (2) the outermost
tertial pattern, and (3) the width of the dark crown. Two other
characters were useful: (4) the side/flank coloration and (5) the
facial pattern. I was able to find no consistent difference in vent
(between the legs) pattern, and I believe that the differences in
back color are probably of little value in the field. Fig. 1 provides
an overview of terminology (greater coverts, tertials, flanks), and
Table 1 summarizes useful and proposed differences.
Greater Coverts
Carolina Nuthatches have blackish centers to their greater coverts that contrast markedly with the feathers’ edges. Rocky
Mountain Nuthatches are variable in this character, ranging
from rather uniformly marked greater coverts to coverts with
dusky (but not black) centers. In my museum and photographic studies, I found that birds from the northwestern portion of
30
this taxon’s range seem to show the plainest greater coverts,
whereas birds from Arizona and New Mexico average darkercentered greater coverts. Some Rocky Mountain Nuthatches
have almost blackish centers to their greater coverts, but these
are never as dark as those of Carolina Nuthatches, and when
present, the dark center is narrower. Slender-billed Nuthatches
usually have plain greater coverts, with little or no contrast.
These differences would seem to make the identification of
Carolina Nuthatches straightforward. However, feather positioning can confound interpretation, as the scapulars or side
feathers sometimes partly veil the greater coverts, making the
dark centers less apparent. Also, the centers of the greater coverts are less evident if these feathers are crowded together and
more evident if they are spread apart.
Tertials
On all three taxa, only the inner web of the smallest (innermost)
two tertials is black or dusky. On the largest (outermost) tertial,
Carolina Nuthatches are black on the inner and outer webs,
Rocky Mountain Nutatches are dusky (occasionally black, especially on southern birds) on the inner web and dusky on the
outer web, and Slender-billed Nuthatches are gray to dusky
(rarely blackish) on both the inner and outer webs. Again,
feather positioning can make this character difficult to assess.
Crown and Face
Carolina Nuthatches have a rather broad dark crown when
viewed from above. In Rocky Mountain Nuthatches, the crown
stripe is relatively narrow. Slender-billed Nuthatches have a
crown width similar to that of Carolina Nuthatches, but may
average a bit narrower. When viewed from the side, the broader
crown of Carolina and Slender-billed nuthatches tends to bring
the crown edge closer to the eye than in Rocky Mountain Nuthatches. Be advised that the posture of a bird can affect the ap-
# * 3 % * / (t.":+ 6 / &
pearance of crown width, particularly
with side views.
Many, perhaps most, Carolina Nuthatches have a narrow black line extending backward from the eye, sometimes short, sometimes rather long.
This mark is seemingly never present
on Rocky Mountain Nuthatches. Some
Slender-billed Nuthatches appear to
show a very brief dark mark behind
the eye.
The H-word
Sides/Flanks
I owe a debt of gratitude to The Field
Museum of Natural History and the
University of Puget Sound’s Slater Museum of Natural History for their generosity in allowing me to ponder their
specimens ad nauseam. Additionally,
Scott Carpenter and David Irons are to
be thanked for making so many images
of Slender-billed Nuthatch available to
me. Finally, but not least, many thanks
to Tony Leukering, Mark Lockwood,
and Michael O’Brien for helpful comments on an earlier version of this manuscript.
The sides of Carolina and Slender-billed
nuthatches are whitish or pale blue–gray,
sometimes washed in orange–buff. The
sides of Rocky Mountain Nuthatches are
pale to medium blue–gray, sometimes
nearly as dark as the back, rarely washed
in orange–buff.
Bill
There is extensive overlap in bill size,
with bill length ranging from 15.4 to
19.5 mm in Carolina Nuthatch, from
16.9 to 21.7 mm in Rocky Mountain Nuthatch, and from 16.3 to 19.2
mm in Slender-billed Nuthatch (Pyle
1997). In Colorado, where Carolina
and Rocky Mountain nuthatches occur,
I have found bill length to be of little
use in the field. Slender-billed Nuthatches do, however, appear to have a
shorter and, in particular, slimmer bill;
bill shape of this taxon may well be
useful for field identification.
Where the ranges of different Whitebreasted Nuthatch populations intersect, it is quite possible, if not likely,
that some hybridization occurs. One
should always be open to the possibility of hybridization when confronted
with potential vagrants or in regions
where the ranges of two taxa approach
or overlap one another.
Acknowledgments
4a
4b
4c
Literature Cited
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Final Thoughts
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Recent evidence suggests that three or four species are represented by what we currently label the White-breasted Nuthatch. Most individuals of these “new” species are identifiable with careful study of calls and plumage.
Correct identification requires that observers understand the effects of posture, plumage wear, and lighting—as
well as the somewhat subjective nature of human hearing and seeing. As always, an appreciation for variation in
field characters—aural and visual—is essential. Photos and audio recordings may be necessary for establishing
the identity of birds well out of range.
Many such “cryptic” species have been recognized over the past decade or two, mainly in the tropics, but also
right here in the ABA Area. Perhaps the most famous is the Gunnison Sage-Grouse, described to science only
in 2000. A very recent example is that of the erstwhile Sage Sparrow, now considered to comprise two species,
the Bell’s and Sagebrush sparrows; conceivably, a third species is involved. Genetic data are certainly part of the
elucidation of cryptic species, but phenotypic characters play a role, too. For example, the differences between
what we today know as the Bell’s and Sagebrush sparrows were well understood by the ornithologists of the
19th century. And the recent discovery of the Gunnison Sage-Grouse was based on a broad suite of evidence:
behavioral, vocal, physiological, and of course genetic. By being more aware of birds at the subspecies—or
apparent subspecies—level, rather than simply at the species level, savvy birders can contribute to our understanding of biogeography as well as identification criteria.
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