Since 1975, Helmeted Friarbird Philemon buceroides (Swainson, 1838) sensu lato has often been considered a widespread, polytypic species comprising up to 11 subspecies across the Indo-Pacific in Indonesia, northern Australia and Papua New Guinea (Schodde 1975, Schodde et al. 1979, Schodde & Mason 1979, Christidis & Boles 2008, Dickinson & Christidis 2014, del Hoyo & Collar 2016, Clements et al. 2022, BirdLife Australia 2022; Fig. 1). It has been considered a member of what was long known as the Black-faced Friarbird P. moluccensis s. l. group comprising P. buceroides s. l. and several other Philemon species (Mayr 1944, Schodde & Mason 1999). The group's nomenclature is currently unsettled (e.g., Higgins et al. 2008, Gregory 2017, Eaton et al. 2021, Joseph 2021, Gill et al. 2023). This in turn reflects an old challenge in avian systematics: how many species are there among very closely related geographically isolated populations exhibiting low phenotypic diversity? This challenge is especially pertinent to birds such as the friarbirds discussed here and found in Indonesia's island archipelagos, Australia and New Guinea (for other examples see Parker 1982, Andersen et al. 2015, Rheindt et al. 2020, Johnstone et al. 2022, Ó Marcaigh et al. 2022, Wu et al. 2022).

Here I review key points in the nomenclatural history of P. buceroides s. l. My first aim is to understand the origins of the current nomenclatural flux. I then focus on the Australian populations because their position at the geographical centre of the group's distribution is a useful pivot from which to achieve a second aim of deriving key questions requiring research. Answers to these questions should help bring stability though improved understanding of relationships among the entire group.

Nomenclatural background

The type locality of Philedon buceroides Swainson, 1838, the earliest species-group name applicable to any populations considered part of this complex, was given by Swainson (1838) as New Holland, i.e., Australia. For nearly a century thereafter, Australian populations were known only from Queensland on the continent's east coast (Fig. 1). After the spelling of Philedon was emended to Philemon, these Queensland populations were therefore known as Philemon buceroides (e.g., Gould 1865, Mathews 1912a,b, 1913). Swainson's type locality was long ago shown to be in error (Hellmayr 1916). Jansen (2018) fixed it to Kupang Bay, Kupang, East Nusa Tenggara in present-day West Timor, Indonesia.

Figure 1.

Distribution of the Helmeted Friarbird Philemon buceroides sensu lato complex and patterns in molecular data modified from (A) Jønsson et al. (2016; mtDNA) and (B) Peñalba et al. (2019; nuclear DNA). Colours in the map at the right in (A) summarise and match the initial phylogeographic structuring of the tree at the left (but note paraphyly among P. b. neglectus at left). Jønsson et al. (2016) adopted a three-species hypothesis in labelling their tree; epithets shown on the map in (A) follow Dickinson & Christidis (2014) where brevipennis was synonymised with novaeguineae as indicated. The dotted line highlights the need to clarify relationships among yorki and New Guinea taxa. Jønsson et al. (2016) listed a specimen held in the Naturalis Biodiversity Centre, Leiden, RMNH.AVES.75014, as gordoni; examination of the specimen and its locality data ( https://www.naturalis.nl/) show it to be ammitophilus as here depicted in (A), gordoni being shown as not sampled in (A). The star shows the location of Darwin and the square in south-east New Guinea shows the approximate location of playback observations referred to in the text and Joseph (2021). Modified from Joseph (2021); prepared by Julian Teh.

Linkage of the epithet buceroides to Lesser Sunda populations was thus cemented. Hellmayr (1916) also firmly aligned the epithet yorki Mathews, 1912, with Queensland populations. Mathews (1912b) named the coastal Northern Territory populations in central-north Australia (Fig. 1) then still known only from the Tiwi Islands (Fig. 1) as P. buceroides gordoni and Salomonsen (1967) followed this 55 years later. Hellmayr (1916) and Mathews (1927) implicitly assumed gordoni to be closely related to Queensland populations as indicated by their use of P. yorki gordoni (hereafter epithets alone will be used when feasible or necessary).

Therefore, for much of the 20th century after 1916, P. buceroides comprised several subspecies in the Lesser Sundas of Indonesia and the coastal Northern Territory population. After Hellmayr (1916; see above) and until 1975, Queensland populations were mostly known as P. yorki. Schodde (1975) and later reviews, albeit with reservations (Schodde & Mason 1999, Schodde et al. 1979), included P. yorki of Queensland and New Guinea Friarbird P. novaeguineae (Müller, 1843) and the latter's various nominal subspecies in what became the broadly if not unanimously accepted view of Philemon buceroides s. l. as a polytypic species widespread across Indonesia, northern Australia and Papua New Guinea (e.g., White & Bruce 1986, Coates & Bishop 1997, Higgins et al. 2008, Beehler & Pratt 2016, cf. Sibley & Monroe 1990; Fig. 1).

Given these foundations, current nomenclature of the three Australian subspecies can be further probed, and can be used to suggest a research pathway to improve understanding of the entire group.

A focus on Australian populations

Queensland populations.—Since Schodde (1975), P. buceroides yorki has prevailed for Queensland populations (Higgins et al. 2001, 2008, Christidis & Boles 2008, Pizzey & Knight 2012, Menkhorst et al. 2017, 2019, BirdLife Australia 2022). Recent literature has seen reversion to P. yorki at species rank (e.g., Gill & Wright 2006, Jønsson et al. 2016, Marki et al. 2017, Davies et al. 2022) sometimes with openly stated reservations (e.g., Gill et al. 2023) but I know of no explicitly argued defence.

Alternatively, some authors have treated Queensland populations as conspecific with, or at least more closely related to, New Guinean Friarbird P. novaeguineae by listing them as P. novaeguineae yorki or P. novaeguineae (e.g., Mayr 1944, Keast 1961, Salomonsen 1967, Slater 1974, Sibley & Monroe 1990, MacDonald 1992, Gregory 2017). Note that the epithet novaeguineae (S. Müller, 1843) has priority over yorki Mathews, 1912 (see Salomonsen 1967, Dickinson & Christidis 2014).

A key issue here is conspecificity of bird populations in New Guinea and northeast Queensland where yorki occurs. This is a common pattern in Australo-Papuan biogeography (e.g., Schodde & Mason 1999, Beehler & Pratt 2016, Joseph et al. 2019). New Guinea populations of these friarbirds, however, may well comprise more than one species (Jønsson et al. 2016, Gregory 2017, Marki et al. 2017), which issue too requires more analysis and discussion. The question arises as to which New Guinea populations might be conspecific with, or at least most closely related to, Queensland populations (e.g., cf. Ptiloris riflebirds; see Beehler & Pratt 2016). Jønsson et al.'s (2016) mitochondrial DNA data (Fig. 1; essentially reanalysed in Marki et al. 2017) suggest southern New Guinea populations are most closely related to yorki; so, too, do my anecdotal observations of responses of southern New Guinea birds to playback of yorki reported in Joseph (2021; see Fig. 1). If yorki was judged to be conspecific with those populations, Queensland birds would be known as P. novaeguineae yorki.

The point here is not to deny merit in separating yorki from P. buceroides; it is that the relationships of yorki to all other populations within P. buceroides s. l. but especially to New Guinea populations and how many species they too comprise must be rigorously assessed. This will determine to which species the Queensland populations belong and thus their appropriate taxonomic name and rank. Analyses of data in Jønsson et al. (2016), Marki et al. (2017) and Peñalba et al. (2019) comprise an excellent foundation here (see Fig. 1) but are limited in their sampling of subspecies, populations and nucleotides.

Northern Territory populations.—These have had a turbulent taxonomic and nomenclatural history. There are coastal and inland populations (Fig. 1). I treat these separately but note the possibility of geographic and genetic connections between the two populations via riparian habitats (see also Schodde et al. 1979).

Excitement met the possibility in 1962 that what was then known as Melville Island Friarbird Philemon gordoni (e.g., Officer 1964) thought to occur only in the Tiwi Islands might also occur on the mainland in Darwin (Fig. 1). Officer (1968) eventually confirmed that P. gordoni had been recorded at two localities in Darwin. (Ornithological field work around Darwin in 1962 would have been much harder than now.)

Officer (1968, 1975) evidently overlooked Deignan's (1964) account of the 1948 expedition from the United States National Museum. Deignan had collected specimens in suburban Darwin and at Yirrkala c.600 km east of Darwin. Deignan reported these specimens as Philemon moluccensis gordoni. His use of gordoni indicated that he knew they were what had until then been known as Melville Island Friarbird Philemon gordoni (or Philemon yorki gordoni—see above). His use of moluccensis indicated that he treated gordoni as a subspecies of what was then considered another Indonesian species the Black-faced Friarbird P. moluccensis.

Concerning inland populations, Deignan had collected another friarbird of this group from sandstone escarpments of Arnhem Land near today's Kakadu National Park well inland from the more coastal habitat of gordoni (Fig. 1). This too he classified as Melville Island Friarbird P. moluccensis gordoni. In 1968, the British Museum's Harold Hall Expeditions collected more friarbirds from Arnhem Land's sandstone escarpments (Colston 1974). Parker (1971) concluded that the sandstone populations were not Melville Island Friarbirds but that they were P. buceroides buceroides, i.e., the same form of P. buceroides as on Timor and Savu. Indeed, he explicitly stated that two species were present in Northern Territory, coastal P. gordoni and P. b. buceroides of sandstone escarpments. Eventually, Schodde et al. (1979) named Arnhem Land populations P. buceroides ammitophila, so completing discovery and naming Australia's three component taxa in the complex. These taxa and P. b. neglectus form a trichotomy in Jønsson et al.'s (2016) findings (Fig. 1) so Parker's (1971) surprising hypothesis may be worth revisiting.

A nomenclatural note concerning the gender of the species-group epithet ammitophila is warranted. Although ammitophila was intended as a noun in apposition when published (R. Schodde pers. comm. 28 January 2023), its current use in masculine form ammitophilus evidently traces to Dickinson (2003) who changed it reasonably but without explicit comment. Reasons were later elaborated by David & Gosselin (2011) who, in turn, cited Liddell & Scott (1996): once Latinised, a compound species-group name derived from Greek and ending in -phila, such as ammitophila, is to be treated only as an adjective (ICZN 1999: Art. 31.2, 34.2) and does not fall under Art. 31.2.2.

Linkage of P. yorki to ‘Hornbill’ Friarbird.—Until the taxonomic revision of Schodde (1975), the English name “Helmeted Friarbird” had been applied mostly without qualifiers and in effect exclusively to Queensland populations of these friarbirds (e.g., Gould 1865, Mathews 1912a, 1913, 1925, 1927, the index to the first 50 years of The Emu [1901–50], Officer 1964, 1975).

Hornbill Friarbird was introduced by Gill & Wright (2006) as an English name for Queensland populations, which they also recognised as Philemon yorki, but they proffered no basis for either decision. Hornbill Friarbird has since entered popular usage (e.g., del Hoyo & Collar 2016, Gregory 2017, BirdLife Australia 2022, Clements et al. 2022, Davies et al. 2022). Reflecting the discussion above, Gill et al. (2023) noted that the species status of ‘Hornbill Friarbird P. yorki’ is dubious and they wisely called for more study. Officer (1975) gave the English name Sandstone Friarbird to what has been known mostly as P. buceroides ammitophilus.

Molecular data and relationships: a way ahead?

Jønsson et al. (2016; reanalysed in Marki et al. 2017) derived mitochondrial DNA data from Philemon friarbirds generally. Their primary purpose was not a thorough assessment of relationships within P. buceroides s. l., so their data were understandably limited in sampling of individuals and populations. Nonetheless, their mtDNA data imply a close relationship between southern New Guinea and Queensland populations and, as noted, raise the possibility of several species in New Guinea (Fig. 1). (Note, however, Joseph's 2021 corrected identification as ammitophilus of the Northern Territory sample cited by Jønsson et al. 2016 as gordoni; Fig. 1.) Eaton et al. (2021) have already used Jønsson et al.'s (2016) results to advocate recognition of Indonesian populations west of New Guinea as Tenggara Friarbird P. buceroides. In contrast, nuclear DNA data (Peñalba et al. 2019; Fig. 1), similarly derived for other purposes, might be seen as more suggestive of separate species rank for yorki, but I note the similarly limited sampling of New Guinea populations on which those data are based.

Lastly here, and pending improved understanding of character evolution in Philemon friarbirds generally, Schodde & Mason (1999) noted the possibility of yorki aligning with the Silver-crowned Friarbird P. argenticeps also of northern Australia (see Mayr 1944).

Conclusion

Clarification is needed of relationships among populations and subspecies of the entire P. buceroides s. l. group from Indonesia to Papua New Guinea (Eaton et al. 2021, Joseph 2021) and of course to other friarbirds such as P. moluccensis s. l. and P. argenticeps. Key questions are (1) whether the three Australian taxa should be recognised as one, two or three species, (2) to which other populations they are each most closely related in Indonesia, Papua New Guinea and within Australia, and (3) how many species are present in Indonesia, Australia and Papua New Guinea? These tasks require a robust phylogenetic framework with which to better understand character evolution and defend any separations of taxa at species rank in a taxonomic revision.

These questions are challenging and will require field, museum and laboratory work spanning states, territories, provinces and regencies in Australia, Indonesia and Papua New Guinea. Analysis of DNA and vocalisations as well as traditional and novel methods for scoring and analysing plumage and morphometric traits are needed. Notwithstanding the foundation provided by Jønsson et al. (2016) and Peñalba et al. (2019), thorough sampling across the entire geographical range of populations (e.g., improved sampling of gordoni, yorki and jobiensis), individuals and, for DNA work, nucleotides will be needed. This will surely entail study of DNA extracted from toe pads of older museum specimens. Only then would we achieve an understanding of patterns of relationships and present and past gene flow as sea levels have changed (see Peñalba et al. 2019) and move towards a stable taxonomy and nomenclature for these friarbirds.