Glow-throated Hummingbird Selasphorus ardens Salvin, 1870, is a poorly known species endemic to the humid highlands of west-central Panama. Very little has been published concerning its life history, ecology or possible movements (Collar et al. 1992, 1994) and the taxon has recently been classified as Endangered (BirdLife International 2013). There are very few recent reports of S. ardens in life (Collar et al. 1992, Angehr et al. 2008) and we know of no unambiguous photographs. Uncertainty regarding its identification criteria and distribution has led to confusion with the widespread Scintillant Hummingbird S. scintilla and has hampered efforts to locate and identify the species in the field. S. ardens is known from very few localities, and vagueness and unreliability of locality data have also contributed to the uncertainty of identification criteria. Female plumage has been described from a very small number of putative specimens, all with ambiguous label data, and including some that are certainly misidentified (Table 1). There is no material basis for published descriptions of female plumage, and therefore the female plumage is unknown. In this study, we review the available specimen material of S. ardens and compare it with published descriptions in the hope of clarifying diagnostic characters for female plumage.

History

Salvin (1870) described Selasphorus ardens from two undated specimens taken by Arcé in the highlands of western Panama (Serranía de Tabasará) at localities believed to be in present-day Veraguas province (Wetmore 1968, Siegel & Olson 2008; Fig. 1). The two syntypes comprise an adult male labelled as being from ‘El Castillo’ (Natural History Museum, Tring, NHMUK 1887:3.22.1076; Fig. 2) and an immature male from ‘Calovévora’ (NHMUK 1887.3.22.1077; Fig. 2). Salvin (1870) provided a description of plumage characters to distinguish the adult male from Volcano Hummingbird S. flammula and S. torridus (now S. flammula torridus), including tail pattern, purplish-red throat and absence of elongated throat feathers, but he did not distinguish S. ardens from S. scintilla.

Selasphorus hummingbirds collected at Volcán Barva in central Costa Rica during the late 19th century were included in S. ardens until Carriker (1910) described these as S. simoni (Fig. 3), a restricted-range endemic now treated as a subspecies of S. flammula following Stiles (1983). The Volcán Barva birds have also been referred to S. ardens underwoodi, but Stiles (1983) demonstrated that the type of underwoodi is not of this taxon, but rather that it probably represents a hybrid between S. scintilla and S. flammula.

TABLE 1

Data and identification of specimens catalogued as S. ardens in natural history museums or identified as S. ardens in the literature (excluding ‘S. a. underwoodi’). For measuring protocols see Methods and materials. Measurements in mm. All specimens examined by both authors and measured by DD, except NHMUK material, which was examined by ACV and measured by Hein van Grouw, and the SFN specimen measured by Thais Zanata, who also measured AMNH specimens with closely similar results to DD. For acronyms see Acknowledgements.

Figure 1.

Map of western Panama showing localities mentioned in the text: (1) Volcán Barú; (2) Cerro Santiago; (3) Santa Fé; and (4) Cerro Hoya. Grey shading denotes areas above 700 m elevation.

Elliot (1878) published the first description of the female, which included ‘middle tail feathers bronzy-green, lateral ones buff with a black bar across their central part’. Elliot's description was repeated in full by Sharpe (1887) together with Gould's illustration of the syntypes. Boucard (1894–95) described both sexes using wording very close to that of Elliot (1878) and his description of the female tail is identical. Ridgway (1911) provided a detailed description of adult female S. ardens but remarked in a footnote that this was based on a single specimen and that he was ‘very doubtful... whether this is not in reality a female of S. scintilla.’ No other description of female plumage appeared in the literature until Wetmore (1968) reported his examination of 12 specimens of S. ardens, which from his account comprised nine males and three females. None of these authors stressed characters that distinguish S. ardens from S. scintilla. Stiles (1983) provided a diagram of female Selasphorus rectrix shape and markings. Brief descriptions of the female are also given in Ridgely & Gwynne (1989), Angehr & Dean (2010) and Stiles (1999).

Figure 2.

Specimens of Glow-throated Hummingbird Selasphorus ardens at Tring museum: (a) NHMUK 1887.3.22.1076 dorsal, syntype; (b) NHMUK 1913.3.20.588 dorsal; (c) NHMUK 1887.3.22.1077 dorsal, syntype; (d) NHMUK 1887.3.22.1076 ventral, syntype; (e) NHMUK 1913.3.20.588 ventral; and (f) NHMUK 1887.3.22.1077 ventral, syntype (Hein van Grouw, © Natural History Museum, London)

Figure 3.

Plumages of the southern Selasphorus hummingbirds: (a) Volcano Hummingbird S. flammula simoni female; (b) S. flammula simoni male; (c) S. f. flammula female tail; (d) S. f. flammula female; (e) S. f. flammula male; (f) S. f. torridus male; (g) S. f. torridus male variant; (h) S. f. torridus male variant; (i) S. f. flammula male tail; (j) probable G low-throated Hummingbird S. ardens female tail based on AMNH 37834; (k) probable female S. ardens based on AMNH 37834; (1) S. ardens male; (m) S. ardens male tail; (n) Scintillant Hummingbird S. scintilla female tail; (o) S. scintilla female; (p) S. scintilla male; and (q) S. scintilla male tail (Dale Dyer)

Methods and materials

We searched museum collection databases, reviewed relevant literature and consulted specialists (see Acknowledgements) to compile a list of specimens catalogued as S. ardens, exclusive of ‘S. ardens simoni’ or ‘S. ardens underwoodi’. We examined all of these personally or via photographs, and where possible we obtained measurements of bill length (exposed culmen), wing chord and tail length (base to tip of longest rectrix). In an effort to verify published descriptions of female plumage, we compared these with available specimen material.

Results

We located 17 specimens identified as S. ardens in the literature or museum catalogues, and determined 12 of these to be S. ardens or possibly S. ardens (Table 1). The majority (≥ 11) of specimens are male. The single possible female does not bear complete or unambiguous label data that includes collector, a traceable locality and date, and thus its identity is uncertain. Identification of adult males is relatively straightforward and uncontroversial using the characters proposed by Salvin (1870), Ridgway (1911) and Stiles (1983), but in one case a recent series of male S. scintilla was misidentified as S. ardens (McGuire et al. 2014).

Discussion

Tracing localities.—Salvin (1870) provided a map of western Panama, but was unable to place marry of Arcé's localities, including ‘El Castillo’, and equally others have remained unresolved (Siegel & Olson 2008). One syntype (NHMUK 1887:3.22.1076) is denoted as being from ‘El Castillo’, but there are several localities bearing this name in western Panama. Cerro El Castillo, east of Santa Fé and reaching 1,297 m, is thought probably to be the locality concerned (G. Angehr in litt. 2016). A label on the second syntype (BMNH 1887.3.22.1077) is marked ‘Calovévora’ (although ‘El Castillo’ is also given, struck through). Salvin's map (1870) placed Pico Calovévora north of Santa Fé (Fig. 1), and also shows a ‘Rio Calovévora.’ Siegel & Olson (2008) traced Pico Calovévora to near what is now called Cerro Tute, and gave its elevation as 1,400 m, but this is doubtful. Cerro Cabeza de Toro (1,412 m), 8 km due west of Santa Fé and part of the same massif as Cerro Tute, is the peak that actually lies above the headwaters of the río Calovébora and is therefore perhaps the best candidate for ‘Pico Calovébora’ based on the name. There are, however, several other peaks nearly due north of Santa Fé that are also potential candidates, the highest being Saro at 1,518 m. Six other (Arcé?) specimens are labelled ‘Veragua’. We note, however, that for 19th century naturalists, ‘Veragua’ referred to the entirety of modern-day west Panama to the Costa Rica border (Siegel & Olson 2008). Boucard (1894–95), for instance, wrote that the species ‘was discovered on the Volcano of Chiriqui, Veragua (Colombia) by Mr. Arcé.’ The name ‘Chiriqui’ is also problematic as it can refer to any of several features including the province or the volcano (now Volcán Barú).

In the American Museum of Natural History, New York, an adult male (AMNH 484754) is marked ‘Chiriquí’, and two adult males (AMNH 37832–833) and another individual labelled a female (AMNH 37834) are marked ‘Costa Rica’. Of these specimens Wetmore (1968) wrote that the locality data is ‘unquestionably incorrect. In view of the known range it is believed that all 3 [sic] are from Veraguas’. Given how little is known of the species' morphology, and present or past distribution, Wetmore's reasoning appears circular, and as to Chiriqui province it is contradicted by two specimens mentioned in his own account (see below). The AMNH males labelled ‘Costa Rica’ are morphologically consistent with the Veraguas and Chiriqui S. ardens, and not with S. f. simoni. While we accept Wetmore's (1968) assertion that identification of these males to S. ardens is correct, we cannot infer that their locality is ‘unquestionably incorrect’. Annotators have deleted ‘Costa Rica’ on the labels of AMNH 37832 and 37834 and written ‘Veragua’. Although AMNH 37832–835 were acquired by the museum from Elliot in 1888, Elliot (1878) listed ‘Hab. Veragua’ with no mention of Costa Rica.

In 1924, two adult males (AMNH 182682, 182684) were collected by Griscom at ‘3,600 ft.’ (c.1,100 m) on Cerro Flores (1,605 m) in eastern Chiriquí province. These are the only specimens with elevational data. The site is near Cerro Santiago (Siegel & Olson 2008), which is within modern-day Comarca Ngäbe-Buglé, west of Veraguas, In addition, Griscom (1935) identified a male collected on Cerro Flores (also at ‘3,600 ft.’) as S. scintilla (AMNH 182683). This established an eastern range extension for S. scintilla and the first documentation of sympatry between S. scintilla and S. ardens. Stiles (1983) doubted that S. scintilla was resident on Cerro Flores, supposing that two specimens (which must be AMNH 182683 and AMNH 182685) ‘could have arrived via post breeding dispersal.’ In recent years, S. scintilla has been regularly reported from nearby Cerro Colorado (G. Angehr in litt. 2016). We know of no basis for Griscom's (1935) inclusion of Veraguas in the range of S. scintilla.

In 1994, F. S. Delgado collected an immature male Selasphorus showing characters consistent with S. ardens at Cerro Hoya, Veraguas, on the Azuero Peninsula (Engleman 1994). The specimen was examined by G. Angehr and R. S. Ridgely, but has subsequently been lost (G. Angehr in litt. 2016). If the identity of the specimen was confirmed as S. ardens this would establish a new locality for the species, 75 km from the type localities and separated by lowlands (Fig. 1). In 2011, a small male hummingbird matching well with S. ardens was photographed on Cerro Hoya (Miller et al. 2015). Those authors concluded, however, that ‘unless a specimen is collected and deposited in a formal natural history collection, we recommend removing Selasphorus from the list of species occurring in Cerro Hoya’ (2015).

There have been sporadic sight reports of S. ardens from the Cerro Colorado area in recent decades. These observations are mainly from above 1,200 m on the road to Cerro Colorado (G. Angehr in litt. 2016). There were sight reports from near Cerro Tute, of a male Selasphorus at 800 m in 1974 (F. G. Stiles in litt. 2016) and another male in 1982 (Collar et al. 1992) but there are no recent reports from this region (Angehr et al. 2008; G. Angehr in litt. 2016). The species has not been reliably reported from any other locality since the 19th century, unless the Cerro Hoya birds prove to be of this species, In conclusion, while the collecting localities of the two syntypes are believed to be in central Veraguas province, and we can be confident in tracing the two Griscom specimens, the geographic provenance of the remaining eight known specimens remains obscure.

Female specimens and plumage characters.— We are unable to reconcile the currently available specimen material with that referred to in the literature. No previous authors have provided specimen numbers to identify their material unambiguously, In the following, we review putative female specimens in relation to published descriptions of female plumage.

Among 19th century skins marked ‘Costa Rica’ is one (AMNH 37834) also marked ‘female’ (Fig. 3). It is consistent in preparation and label style with males of S. ardens labelled ‘Costa Rica’ (AMNH 37832–833, collected by Arcé). Based on the AMNH catalogue, this specimen came into the museum's collection from Elliot along with a male S. ardens (AMNH 37835), and both had previously been purchased from Boucard. Elliot's (1878) brief description is fairly consistent with this bird. This ‘female’ was accepted as S. ardens by Wetmore (1968). The tail diagram presented in Stiles (1983) also is a fair match for the same specimen, but the pale tips to the outer rectrices of the specimen appear larger. Its rectrices are thin and weak, so it may be a juvenile, and therefore cannot be determined as certainly female based on plumage. The bill (12.7 mm), however, is longer than any of the S. ardens males, which is consistent with the pattern of dimorphism in Selasphorus (cf. Stiles 1983). Also, according to our observations immature male Selasphorus often show some spots of glittering colour on their throats, which this bird lacks. The specimen is distinctive in several respects from females of S. scintilla, which is sympatric with S. ardens in Panama and which occurs also in Costa Rica (Fig. 3). Wing chord (40.5 mm) is longer than in female scintilla (36.2–38.8 mm; Wetmore 1968). Its central rectrices are mostly green with narrow rufous basal margins, whereas the typical pattern for female scintilla is mostly rufous with a narrow green median stripe. This specimen's outer rectrix tips are paler than most S. scintilla. Compared to most female S. flammula, it is more rufous above and below, has a less spotted throat, more cinnamon-tinged outer rectrix tips, and although its tail is incomplete it appears to lack the pointed central rectrices of adult female flammula (Fig. 3). Each of these characters is shared by some S. flammula specimens, especially juveniles. Its wing chord is comparable to female S. flammula, as expected for female S. ardens, because male ardens are within the size range of male S. flammula. The specimen lacks undertail-coverts. Based on morphology alone, we cannot eliminate the possibility that it is S. flammula. S. ardens and S. flammula are believed to be allopatric but, without faith in the data, identification cannot be based on locality.

Boucard was a dealer, and at least three of the known specimens passed through his hands (AMNH 37834–835, and Senckenberg Forschungsinstitut und Naturmuseum Frankfurt, SFN 81965). In 1892, he wrote ‘I have two very fine males and one female of this rare species.’ However, by 1888 AMNH 37834 (the ‘female’) and AMNH 39835 had passed into the New York collection via Elliot, and so he could not have been referring to these birds. We cannot ascertain if the specimens referred to by Boucard (1894–95) are extant.

We cannot determine what specimen Ridgway's (1911) female description was based upon, but we share his concern as to its identity. Wetmore (1968) stated ‘Ridgway had available only two males and female from the American Museum’, but we doubt that his descriptions were based on AMNH birds. The museum at that time had four specimens, including three males, and he probably would have measured all three. He did not mention ‘Costa Rica’, as three of those AMNH skins are labeled—the only localities he gave are those of the syntypes and Volcán Chiriquí. We have not discovered any specimen of S. ardens from Volcán Chiriquí, which lies 100 km west of any documented locality, and no subsequent author has included the Chiriquí / Talamanca range in the range of S. ardens. Ridgway (1911) gave the wing chord for his female as 1 mm shorter than that for the AMNH female (37834), although his bill measurement matches well. Most significantly, he included ‘under tail-coverts pale cinnamon-buff’ in his description, but AMNH 37834 lacks undertail-coverts. We cannot be certain what individual Ridgway was describing—perhaps a bird from Volcán Chiriquí (where both S. scintilla and S. flammula occur)—but it was probably not, as he feared, an individual of S. ardens.

Griscom collected a female (AMNH 182685) on Cerro Flores initially identified as S. ardens. Wetmore (1968) accepted this identification, but we do not and neither did Stiles (1983, see above). Its wing chord is 38 mm, large for a S. scintilla but smaller than all but one known specimen of S. ardens (a male). Because females are larger than males in Selasphorus (Stiles 1983), these measurements are probably too small for a female S. ardens. Its plumage characters, including central rectrix pattern, are consistent with female S. scintilla, and differ from the ‘Costa Rica’ female (AMNH 37834) mentioned above. Stiles (1983) determined it as a juvenile female, but we cannot confidently age it.

McGuire et al. (2014) presented a phylogenetic hypothesis for the Trochilidae based on molecular data and including four tissue samples listed as S. ardens (Table 1). These samples are from four birds taken by J. T. Weir in 2003 at Cerro Colorado and deposited at the Louisiana State University Museum of Zoology, Baton Rouge (LSUMZ), and Burke Museum University of Washington, Seattle (UWBM). One is a female. All were found in their analysis to be close to an individual of S. scintilla. We examined the three males (LSUMZ 177697–699), two adults and one immature, and determined them as S. scintilla based on their small size and orange-red gorget. A fourth specimen from this series, the female, was deposited at UWBM (113266). Our examination indicated that it is also too small for any known S. ardens (wing 36.8 mm). It is consistent with S. scintilla in size, and also in central rectrix pattern. We believe that the Burke Museum female is also an example of the widespread S. scintilla.

Following the discussion of the syntypes (males) Wetmore (1968) wrote that he had ‘examined these 2 specimens and also a female, all collected by Arcé, in the British Museum’. However, there are currently three males and no female S. ardens at NHMUK. Wetmore gave measurements of females based on ‘3 from eastern Chiriquí and Veraguas’. It is possible that he placed his data for one male specimen in the wrong set. His means are not halfway between his longest and shortest measurements, and therefore a third bird must have been included. It would be highly desirable to see a female collected by Arcé from a topotypical locality, but Wetmore (1968) offered no more information concerning his supposed NHMUK female.

Like Wetmore's (1968) account, Stiles' (1983) data for females are puzzling. His Table 1 gives three as the number of adult females measured. These three probably included the AMNH ‘Costa Rica’ skin (AMNH 37834), but we cannot determine what, if any, additional material he possibly examined. None of the other collections Stiles consulted possess skins of S. ardens. Stiles' measurements closely recall, but are not identical, to Wetmore's (1968).

While we have arrived at the same number of specimens for the species (12) as Wetmore (1968) and Stiles (1983), our list is different as to material included, and Wetmore's and Stiles' differ from each other. While Wetmore (1968) and Stiles (1983) both give three as the number of females examined, the individual specimens referred to in these two accounts cannot be reconciled with each other.

Conclusions

Definite localities for S. ardens are all in the highlands of central Veraguas province and what is now eastern Comarca Ngäbe-Buglé (Fig. 1). The species is rare in collections and most specimens are male. Three specimens (AMNH 37832–834), including the only probable female (AMNH 37834), bear a dubious locality attribution (‘Costa Rica’). Despite the confident assertions of earlier workers, no unambiguous characters documented from extant specimens are known to distinguish females of S. ardens from females of other Selasphorus. Until future collecting efforts can secure a female specimen, or a female specimen is confirmed to be S. ardens using molecular techniques, the female plumage of S. ardens remains unknown.