Proposal (885) to South American Classification Committee
Generic classification of the Xolmiini
Two recent studies proposed alternative generic classifications for the Xolmiini: Ohlson et al 2020 and Chesser et al 2020. Their proposals were largely coincident but differed in how broad the genus Nengetus should be. Our subproposals here broadly coincide with these two papers but differ in the treatment of Nengetus.
We recommend a YES vote to all of the subproposals.
A) Include pyrope in Pyrope (YES) or do something else (NO)
The highly distinctive Patagonian forest pyrope was found as stemming from a deep node in both studies, supporting the recognition of the old genus Pyrope, of which it is the type species.
B) Include rufipennis in Cnemarchus (YES) or leave in Polioxolmis (NO)
Both species in this clade are rather elongated, upright-perching, high-altitude specialists that deliver shrill whistles and share the same tail pattern. We favor merging them in a single genus, which would emphasize the common theme between them in plumage, habitat associations and vocalizations. This treatment has been followed in the past (see table from Chesser et al 2020).
C) Restrict Xolmis to irupero and velata (YES) or do something else (NO)
This is a rather straightforward decision, as both species formed a coherent clade in both studies and the type species of Xolmis is irupero.
D) Use Myiotheretes for 4 species (YES) or merge Myiotheretes under Nengetus (NO).
Myiotheretes is a very homogeneous genus comprising four Andean species of reddish/brownish birds that deliver simple monotone flat whistles and inhabit the tree-line zone. The continued recognition of Myiotheretes implies stability and is consistent with the recovery of this clade in both studies.
Ohlson et al. (2020) proposed to merge Myiotheretes in an expanded (and heterogeneous) Nengetus, stemming from the placement of cinereus in their phylogeny. However, this merger is not consistent with the position of cinereus in the work by Chesser et al. (2020) and would create a large and very heterogeneous grouping of birds. The best way out of this is to recognize Myiotheretes, which would imply no change in our classification.
E) Use Nengetus only for cinereus and Neoxolmis for coronatus, rubetra, salinarum, and rufiventris (YES) or use Nengetus for all the taxa (NO)
As in the case of Myiotheretes, the rather obscure genus Nengetus is again at the center of conflict. Both studies advocate a rather broad use for Nengetus, but we believe this is not good for two reasons 1) it creates too heterogeneous groupings and 2) it disrupts stability.
Reasons for restricting usage of Nengetus for cinereus
The type of Nengetus is cinereus, a widespread species with distinctive plumage that lives in open savanna areas. This distinctiveness is also evident in the deep genetic divergence with other species in the clade. Besides these aspects, cinereus does not walk on the ground regularly or at all (unlike the other species usually placed in Neoxolmis) and inhabits more humid and generally lowland areas (reaching higher altitude in southern Brazil, a pattern exhibited by many "Pampas" birds), whereas the other species are all exclusive to arid areas mostly in the extreme southern portion of the continent. Finally, the position of cinereus might be open to discussion (see contrasting results in Ohlson et al 2020 and Chesser et al 2020 discussed also in Subproposal D); thus, we do not recommend expanding Nengetus any further beyond its type species, cinereus.
Reasons for placing four species in Neoxolmis
The four species that would be included in Neoxolmis (rufiventris, rubetra, salinarum and coronatus) are all species of dry, extreme areas that regularly walk on the ground (less often in coronatus). Their nests are made of sticks on the ground or in shrubs are remarkably similar, as are their eggs. They form a monophyletic group in both the Ohlson et al. (2020) and Chesser et al. (2020) studies. Traylor (1979) and Lanyon (1986) already recognized a close relationship of rufiventris+rubetra/salinarum when placing them together in the genus Neoxolmis (see table from Chesser et al 2020 above). In sum, they form a far more coherent grouping when cinereus is excluded. Moreover, using Neoxolmis has the advantage of using a name that has already been in wide use and which recognizes that they are in some way akin to "Xolmis". Having direct field experience with all the Xolmiini species, and with 3/4 species being endemic breeders to Argentina, we are convinced that using Neoxolmis is the best possible solution, which is consistent with genetic evidence, supported by field data and which best serves stability.
We think that, based on the deep genetic divergence and plumage differences, the Argentine endemic breeder coronatus may merit a new genus. However, no such genus name is available and at present it seems easier and less disrupting to place it in Neoxolmis together with the other species in this clade of austral birds.
F) Recognize Syrtidicola for fluviatilis (YES) or keep in Muscisaxicola (NO)
A deep divergence time, possible sister relationship to Satrapa and morphological and ecological differences (e.g., very short tail, lowland inhabitant of riverine sand bars) lead Chesser et al. (2020) to describe a new genus for fluviatilis.
G) Sequence. The sequence we need to adopt would be this one (YES) or another one (NO):
G) Recognize Heteroxolmis. The Black-and-white Monjita was found to not even belong in the Xolmiini, so a return to the genus Heteroxolmis is warranted. It was found to be more closely related to other "warm" grassland specialists (Alectrurus spp. and Gubernetes yetapa) in the Fluvicolini. Ohlson et al 2020 stated:
"Recognize Heteroxolmis Lanyon, 1986 (type = Tyrannus dominicanus (Vieillot) for Xolmis dominicanus and remove it from Xolmini to Fluvicolini. The distinctiveness of dominicanus from other Xolmis species was recognized by Lanyon (1986) based on morphological characters of the nasal capsule and syrinx, but he regarded the two genera as close relatives. Among the character states that motivated a separation from Xolmis is a fully ossified nasal capsule, including alinasal walls and turbinals, which is also found in Alectrurus Vieillot, Gubernetes Such, Fluvicola Swainson and Arundinicola d’Orbigny (Lanyon 1986). As the gender of Heteroxolmis is feminine (see Lanyon 1986, fig. 24), the name of the species becomes Heteroxolmis dominicana.”
A YES vote would resolve Xolmis dominicanus as Heteroxolmis dominicana and place it in the sequence between Alectrurus and Gubernetes.
Nacho Areta & Mark Pearman, September 2020
Chesser, R.T., M. G. Harvey, R. T. Brumfield and E.P. Derryberry (2020) A revised classification of the Xolmiini (Aves: Tyrannidae: Fluvicolinae), including a new genus for Muscisaxicola fluviatilis. Proceedings of the Biological Society of Washington 133:35–48
Ohlson J.I., M. Irestedt, H. Batalha Filho, P.G.P. Ericson and J. Fjeldså (2020) A revised classification of the fluvicoline tyrant flycatchers (Passeriformes, Tyrannidae, Fluvicolinae). Zootaxa 4747: 167–176
Tree from Ohlson et al 2020
Tree from Chesser et al 2020
Table from Chesser et al 2020
Comments from Robbins: “YES to all proposals except E. With regard to E, looking at the genetic data it does not make sense to break up this clade by separating cinereus from the others. If one was going to apply a consistent treatment (note the branch length of coronatus is similar to cinereus), one either treats these as three different genera or all the same. The latter seems more reasonable to me.”
Comments from Zimmer: “Each of these subproposals seem reasonable to me, given the phylogenetic data as well as ecological, morphological, and vocal distinctions and similarities between the various groups. The fly-in-the-ointment does appear to be the Gray Monjita, as evidenced by the differing results in the Ohlson et al. (2020) study versus the Chesser et al. (2020) study. I remain a pretty consistent proponent of more homogeneous, tightly defined genera when making borderline calls, and, in this case, I can’t bring myself to go for the greatly expanded Nengetus suggested by the Ohlson et al. results, and treating cinereus as part of the same genus as coronatus, rubetra, salinarum and rufiventris doesn’t make as much sense from the standpoint of ecology and biogeography (as suggested by the authors of the Proposal) as does treating cinereus as distinct (at the generic level) from the other species in the clade.