Chestnut-necklaced Partridge – one species or two?

One of the many highlights of 2015 for me was the opportunity to observe Chestnut-necklaced Partridges in Sabah and Peninsular Malaysia within a few weeks of each other.

The Clements and IOC checklists treat these two forms as subspecies – Arborophila charltonii charltonii (range: S Thailand to S Myanmar and Malay Peninsula) and A.c. graydoni (range: N Borneo (Sabah)). A third subspecies is A.c. atjenensis (range: N Sumatra (Aceh province)). However, Birdlife International and HBW split charltonii and atjenensis from graydoni as Chestnut-necklaced Partridge and Sabah Partridge respectively.

HBW describes the ways in which graydoni differs from charltonii  as follows:

“Formerly treated as a subspecies of A. charltonii, but differs in its plain dirty white vs plain tawny-buff ear-coverts (2); absence of a distinct black upper necklace from the lower throat around below the ear-coverts (2); much darker, richer chestnut on upper breast with black vs greyish-brown lower breast feathers, more narrowly, evenly and distinctly barred with buff (3); belly with darker rufous and rufous-chestnut coloration (ns[1]); and darker upperparts (ns[1]).”

Whereas Sabah Partridge has a Red List threat classification of Least Concern (LC), Chestnut-necklaced is considered Vulnerable (VU).

I had the chance to watch and photograph a single graydoni from a boat on the Kinabatangan River, as it strutted and called along the bank of the river just feet away, for over an hour.

Aborophila (charltonii) graydoni. Compare this with the points outlined in the HBW description above.

Not long afterwards, I had a memorable encounter with charltonii in Peninsular Malaysia, while sitting in a hide. The bird spent over two and half hours in front of me, down to about 5 feet away, seemingly oblivious to my presence!

Charltonii. Both these images are uncropped.

Here are a couple of short videos of the bird calling.

<p><a href=”https://vimeo.com/150893762″>MVI_7245 Chestnut-necklaced Partridge</a> from <a href=”https://vimeo.com/user23184508″>Dave B</a> on <a href=”https://vimeo.com”>Vimeo</a&gt;.</p>

<p><a href=”https://vimeo.com/150896550″>MVI_7358</a&gt; from <a href=”https://vimeo.com/user23184508″>Dave B</a> on <a href=”https://vimeo.com”>Vimeo</a&gt;.</p>

These are best viewed fullscreen and with the resolution pumped up to 720p (click on the HD symbol at the bottom right of the screen)

Having so recently seen the Sabah version, I was pretty surprised to see how different the Peninsula form looked. I made a few composite montages to make comparison easier. The Sabah version is always on the left in the pictures below. In making the following observations I realise that I am drawing on a sample size of one in each case, too small to make generalisations. Nevertheless, differences are suggestive.

The most obvious difference at first sight, apart from the ear covert patch, is the difference in the colour of the bill base and eyering – nondescript dark brownish-grey in the Sabah example and vibrant reddish-pink in the Peninsula bird. Wells mentions some variation in this feature on charltonii:

“Periorbital skin reddish-orange; bill black in four specimens, black with a reddish base and yellow-green tip to lower mandible in one, all olive-green in one, all brown in one and all yellow-brown in one – variation that is not yet understood.”

The throat feathering on charltonii seems to have much less black on the feather tips compared to graydoni.

When seen in detail, the head patterns are fairly different from each other. The Sabah example differs in having:

• a darker, grey-brown crown
• whiter, less-streaked supercilium
• a white loral spot
• the whitish-grey ear coverts bissected by a dark vertical line
• more evenly dark-tipped throat feathers, lacking a distinct lower necklace

Other differences are:

• darker, greyer-brown breastband with thinner, paler transverse barring
• Belly and flanks dark, rich chestnut, being similar to the colour of the lower neck (much paler peach on the Peninsula bird)

Although the Sabah bird is in shade and the Peninsula one in bright sunlight, the upper parts of the Sabah example do seem to be darker, greyer, and more thinly barred.

Structurally, the Sabah bird seems longer-bodied, longer-necked and larger-headed.

The flanks are distinctly different – plain on the Sabah form and heavily barred on the Peninsula bird.

The profile view seems to add weight to the structural differences suggested above. The Sabah bird looks longer and ‘rangier’ compared to the dumpy and rotund Peninsula one.

Lastly, a close-up of the head in profile shows that the bills are not only different in colouration but also in shape, with the Sabah bird having a longer, more tapered bill compared to the thick, almost bulbous bill of the Peninsula individual.

The Peninsula bird was only ever heard to make the single-noted repetitive crooning trill which can be heard in the videos, and never the rising and falling see-saw shriek which Sabah birds make when in full cry. However, that may be explained by the fact that the bird was alone, rather than part of a pair. The louder calls are more usually heard from antiphonally duetting pairs of graydoni.

Whether or not the two forms represent two full species or not, it was certainly a privilege to be able to observe both at such close range within such a short time. I’ll finish with a few more shots of the bird on the Peninsula, which came too close to focus on at times.