Mexican Woodnymph Thalurania ridgwayi Nelson, 1900, was described as a species based on a single specimen collected in Jalisco, western Mexico. It was lumped with Fork-tailed Woodnymph T. furcata (J. F. Gmelin, 1788) and treated as a subspecies of the latter by Peters (1945) without any taxonomic discussion. Subsequently, it was regarded as a subspecies of Crowned Woodnymph T. colombica (Bourcier, 1843) (AOU 1983) until Escalante-Pliego & Peterson (1992) raised it to species level based on differences in plumage colour and pattern from Mesoamerican members of T. colombica.

Phylogenetic analysis of multilocus molecular data has placed T. ridgwayi as sister to the four species of Eupherusa Gould, 1857, and distant from other species of Thalurania Gould, 1848 (McGuire et al. 2014). This was surprising given the morphological similarities of T. ridgwayi to other Thalurania (del Hoyo et al. 1999), including the green throat and chest, dark belly and bright blue-violet crown. However, the phylogenetic evidence clearly demonstrates that T. ridgwayi cannot be maintained in Thalurania because this would result in a polyphyletic taxon.

There are two main ways to reconcile the close relationship of T. ridgwayi to Eupherusa with the requirement that all genera are monophyletic. First, T. ridgwayi might be transferred to Eupherusa (Stiles et al. 2017, Chesser et al. 2020). Stiles et al. (2017) argued that the blue crown of male T. ridgwayi is quite similar to that of Blue-capped Hummingbird E. cyanophrys Rowley & Orr, 1964. However, a blue crown does not clearly group T. ridgwayi with Eupherusa because three of the four species of Eupherusa lack a blue crown and several other species of emerald hummingbirds (Trochilini) also have a blue crown (e.g., Violet-capped Hummingbird Goldmania violiceps, Violet-capped Woodnymph Thalurania glaucopis, Blue-headed Sapphire Chrysuronia grayi, Andean Emerald Uranomitra franciae, Long-tailed Sabrewing Pampa curvipennis excellens). A blue crown has evidently evolved multiple times independently in emerald hummingbirds and carries little weight in grouping taxa into genera. Stiles et al. (2017: 406) also stated that ‘the blackish underparts’ of male T. ridgwayi are shared with male Black-bellied Hummingbird E. nigriventris Lawrence, 1868. We disagree that this is a shared character state because, unlike male E. nigriventris, male T. ridgwayi actually has most of the underparts dusky green, not black (Howell & Webb 1995), and only part of the belly is black (Escalante-Pliego & Peterson 1992). Stiles et al. (2017: 406) further stated that females of T. ridgwayi and Eupherusa share grey underparts and ‘differ mainly in tail patterns’. However, grey underparts occur in the females of several other species of emerald hummingbirds (e.g., Golden-crowned Emerald Cynanthus auriceps, Napo Sabrewing Campylopterus villaviscensio, Antillean Crested Hummingbird Orthorhyncus cristatus, White-vented Plumeleteer Chalybura buffonii, Long-tailed Woodnymph Thalurania watertonii) and the presence of grey underparts is clearly also not a reliable way to define genera. Importantly, females of T. ridgwayi do not differ mainly in tail pattern from Eupherusa but also, clearly, by the absence of red on the secondaries and tertials (present in Eupherusa). Overall, the arguments listed to lump T. ridgwayi with Eupherusa are not compelling.

Figure 1.

External morphology of males (upper row) and females (lower row) of ‘Thalurania’ ridgwayi and the four species of Eupherusa, illustrating the forked blue tail, lack of a distinct red patch on the secondaries and tertials, and lack of white in the tail of the former. Illustrations by Norman Arlott (used with permission from Birds of the world / Cornell Lab of Ornithology).

The second way to address the distant relationship of T. ridgwayi to the morphologically similar Thalurania and its close relationship to the morphologically different Eupherusa is to place T. ridgwayi in a monotypic genus. Whether or not to ‘chop up’ a dichotomous phylogeny into two genera has often depended on the extent of the morphological differences between groups. There are many examples of avian sister taxa that are recognised as different genera explicitly or implicitly based on morphology, including Tetrao / Lyrurus (Phasianidae), Rostratula / Nycticryphes (Rostratulidae), Sterna / Thalasseus (Sternidae), Fulmarus / Macronectes (Procellariidae), Phyllaemulor / Nyctibius (Nyctibiidae), Eudyptes / Megadyptes (Spheniscidae), Drymornis / Drymotoxeres (Dendrocolaptidae), Parus / Pseudopodoces (Paridae) and Cardellina / Myioborus (Parulidae). Sister genera in Trochilini also differ by multiple morphological character states, including Abeillia / Klais, Anthocephala / Stephanoxis, and Leucippus / Phaeochroa.

In this case, there are multiple prominent differences between T. ridgwayi and the four species of Eupherusa (Fig. 1). T. ridgwayi (i) lacks red on the wing (clearly present on the secondaries and tertials in both male and female Eupherusa), (ii) has a forked tail (square in Eupherusa), which is (iii) mainly blue (black in Eupherusa) and (iv) lacks white on the outer rectrices in males (present and extensive in Eupherusa).

We believe the differences between T. ridgwayi and the four species of Eupherusa are best reflected taxonomically by the recognition of two genera. T. ridgwayi shares no unique character state with the four species of Eupherusa and its inclusion in Eupherusa would make that genus unnecessarily heterogenous. Eupherusa would no longer be diagnosable by any single character, unlike the present classification in which Eupherusa can be distinguished from all other emeralds by the presence of red on the tertials. We feel that the multiple clear differences between T. ridgwayi and the four Eupherusa strongly outweigh the debatable similarities between T. ridgwayi and individual species of Eupherusa, and therefore separate genera are warranted. Thus, a separate genus name is required for T. ridgwayi. Because no such name is available, we propose: