W. Ross Silcock | email@example.com | 17 July 2019
Although introgression between Spotted and Eastern Towhees on the Great Plains is well-known (Sibley and West 1959, Short 1961, Bartos Smith and Greenlaw 2015), the two are currently considered separate species (Banks et al 1995). Strong genetic differences between Eastern and Spotted Towhees, including absence of shared haplotypes, were demonstrated by Ball and Avise (1992), although, since their samples were from western and eastern extremes (Washington, South Carolina, and Georgia) of the North American range, this result was not unexpected. Re-evaluation of data in Sibley and West (1959) that showed 56% of individuals in the “hybrid zone” were presumed phenotypic Eastern and Spotted Towhees was considered indicative of assortative mating and supported species status (Banks et al 1995).
Sibley and West (1959) suggested that one factor leading to assortative mating might be correlation of degree of back spotting pattern with climatic differences, in particular the amount of shade in towhee habitat (more in eastern forests than in western open brush). Anecdotal information supporting this theory was provided by T. J. Walker (personal communication), who observed that in his southeastern Lincoln County survey area “I have noted for years that the [Spotted Towhee] songs vary quite a bit – in areas with more deciduous woodland and cedar coverage most of the songs, while still typical of Spotted, have a little hint of “Eastern” to them. In the areas of the canyons with very little woodland, where the landscape is mostly grassland with shrub thickets the songs are quite “dry” and clearly Spotted.”
Sibley and West (1959), despite their study’s finding no difference in “hybrid index” between birds at Blair, Washington Co and in Lincoln Co, pointed out that contact between Spotted and Eastern Towhees in Nebraska was not on a broad front but limited to riparian corridors, and surmised that this resulted in “greatly restricted” gene flow. Currently, however, some 60 years later, riparian corridors over most of Nebraska are significantly more vegetated and apparently not as restrictive to gene flow as in 1959 (Williams 1978, Johnson 1994, Knopf 1986). Johnson (1994) found that by 1986 the entire Platte River Valley system was vegetated whereas in the 1930s only the North and South Platte River Valleys were vegetated.
During the last 60 years, and especially since around 2000, increasing reports of phenotypic Eastern and Spotted Towhees well within the Nebraska summer range of the other as well as birds showing song and plumage characteristics of both species suggests gene flow resulting in increasing introgression but also that assortative mating contnues to be an important factor limiting even more extensive introgression as seen in Northern Flicker in Nebraska. Mixed and unusual towhee songs are common over most of the state. Scharf (2005), during a four-season nesting season netting study along the Platte River in Dawson Co 2001-2004 found 20 Eastern Towhees of phenotype category 0 (“pure” Easterns; Sibley and West 1959) among 107 towhees captured (18.7%) and concluded that this was a result of westward expansion of the Eastern Towhee range; Sibley and West (1959) had found three category 0 towhees among 33 captured (9.1%) at Elm Creek, some three miles east of Dawson Co. Scharf (2005) noted, however, that notable variation between years in the relative proportions captured of the two species was characteristic of a “tension zone maintained by a balance between dispersal and selection against hybrids.” In the Panhandle, there were several reports in May of 2013 and 2014 from Scotts Bluff Co of phenotypic Eastern Towhees, including males singing Eastern songs, a male “unspotted, singing an Eastern song”, and six at two locations 4-21 May 2013, one of which was still present 21 Jul. In spring 2014, at least one phenotypic Eastern male was near Mitchell, Scotts Bluff Co 12 May; one with a “black back except for the white wing spot” was in the same yard 6 May, and a singing male was at Gering Cemetery 9 May. There had been no “verified records” of birds resembling phenotypic Eastern Towhees for the Keith Co area until 2000 (Brown and Brown 2001); since then, one was reported at Lake McConaughy 21 Apr 2007, one was in Dawson Co 25 May 2010, and one was found in southeast Lincoln Co 5 Jun 2004, where, however, the “vast majority are Spotted Towhees” (eBird.org).
The status of Pipilo towhees in Nebraska was summarized by Bartos Smith and Greenlaw (2015) and Greenlaw (2015); Greenlaw (2015) indicated in his Figure 2 that essentially all “black” Pipilo towhees in Nebraska are introgressants, and that, within the “hybrid zone” (most of the state), Spotted Towhee genes predominate in the Panhandle, the northwestern Loup drainage, and along the northern edge of the state following the Niobrara River Valley and Missouri River Valley east to the Yankton area, while Eastern Towhee genes predominate elsewhere, particularly westward in the Platte River Valley to Garden Co. Genotypic Spotted Towhees probably occur only in the Panhandle and genotypic Eastern Towhees only in the extreme east (Bartos Smith and Greenlaw 2015).
In the east, phenotypic male Spotted Towhees singing Spotted songs were carefully studied at Rose Creek WMA, Jefferson Co 27 Jul 2016, and another was near Steele City, Jefferson Co the same day. A male at Schramm SP, Sarpy Co 21-29 May 2005 that looked and sang like a Spotted Towhee possessed about half of the white primary patch typical of Eastern Towhee.
It is interesting that phenotypic Eastern Towhees are reported in the Panhandle in May, the same time Spotted Towhees arrive on their Panhandle breeding range and about a month later than Eastern Towhees arrive in the southeast. It is tempting to consider these later migration dates as evidence that these birds are introgressants with a predominance of Eastern Towhee genes. The role of epigenetics in expression of phenotypes was reviewed by Fresard et al (2013); whether this mechanism is operative among Pipilo towhees in the Nebraska hybrid zone is as yet conjectural.
Recommendations to Observers
Since many readers of Birds of Nebraska- Online (www.birdsofnebraska.org) are eBirders as well, the above outline of the best current science regarding Eastern and Spotted towhees (EATO, SPTO) in Nebraska should be kept in mind when reporting these species to eBird. Currently, eBird offers four options for submitting checklists: Spotted X Eastern Towhee (F1 hybrids), Spotted/Eastern Towhee (introgressants/ backcrosses), Spotted Towhee, and Eastern Towhee. Our Nebraska eBird reviewers set their filters to zero for several regions and seasons, which requires observers to provide documentation. For example, reporting an Eastern Towhee in the Panhandle will flag and the reviewer will request further details. In general, however, these are the guidelines that reviewers follow:
Spotted Towhee X Eastern Towhee: This option should be avoided and deleted from all filters, since it refers to first generation (F1) crosses, which would be extremely rare and, in any case undocumentable in Nebraska given the plethora of introgressants.
Spotted Towhee/Eastern Towhee: This category is for introgressants/backcrosses, the vast majority of Nebraska Pipilo towhees. Except for regions where Spotted Towhee and Eastern Towhee are expected (extreme northwest and southeast, see below), and unless documented as Spotted Towhee or Eastern Towhee, all eBird reports should be in this category.
Eastern Towhee or Spotted Towhee: Although at certain seasons and regions determined by reviewers, reporting towhees as species will not flag, the vast majority of Nebraska towhees are introgressants and should be reported as such (Eastern Towhee/Spotted Towhee) unless a particular bird can be documented (photo, recording, description) as Eastern Towhee or Spotted Towhee.
Figures 1 and 2 below show the ranges of both species, as well as the hybrid zone.
Ball, R.M. Jr., and J.C. Avise. 1992. Mitochondrial DNA phylogeographic differentiation among avian populations and the evolutionary significance of subspecies. Auk 109: 626-636.
Banks, R.C., J.W. Fitzpatrick, T.R. Howell, N.K. Johnson, B.L. Monroe Jr., H. Ouellet, J.V. Remsen Jr., and R.W. Storer. 1995. Fortieth supplement to the American Ornithologists’ Union Check-list of North American Birds. Auk 112: 819-830.
Bartos Smith, S., and J.S. Greenlaw. 2015. Spotted Towhee (Pipilo maculatus), version 2.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, New York, USA. https://doi.org/10.2173/bna.263.
Brown, C.R., and M.B. Brown. 2001. Birds of the Cedar Point Biological Station. Occasional Papers of the Cedar Point Biological Station, No. 1.
Fresard, L., M. Morisson, J.-M. Brun, A. Collin, B. Pain, F. Minvielle, and F. Pitel. 2013. Epigenetics and phenotypic variability: some interesting insights from birds. Genetics Selection Evolution 45:16.
Greenlaw, J.S. (2015). Eastern Towhee (Pipilo erythrophthalmus), version 2.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.262.
Knopf, F.L. 1986. Changing landscapes and the cosmopolitanism of the eastern Colorado avifauna. Wildlife Society Bulletin 108- 132-142.
Scharf, W.C. 2005. New westward breeding records for Eastern Towhees in Central Nebraska. NBR 73: 26-28.
Short, L.L., Jr. 1961. Notes on bird distribution in the central Plains. NBR 29: 2-22.
Sibley, C.G., and D.A. West. 1959. Hybridization in the Rufous-sided Towhees of the Great Plains. Auk 76: 326-328.
Williams, G. P. (1978). The case of the shrinking channels: The North Platte and Platte Rivers in Nebraska (Vol. 781). Department of the Interior, Geological Survey.