Leybold’s Firecrown was described in 1870 as a full species, but was later downgraded to subspecific level under the Juan Fernández Firecrown and is now known as Alejandro Selkirk Firecrown (Sephanoides fernandensis ssp. leyboldi (Gould)).
Let’s just start with something completely different to make all of this even more complicated!
The Juan Fernández Archipelago consists of three islands, which were originally named in 1574 as Más a Fuera, Más a Tierra, and Santa Clara.
The archipelago is primarily known for having been the home to the marooned sailor Alexander Selkirk in the very early 18th century, which again was the template for Daniel Defoe’s Robinson Crusoe. The islands are now named Isla Alejandro Selkirk, Isla Robinson Crusoe, and, well, Santa Clara respectively. Robinson Crusoe Island is the largest of the three, while Santa Clara, about 1 km southwest is merely a snipped crumb of it. Alejandro Selkirk, however, is located 180 km further west.
The islands contain many endemic animals and plants, and, among the birds are home for at least two species of hummingbirds, the endemic Juan Fernández Firecrown (Sephanoides fernandensis (King)) and the Green-backed Firecrown (Sephanoides sephaniodes (Lesson, RP & Garnot)), which inhabits Argentina and Chile and comes to the Juan Fernández archipelago only during the warmer parts of the year (… perhaps only to annoy the endemic species!).
Let’s now move the Juan Fernández Firecrown itself, which for some time was also somewhat a mysterious bird.
The species was described in 1831, yet both sexes are so dissimilar that they immediately were described as two distinct species, the male as Trochilus fernandensis and the female as Trochilus stokesii (both by Capt. Philip P. King in „Proceedings of the Committee of Science and Correspondence of the Zoological Society of London 1: 30. 1831“).
John Gould in his hummingbird monograph  followed that by naming them es Eustephanos fernandensis and Eustephanos stokesii respectively, and, somewhat funny, wondered if he would ever get to see a female E. stokesii.
„Some of them are more richely coloured than others, but all have the metallic green crown; the question then arises, has the female this character as well as the male, or did Mr. Bridges procure males only?“ 
Well, no, they all were females!
He corrected himself later in the supplement of his monograph , and by the way also introduced the second (sub)species.:
„… such a case is the present, in which, from lack of the requisite knowledge, I figured the two sexes of the Cinnamon Firecrown as distinct species, under the names of E. fernandensis and E. stokesii; and now that I am giving for the first time a Plate of the more recently discovered E. leyboldi, I have seized the opportunity to refigure the two sexes of E. fernandensis.“ 
Let‘s talk about the differences between the two forms.:
S. f. ssp. leyboldi:
male: the glowing golden feathers on the crown stretch onto the neck
female: the back is more yellowish green instead of bluish green to green; the tail feathers with a far less amount of white.
The nominate race is endemic to Más a Tierra (Robinson Crusoe), while the ssp. leyboldi is supposed to have been endemic to Más a Fuera (Alejandro Selkirk).
Let’s now take a look at the archipelago’s wonderful flora …! 
These are the plant species that are native (mostly endemic) to Isla Robinson Crusoe and that are known to be utilised by the Juan Fernández Firecrown.:
Centaurodendron dracaenoides Johow
Cuminia fernandeziana Colla
Dendroseris litoralis Skottsb.
Dendroseris macrantha (Bertero ex Decne.) Skottsb.
Eryngium bupleuroides Hook. & Arn.
Escallonia callcottiae Hook. & Arn.
Nicotiana cordifolia Phil.
Notanthera heterophyllus G. Don.
Ochagavia elegans Phil.
Raphithamnus venustus (Phil.) Rob.
Sophora fernandeziana (Phil.) Skottsb.
The most important endemic plant for the hummingbirds is Raphithamnus venustus (Phil.) Rob., a beautiful shrub with violet flowers, because it is the only native plant that flowers during the winter, thus is extremely important for the survival of the Juan Fernández Firecrown. The males often build their territory around such a shrub, and vigorously chase away other birds, including females except in the breeding season.
And these are the native plant species that occur (native or endemic) on Isla Alejandro Selkirk..:
Dendroseris regia Skottsb.
Eryngium sarcophyllum Hook. & Arn.
Nicotiana cordifolia Phil.
Raphithamnus venustus (Phil.) Rob.
Sophora masafuerana (Phil.) Skottsb.
They are not always conspecific, but at least congeneric, and Raphithamnus venustus does occur on this island as well, so …?
By the way, the first real inventure of Masafuera’s flora was made by Carl Skottsberg at the beginning of the 20th century , the island was already nearly completely denuded at that time by introduced goats, and there were only traces left of the former forests. It is thus no wonder that no hummingbird has survived there.
What do others think?:
Federico Johow, either Friedrich Richard Adalbert Johow, the German-Chilean botanist and biologist born 1959 and now (in 2018) 159 years old, or, more likely, someone with just the same name, has done some research on the same matter in 2007, and has found only four persons who have claimed to have seen hummingbirds on Isla Alejandro Selkirk.:
Augustín Guajardo, a botanist that has worked in the archipelago in 1869, Friedrich Johow (named as Federico Johow), the one mentioned above, Carl Skottsberg, and finally R. A. Philippi Bañados, a zoologist that has worked on the islands in 1928.
I could only find two of the abovementioned accounts, and I will generously share them with you here. 😉
„Eustephanus galeritus Mol. – Masatierra. En chile solamente durante el invierno. – Nombre vulgar en Juan Fernandez: Picaflor chico.
Eustephaus fernandensis King. – Endémico en Masatierra. – Nombre vulgar: Picaflor grande.
Eustephanus leyboldi Gould. – Endémico en Masafuera (*)
(*) Opinamos que esta especie, fundada en ejemplares mal conservados, es idéntica con el E. fernandensis de Masatierra. A lo ménos, no nos fué posible descubrir diferencia alguna entre los ejemplares que cazamos en una i en otra isla.“
„Eustephanus galeritus Mol. – Masatierra. In Chile only during the winter. – Common name in Juan Fernandez: Picaflor chico.
Eustephaus fernandensis King. – Endemic in Masatierra. – Common name: Picaflor grande.
Eustephanus leyboldi Gould. – Endemic in Masafuera (*)
(*) We believe that this species, founded on poorly preserved specimens, is identical with E. fernandensis of Masatierra. At least, it was not possible for us to discover any difference between the specimens we hunted on one island and another island.“
„Die Kolibris gehören auf Masatierra zu den bedeutungsvollen Bestäuber[n]. Früher kam E. fernandensis – denn E. Leyboldii ist höchstwahrscheinlich mit ihm identisch – ebenfalls auf Masafuera vor, wo ich im August 1908 mehrere beobachtete. Im [Jahr] 1917 bekamen wir keinen einzigen zu sehen; er scheint hier ausgestorben, was übrigens keinen, der das Schicksal Masafueras kennt, überraschen wird.“
„The hummingbirds belong on Masatierra to the significant pollinators. Formerly, E. fernandensis – because E. Leyboldii is most likely identical with it – also occurred on Masafuera, where I observed several in August 1908. In 1917 we did not get to see a single one; it seems to be extinct here, which by the way, will not surprise anyone who knows the fate of Masafuera.”
The Juan Fernández Firecrown is said to be quite variable, especially the females, I personally have no access to any specimens nor do I know of any scientific assay that deals with that matter; and of course, the specimens used for the description of Leybold’s Firecrown are lost now, so my last words in this case are: Leybold’s Firecrown very likely should not be treated as a full species but indeed as a subspecies, it has in fact existed, and it is now extinct.
… uff, that was quite an amount of letters …. 😛
 John Gould: A Monograph of the Trochilidae, or family of humming-birds. London: Printed by Taylor and Francis 1861
 John Gould: A Monograph of the Trochilidae, or family of humming-birds. Supplement. London: Henry Sothern & Co. 1887
 Friedrich Johow: Estudios sobre la flora de las islas de Juan Fernandez. Santiago de Chile, Imprenta Cervantes 1896
 Carl Skottsberg: Pollinationsbiologie und Samenverbreitung auf den Juan Fernandez-Inseln. 503-547. 1928. In: The Natural history of Juan Fernandez and Easter Island, edited by Carl Skottsberg. Vol. 2., Botany. Uppsala: Almqvist & Wiksells Boktryckeri, 1920-1953
 C. Skottsberg: Derivation of the Flora and Fauna of Juan Fernandez and Easter Island, Part 1. The Juan Fernandez Islands. 193-438.In: The Natural history of Juan Fernandez and Easter Island, edited by Carl Skottsberg. Vol.1., Geography, Geology, Origin of island life. Uppsala: Almqvist & Wiksells Boktryckeri, 1920-1956
 Michael S. Roy, Juan Carlos Torres-Mura, Fritz Hertel, Marina Lemus and Renate Sponer: Conservation of the Juan Fernandez ﬁrecrown and its island habitat. Oryx 33(3): 223-232. 1999
 Federico Johow: El picaflor de la Isla de Masafuera: ¿Subespecie extinta, nunca existió o Mistery Bird?. Boletín Chileno de Ornitología 13: 28-41. 2007