A Study of Two White-breasted Nuthatch Subspecies


In 2023, Emma Arulanantham was one of four recipients of a WFO Student Research Grant to support their field studies. In summer of that year, she studied two subspecies of the White-breasted Nuthatch—Sitta carolinensis aculeata and S.c. tenuissima—in Northern California. Here is her report on her field work.

Testing vocal responsivity, discrimination, and prezygotic isolation between subspecies of White-breasted Nuthatch
Introduction

The White-breasted Nuthatch (Sitta carolinensis), a common resident bird of North American pine and oak woodlands, is notable due to its high genetic variability among populations, as well as its varied vocalizations. Studies using molecular data have determined that populations fall into four reciprocally monophyletic clades that are largely congruent with current subspecies (Figure 1): A-Eastern, B-Pacific, C-Eastern Sierra Nevada (ESN), and D-Rocky Mountain, Great Basin, and Mexico (RGM). A recent genomic analysis (Spellman and Klicka 2007) suggests the species evolved in situ in regional communities and the isolation of populations in these regions is captured in cryptic genetic variation. However, there are many smaller regions of contact between subspecies that have not been closely examined and which may call into question the extent of genetic isolation between populations.

We examine the contact zone between Pacific (S. c. aculeata) and ESN (S. c. tenuissima) clades across the Modoc Plateau region in Northern California. Specifically, the goal is to more precisely define the ecological and geographic range limits of each subspecies in the contact zone using molecular and vocal data, and to highlight the importance of contact zone sampling in evaluating species or subspecies status. Our mtDNA sequencing analysis of 163 new samples (Figure 2) revealed introgression between the two subspecies in the contact zone, compelling further genomic and vocal analysis of the region.

Recent analyses (Pandolfino and Pieplow 2015) have also shown vocal differences between the subspecies, in both songs and calls. However, no behavioral data have been collected thus far on whether these variations in song influence recognition and thus may contribute to prezygotic isolation. Integrating this component will provide a critical element to understanding how behavioral barriers may drive diversification, reproductive isolation, and incipient speciation (Coyne and Orr 2004, Price 2008).

This grant funded playback experiments to address the following questions.

(1) Is there vocal discrimination between the simple songs of geographically and genetically distinct populations?

(2) If there is vocal discrimination, how does it manifest in the contact zone being studied?

(3) Do the results of these playback experiments mirror the observed genetic structure?

Answering these questions will test the hypothesis that birds will respond more strongly to recordings of their own simple songs (distinct from one subspecies to another), and that this discrimination may contribute to the genetic structure we see in the mtDNA.

Photo: Joan Tisdale

Methods

We studied song recognition and aggressive response by White-breasted Nuthatches at four sites using methods similar to those of Provost et al. (2018).

The sites chosen for this study were as follows.

(1) aculeata away from the contact zone (Palo Alto, Santa Clara County)
(2) aculeata away from the contact zone (Shasta Lake, Shasta County)
(3) tenuissima in east end of contact zone (Warner Mountains, Modoc County)
(4) tenuissima away from contact zone (Markleeville, Alpine County)

In each locality, White-breasted Nuthatches were randomly played the simple song of each subspecies along with a control song from a closely related species (Oak Titmouse, Baeolophus inornatus). We measured three response variables to assess aggression: (1) vocal response (do they sing or not); (2) approach to the speaker; and (3) agitation behavior. In total, 49 birds were sampled.

Emma Arulanantham at Shasta Lake, Shasta County.

Results

(1) The tenuissima birds (sites 3, 4) approached, sang, and responded aggressively to both the aculeata simple song and to their own (Figure 3).
(2) The aculeata birds responded somewhat more to their own song.
(3) Baseline response level was substantially lower for the aculeata birds (sites 1, 2), possibly due to timing of the experiments relative to the breeding cycle.

Discussion

Increased introgression in the ND2 data suggests a potential range expansion of tenuissima westward. We also see a possible trend of overlap between the two subspecies in playback and vocal data. Our analysis confirmed the findings of previous studies showing that aculeata had significantly higher average song frequency (pitch) than tenuissima. Simple song in these two subspecies appears to conform with the acoustic adaptation hypothesis (Morton 1975), but this needs further study. Contact zone songs appear to align more closely with tenuissima in frequency based on previously defined ranges. Surprisingly, tenuissima birds (sites 3, 4) responded with comparable aggression to the aculeata simple song as well as their own. We did observe a marginally higher rate of response by aculeata birds to their own simple song (sites 1, 2). The only song we heard in response to our playback was the slow song (Pandolfino and Pieplow 2015), except for one individual (site 3) who initially sang the slow song, then a disyllabic quank, and eventually transitioned to a faster song.

Our study underscores the importance of in-depth contact zone sampling when assessing species and subspecies distribution and ecology. Whole genome sequencing (in process) will help clarify the genetic makeup of the contact zone as well as subspecies ranges. Further analysis of this data in the context of ecological niche modeling, habitat, and climate will be critical for understanding the geographic patterns of subspecies distribution. Our genomic data will also help to inform future playback experiments. We aim to conduct further playback trials at different times of the year, different field sites, and with different variables (e.g., aculeata ‘tooey’ song, slow vs. fast simple song, and subspecies-unique calls). Additionally, tangential to the main objective of our study, we hope to further investigate the categorization and purpose of the aculeata ‘tooey’ song as well as its potential relationship to the slow simple song (sung by the other three subspecies).

References

Coyne, J.A., H.A. Orr. 2004. Speciation. Sunderland, MA: Sinauer Associates.

Pandolfino, E.R., K.P. Able, J.L. Dunn, D. Lasprugato. 2017. Ranges of the Subspecies of the White-Breasted Nuthatch in California. Western Birds 48:26–34. doi 10.21199/WB48.1.2

Pandolfino, E.R., N.D. Pieplow. 2015. Comparison of vocalizations of the four U.S. subspecies of the White-breasted Nuthatch. Western Birds 46:278–290.

Price, T.D. 2008. Speciation in birds. Boulder, CO: Roberts.

Price, T.D. 1998. Sexual selection and natural selection in bird speciation. Philis. Trans. R. Soc. Lond. B 353: 251–260.

Provost, K.L., W.M. Mauck III, B.T. Smith. 2018. Genomic divergence in allopatric Northern Cardinals of North American warm deserts is linked to behavioral differentiation. Ecology and Evolution 8:12456–78.

Spellman, G.M., J. Klicka. Phylogeography of the white-breasted nuthatch (Sitta carolinensis): diversification in North American pine and oak woodlands. Molecular Ecology 16:1729–1740. doi 10.1111/j.1365-294X.2007.03237.

Acknowledgments

I thank curators at the University Museum of Vertebrates and Denver Museum of Nature & Science for providing supplemental tissue samples as references for this study. Genetics lab work was supported by the Museum of Vertebrate Zoology Avian Genetics Fund. Thank you to Ed Pandolfino for contributions to the data and literature on this subject and to others who have studied this species in the past. Many thanks as well to Carla Cicero, Lydia Smith, Daniel Wait, Linnea Schaefer, Xinyi (Bella) Qiu, and Rauri Bowie for their contributions to the project. Finally, I am so grateful to the WFO for supporting this study and for providing a platform to share this research.

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  1. Very interesting, Emma! Showing my ignorance here: do the 4 subspecies reliably vary (in plumage or song) such that you might be able to identify the subspecies of the respondent during playback experiments? That is, although you may have been in a zone dominated by aculeata at site 1, your genetic evidence suggests a lot of tenuissima individuals present, too…could you distinguish this during your playback trials? Looking forward to seeing more on this in the future!

  2. Ah, wonderful! Thank you Emma! What could be better than donating WFO funds for an issue that vexed me (and Kimball Garrett) for decades. I first noted the striking call note differences in the early 1980’s between aculeata in Southern CA and up to 3500’ or so on the west side of the Sierra and tenuissma from about 7000’ in the Sierra and up and over the Sierra and to the east. In October 1987 I chased a Crested Caracara at Mono Lake and had lunch with David Gaines and heard all about his upcoming book, Birds of the Yosemite Sierra and the East Slope. He was looking for photos. I sent him a number of photos, a few of which appear in the book (published in 1988). Most importantly we talked about White-breasted Nuthatches and my thoughts then are conveyed in his book. I had met David Gaines only once before, a birding trip to Morongo Valley in April 1971 with Bruce Broadbrooks. We looked forward to working together on his book and beyond and corresponded for the next few months. Then, tragically he was killed in a car accident in a snowstorm on U.S. 395 in January 1988 at Deadman Summit in Mono County. He relayed my nuthatch thoughts posthumously when his book was published.

    I gave long felt that there are likely three species of White-breasted Nuthatches, eastern, Rockies/Great Basin and Pacific. Mainland Mexico and the mountains the Cape District of Baja California are included in the Rockies/Great Basin clade. The eastern clade comes very close 1/4 mile or so) to meeting the the Rockies/Great Basin clade in such locations as the Black Hills (Michael Retter in litt.). Their habitat divides them, riparian woodland for the former, conifers for the latter. Finding the contact zones between the aculeata and Great Basin/Rockies clade has been a challenge. Much as in Carla Cicero’s previous work on titmice the Modoc Plateau, this is a great place to investigate. I would have thought the Warner Mountains would have been a bit of east the contact zone. In the Sierra Nevada the red fir zone on the west slope divides the clades and farther south the Kern River is much of a dividing line with aculeata in the Paiutes and Greenhorn mountains. But at Chimney Creek campground in extreme southeast Tulare County we have had both taxa at the same time. Subsequently I have mainly had aculeata there where there are oaks, but on the conifers on the ridges above perhaps they are tenuissima. In the Transverse ranges to the south, it is all aculeata, even in high elevation habitats that would be perfect for tenuissima.

    A couple of further comments. Subspecies aculeata seems to wander east to Mojave Desert oases than tenuissima wanders. I have recorded tenuissima down to about 6000’ in fall in tall red firs at Crane Flat, Yosemite N.P. And a tenuissima once wintered (1987) in a coastal park in Ventura. Brian Daniels once recorded tenuissima in the Imperial Valley. Second don’t lose focus on the contact notes, delivered year-round and utterly different between the three groups. I should add that my ears detect no difference between tenuissima and nelsoni to the east and south. The yidda chattering calls of tenuissima are so different from the ethereal wheer of aculeata or the low pitched yank of eastern carolinensis, the call that was quoted for so many field guides, East or West, of the past!

    Finally, thanks Emma for sharing your research with our membership. And WFO members I hope you can appreciate the full breadth of where your scholarship donations go. We have also help sponsor another graduate student studying geographical variation in Spotted Towhees on the east side of the Sierra and to the “dry” mountains to the east. There are major vocal differences (songs and calls) from those to the west and south. Emma please don’t hesitate to reach out for any further help. Sorry for the long post. I am jet-lagged in Bangkok.

    Thanks to others previously such as Ed Pandolfino and Ken Able that have investigated these questions.

  3. Another aspect of this issue with these two groups of nuthatches are that despite their very different calls, morphologically they appear almost identical, so identifying birds in the field is totally based on calls. Kimball Garrett says that he had difficulty assigning specimens to taxon at the Los Angeles County Museum, but other researchers have done this. Maybe Emma can help on this too. Thankfully nuthatches are talkative. Eastern birds can be visually identified by bill shape and plumage. If these groups are split into three species in the future, we have pointed out that the accepted English name for aculeata is Slender-billed Nuthatch, a good name in comparing it to carolinensis of the East with a shorter and thicker appearing bill. But tenuissima has the longest bill and appears even thinner I would guess, so perhaps “More Slender-billed Nuthatch.”