Wednesday, January 16, 2013

The cline and fall of Western Fieldwren

The Western Fieldwren (Calamanthus [campestris] montanellus) has long had an on-again, off-again place on the Australian list. Even as the 1926 official RAOU Checklist [1] listed four Fieldwrens - Striated (fuliginosus), Rufous (campestris), Rusty (isabellinus/rubiginosus), and ‘Rock Fieldwren’ (montanellus) – the contemporary list of Mathews [2] listed only Striated and Rufous as species. A parallel situation exists today, with the international taxonomy of the IOC [3] recognizing montanellus as the full species Western Fieldwren, while Clements and the ‘official’ checklist of Christidis & Boles [4] lump it with Rufous. But it’s easy to forget that back then – until 1983 in fact - the ‘lumpers’ routinely aligned montanellus with Striated Fieldwren, on the basis of their shared olive and yellow (and lack of rufous) colouration. 

Given that interest is now stirring over how the ‘official’ Australian list will be maintained in the future, it’s worth a look back over the long-simmering arguments around the species status of the Western Fieldwren. 


Western Fieldwren Calamanthus [campestris] montanellus from the Stirling Ranges. Note the olive toned and heavily-streaked back; creamy-buffy underparts; russet on head confined to forehead; limited rufous in rump and tailbase; brown (not chestnut) lores; dull tail tips. This individual has a little more rufous than a ‘classic’ montanellus, notably in the cap and fringing on the wings.
[Image © Kay Parkin (http://kayparkinbirding.blogspot.com.au/2012/09/cheynes-beach-south-west-wa.html),
used with permission]

 

Arguments FOR splitting Western Fieldwren 


Evidence is a little thin in Milligan’s original type description (1903) [5], splitting it from fuliginosus on the basis of “lustreless dingy brown, rather than shiny greenish-brown” upperparts, whiter underparts, and (relative) lack of sexual dimorphism (we can probably discount his additional claim that montanellus also smells different to fuliginosus, based solely on the anecdote that his “Quail bitch did not… show any canine signs of pleasure”). Obviously a poetic soul, he went on to name it Rock Field-Wren after the “sterile stony tracts” of the Stirling Ranges. 

Although admittedly only on the basis of three skins, Ashby’s 1924 review [6] of Calamanthus argued montanellus was:  
“certainly deserving of full specific rank. The beak, shape of head and markings all show characteristic differences. It was agreed this species is nearer to fuliginosus than campestris.” 
Exactly what is unique about the head shape was never elaborated, though intriguingly head shape is also alluded to elsewhere [7]

The next review of the genus by Parker & Eckert (1983) [7] aligned montanellus closer to campestris than to fuliginosus, which they definitively circumscribed on multiple characters: bill shape, sexual dimorphism, lack of rufous & streaking on upper-tail coverts, and colour of tail tip. One flaw in Parker & Eckert’s analysis is that what they called their “montanellus-group” included the grey-backed but white-bellied SA populations of winiam (90-Mile, Big and Little Deserts) and ‘ethelae’ (from southern Eyre & Yorke peninsula, therefore synonymous today with C. c. campestris). Perhaps as a result, when looking back at their defining criteria for fuliginosus, ‘true’ montanellus might now be judged intermediate on 3 of their 5 species-defining characters (see table below).


Rufous Fieldwren of race campestris from near Whyalla, SA. Note the faint but generalized rufous tones over crown, ear and wing coverts, and tailbase
[http://morphbank.ala.org.au/2079101 Image © The Museum Board of South Australia, shared under Creative Commons CC-BY-NC-SA]


Rufous Fieldwren of race winiam (holotype of syn. ‘parsoni’) from Meningie, SA. Parker & Eckert grouped winiam into their “montanellus-group” based on its lack of rufous tones, but note the grey (not olive-grey) back, white (not cream) belly, white primary bases, and bright white tail tips
[http://morphbank.ala.org.au/2079172 Image © The Museum Board of South Australia, shared under Creative Commons CC-BY-NC-SA]


A specimen of Western Fieldwren montanellus of the type series from the Stirling Ranges. Note the uniformly olive-toned and strongly streaked dorsum, and creamy-yellowish underparts. Also note the ‘fat’ head – is this what Ashby was referring to?
[Images from the Digitised Types collection http://museum.wa.gov.au/catalogues/digitised-types/calamanthus-montanellus – see website for high resolution images. Images © WA Museum, reproduced here under Fair Use terms]

The most recent and comprehensive case for splitting montanellus was put by Schodde & Mason’s Directory of Australian Birds (1999) [8]. Their argument that it “emerges as being as close to fuliginosus as campestris can be summarized in the following table: 


Summary of Schodde & Mason’s (1999) [8] analysis placing montanellus as intermediate between Rufous and Striated Fieldwren. Asterisks (*) indicate Parker & Eckert’s (1983) [7] criteria for delimiting fuliginosus - note on 3 of the 5 criteria montanellus is intermediate; on most plumage details, it is closer to Striated.















It’s worth noting that neither C&B [4] or HANZAB [8] specifically reject Schodde & Mason’s treatment, but rather shelve it as “provisional…pending further studies” (though HANZAB adds hindclaw length as an extra feature linking montanellus with campestris). Perhaps sensibly, both preferred to hedge their bets awaiting genetic studies. 


Arguments AGAINST splitting Western Fieldwren 

Problem #1: Clinal variation 
By contrast, WA Museum taxonomists Ron Johnstone and Glen Storr [10] view variation in the fieldwrens as so gradual – “a smooth, clinal intergradation between the southern olive and northern rufous forms” – that they do not even recognize races in WA, arguing this would just give names to arbitrary points on a cline. Ron’s argument can be paraphrased as ‘tell me how much rufous you want, and I can show you the specimen’. Despite this, the Fieldwren plate of their Handbook of WA Birds [10] claiming to illustrate this smooth cline clearly shows olive-backed/plain-rumped/yellow-bellied montanellus, versus rufous-rumped/white-bellied forms elsewhere (if we take this plate as indicative, clinal/regional variation in the context of montanellus seems mostly restricted to the amount of rufous in the head, lores and ear coverts – and this seems to be corroborated by available web images). 

Problem #2: Gloger’s Rule 
That said, there is clearly a north-south cline of variation across the whole group, and within SA campestris and WA rubiginosus, of forms trending paler/rufous/less-streaked northwards, and darker/greyer/more-streaked southwards. This is problematic taxonomically, because such plumage variation is typical of an ecophenotypic gradient predicted by Gloger’s Rule, which even predicts the appearance of yellow or flavonoid pigmentation in more humid southern environments (thought to improve feather preservation). Schodde & Mason [8] thought that even within montanellus, specimens grade paler northwards (and whiter eastwards). If Gloger’s Rule is at play, much of the similarity of plumage between fuliginosus and montanellus, (and their difference from campestris) can be discounted as ecophenotypic. 

Problem #3: Where do you draw the line 
While there appears to be a decent gap between Nullarbor Plain campestris and the closest montanellus, the distribution up the west coast appears more or less continuous all the way to North-west Cape. A further complication to consider is that the current distribution is now much fragmented by habitat reduction. The northern limit of montanellus has variously been cited as Jurien Bay (Serventy [10]), Kalbarri (Parker & Eckert [6]), or “about Geraldton” (HANZAB [8]; Schodde & Mason map [7]), though none of these appear to match a clear break in distribution or habitat (see map below). Since true species of course do not blend into one another, the status of fieldwrens through this region needs to be much better understood before the taxon can be confidently elevated. 


Overlay of records from Atlas of Living Australia (ala.org.au), which includes Birds Australia Atlas and state Museum records (red dots); the distribution limits from Johnstone & Storr 2004 [10] (blue shading) and Schodde & Mason [8] (black line = montanellus limit). Yellow spots indicate source of the possible intergradient specimens mentioned in HANZAB [9] and Schodde & Mason. Birds from around Kalgoorlie are of unknown race; birds around Kalbarri are disputed and possibly intergradient.

Problem #4: Intergradation 
Kalbarri specimens are of interest as they are described as much paler with a reddish rump, “though still retaining the greenish dorsal cast” of montanellus [7, 9]. Suspected intergrades with inland race campestris (pale and narrow-streaked with a rufous cap) are described by HANZAB from 100km E of Southern Cross [9]. But perhaps more problematic for the taxon, birds from around Kellerberrin & Woolundra – squarely in the middle of what should be montanellus territory - are also paler, finely streaked, and rufous-capped (the ever-eager Mathews separated these as race leakei [12]). 

However Schodde & Mason [9] concluded that intergradient specimens are comparatively few, and often adjacent to sites yielding typical montanellus, suggesting the zone of intergradation is limited. Note also that similarly limited intergradation has been observed between Striated Fieldwren and race winiam of Rufous Fieldwren where they closely abut in South Australia (somewhat counter-intuitively, in modern taxonomic thinking the presence of a narrow and stable hybrid zone (rather than a cline) can actually confirm the presence of some inherent barrier to gene flow [13]). 

Problem #5: Shark Bay island forms 
Western Fieldwren are certainly not the only non-rufous races of Rufous Fieldwren, though their olive taint appears unique. The pallid, grey-toned variants from Dirk Hartog & Dorre Islands (hartogi & dorrie) were once lumped with montanellus by Mathews 1912 [2] & Serventy 1937 [11] respectively, the latter considering them to be relicts of a montanellus-type that retreated south with increasing aridity. However Schodde & Mason [8] considered them to have faint but consistent russet toning, sufficient to mark them instead as pale campestris-type. 

(Incidentally, in a vivid example of gene flow within the group, Ron Johnstone [pers. comm.] tells of seeing threatened hartogi Fieldwrens fly from Dirk Hartog Island to the mainland, thus presumably changing subspecies and threatened-taxon status upon landing). 

Problem #6 : Playback response 
While the calls are not particularly well studied, Ron has also shown that calls of true montanellus from the southwest are immediately recognised by Rufous Fieldwrens in the Shark Bay area and the Gibson Desert [Ron Johnstone, pers. comm.]. While this is anecdotal and inconclusive, it does appear to weaken one likely mechanism by which gene flow might be sufficiently restricted for these to truly represent adjacent (parapatric) species. 

 


So - is Western Fieldwren a species? 


There seems no doubt we must wait for broad genetic analyses to untangle the true nature of variation within the fieldwren complex. But on morphological grounds alone, one perfectly reasonable conclusion is that Western Fieldwren is actually neither a species or a subspecies, but something in-between. After all, evolutionary variation occurs in a continuous spectrum, and does not respect our increasingly tortuous definitions of what is or isn’t a ‘species’. 

According to the species definition guidelines of Helbig et al. [13], Rufous, Striated and Western Fieldwrens (and possibly others) could all be classified as ‘paraspecies’ within a ‘superspecies’ (thus best named in the trinomial, with the middle superspecies term [campestris] in square brackets). The naming of superspecies / paraspecies is more common in Europe & US (e.g. Carrion and Hooded Crow), and is a concept Australians probably need to become more familiar with to deal with some our trickier taxa. 

So the short answer is (in the absence of genetic data at least), if you accept that Striated Fieldwren fuliginosus is a separate ‘species’ from Rufous Fieldwren campestris, you probably should accept Western Fieldwren montanellus on similar and only slightly weaker grounds. The ‘species’ status of Western Fieldwren is undoubtedly a grey area, and serves as a timely reminder of the more conservative nature of the C&B, Clements, and BirdLife lists, versus the more generous species-definition and more fluid taxonomy of the IOC – which do you prefer? 


Where to see Western Fieldwren 

The best place to see Western Fieldwren is in the low heath of the lower slopes of the Stirling Ranges. Further afield, Fitzgerald River and Cape Arid National Parks have high densities. Northwards, they can be found in coastal heaths north of around Lancelin – Lake Thetis at Cervantes, and the lakes on the road into Sandy Cape (north of Jurien) are good sites. Fieldwrens are also common along the cliffs south of Kalbarri, but note these are above the range of montanellus according to HANZAB and Schodde & Mason. 

If you want to contribute to solving the mystery of the Western Fieldwren, be sure to atlas any Fieldwren records from the Kalgoorlie and Geraldton to Shark Bay areas – including subspecies details (and/or photographs) if possible. Even if species-level genetic divergence is found in the future, the nature of interaction at the distribution boundaries will be very important to elucidating its true status. 

UPDATE: Preliminary genetic work by Dolman and Burbidge, so far reported only in conference format, suggests insignificant genetic differences between montanellus and other fieldwrens in WA, but does hint there may be split further east over the Nullarbor.


Typical Western Fieldwren habitat - low coastal heath at Point Ann, Fitzgerald River National Park WA

References: 
[1] RAOU Checklist Committee (1926.) Official Checklist of the Birds of Australia. Melbourne: Gov. Printer. 
[2] Mathews GM (1912). A reference-list to the birds of Australia. Novitates Zoologicae 18: 171-656 
[3] IOC World Bird List. See http://www.worldbirdnames.org/ioc-lists/master-list/ 
[4] Christidis L & Boles WE (2008). Systematics and Taxonomy of Australian Birds. CSIRO Publishing, Canberra. 
[5] Milligan AW (1903) Description of a new Calamanthus and a new Megalurus from Western Australia. Emu 2: 200-203 
[6] Ashby E (1924) A review of the genus Calamanthus. Emu 24: 44-45 
[7] Parker SA & Eckert HJ (1983) Remarks on the taxonomy of the genus Calamanthus (Fieldwrens). South Australian Ornithologist 29: 65-71 
[8] Schodde R & Mason I (1999) The Directory of Australian Birds: Passerines. A taxonomic and zoogeographic atlas of the biodiversity of birds of Australia and its territories. CSIRO Publishing. 
[9] Marchant S. & Higgins PJ, et al. (eds) (1990-2006). Handbook of Australian, New Zealand and Antarctic Birds. (7 vols). Oxford University Press, Melbourne. 
[10] Johnstone RE and Storr GM (1998-2004). Handbook of Western Australian birds (2 vols). Perth, Western Australian Museum. 
[11] Serventy DL (1937). Calamanthus forms of the Shark’s Bay district, Western Australia. Emu 37: 103-105
[12] Mathews (1922) Bull. Brit. Ornithol Club 43: 13 
[13] Helbig AJ, Knox AG et al. (2002) Guidelines for assigning species rank. Ibis 144:518-525 

1 comment:

  1. Surprised there is no comments, this is an excellent article. Thanks

    ReplyDelete