
Over the the course of the last year, I've worked on revising the Birds of the World article for the Volcano Hummingbird. By far the most fascinating part of the account for me was writing about the history of its treatment as a species or two or three. I've quoted some of what I wrote there to share here:

To this, I add there is still some potential uncertainty particularly around the population on Poás and Barva Volcanos which extends up into Juan Castro Blanco National Park. It has the smallest reproductive habitat available and is the most genetically distinct (I think). I found some limited genetic studies on some specimens in the LSU museum but couldn't find the localities for the specimens nor did my consultations to the paper authors receive a response in inquiring about their locations, but I believe they show some level of genetic difference between flammula/torridus and simoni (guessing that's how the specimens break down on biogeographic grounds) however my knowledge of genetics doesn't allow me to opine on the significance of these differences as to whether there is a potential for a future split or not. I think (very) long term their allopatric breeding habitats will obviously lead to speciation but in the present I'm not sure how different they are in terms of whether there is near term potential for a split. Any thoughts on the topic are welcome.The systematics of the southern forms of Selasphorus, including the Volcano Hummingbird, have undergone several changes since the initial descriptions were made between 1850 and 1910. The nominate flammula and subspecies torridus holotypes were collected by Smithsonian collector E. Arce on Irazú Volcano and in the Cordillera de Talamanca, respectively. Osbert Salvin used these specimens and initially described them as separate species in 1864 (flammula) and 1870 (torridus). Salvin then described a putatively new species in 1897 as Selasphorus underwoodii, which is now recognized as a hybrid between the Scintillant Hummingbird (Selasphorus scintilla) and the Volcano Hummingbird. Finally, Melbourne Carriker collected the holotype for the simoni subspecies on Poás Volcano in 1910. Initially believed to belong to Glow-throated Hummingbird (Selasphorus ardens), in consultation with French hummingbird expert Eugene Simon he described this population as a separate species and named it after Simon.
In the same 1910 paper, Carriker treated torridus as a subspecies of flammula, as these seemed to differ only in male gorget color. Ridgway accepted most of Carriker's 1910 suppositions but retained torridus as a separate species based on male gorget color as well as supposed differences in the color of the rectrices of the females. Berlioz and Slud both considered torridus to be simply a color morph of flammula rather than a distinct species or subspecies. Slud retained simoni as a separate species, but considered it possibly a subspecies of Selasphorus ardens based principally on color differences rather than detailed measurement. His conclusion was likely due to not examining their morphology in sufficient detail combined with a small sample size. Wetmore placed torridus as a subspecies of flammula (but did not consider the status of simoni).
Stiles' extensive work on the Selasphorus taxa clarified the species divisions to what is generally accepted today. He considered detailed aspects of morphology, particularly the shape and coloration of rectrices as well as their breeding and display behavior. This resulted in simoni and torridus being treated as subspecies of flammula. This treatment brought simoni into the Volcano Hummingbird complex and removed it from Selasphorus ardens. The taxon underwoodii, of which only one holotype is known, was determined by Stiles to be a hybrid of Selasphorus scintilla and Selasphorus flammula flammula. Further studies of breeding behavior by Clark et al. of Selasphorus scintilla and Selasphorus flammula validated Stiles' classification. Dyer and Vallely also analyzed specimens of Selasphorus ardens and Selasphorus flammula simoni and concurred with Stiles treatment as simoni being part of Selasphorus flammula and not Selasphorus ardens.
Presently, most authors treat the Volcano Hummingbird limits as being settled and recent phylogenetic studies generally supports this conclusion.
