Surprising differences among the superficially similar large-bodied bustards

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The Australian bustard (Ardeotis australis, https://www.inaturalist.org/taxa/108-Ardeotis-australis and https://grasslands.ecolinc.vic.edu.au/fieldguide/fauna/australian-bustard#details) was once assumed to be conspecific with

The full extent of the differences between the Australian and African species have tended to be overlooked, partly because

The aim of this Post is to show that

  • the Australian bustard and the kori bustard are particularly different, and
  • the Australian bustard and the great Indian bustard are more similar than expected given their wide geographical separation.

DIFFERENCES BETWEEN THE AUSTRALIAN AND AFRICAN SPECIES

A reason why several differences have been overlooked is that they are hidden in the tail and in improbably distensible ‘air-sacs’.

The tail is obscured in the normal postures of both sexes of large-bodied bustards. Even in flight, the folded tails look deceptively similar between the Australian bustard and the kori bustard. It is only once the tail is variously raised in sexual, defensive and aggressive excitement that the caudal plumage shows the real differences in birds seen on the ground.

The ‘air-sacs’ lack any outer indications until spectacularly inflated in masculine displays. When the males of large-bodied bustards display sexually, they adopt extreme postures while ‘unfolding’ dark-versus-pale plumage in such outlandish ways that the normally inconspicuous birds are transformed into ‘living flags’. It is only then that the true differences in 'air-sacs' between the species become apparent in the extraordinary distended necks.

The tail is less noticeable in the Australian than in the African species, because:

  • in the Australian bustard, the retrices (stiff tail feathers) are relatively short and plain, whereas
  • in the kori bustard these feathers differ by being longer with pale barring which shows up when the tail is raised and fanned vertically or sideways (see https://www.youtube.com/watch?v=DYZgaYkyVKY ).

Furthermore,

  • in the Australian bustard, the white under-tail coverts and vent plumes remain oddly hidden in the masculine display, whereas
  • in the kori bustard, all these white feathers are fully exposed in any display of the tail.

Although there are no breeding plumages in the usual seasonal sense, the extending, erecting and inflating of complex and usually hidden surfaces and structures makes these normally unobtrusive birds into virtual beacons. And even at a glance it can be seen that these masculine displays are surprisingly different between the Australian and African species.

In the Australian bustard, displaying males inflate what is thought to be the oesophagus to form their main ‘balloon’. This shape-shift enormously distends the skin of the lower neck into a conspicuously pale, pendulous 'stocking', which nearly trails on the ground as it swings (see https://www.youtube.com/watch?v=f5YJYmRt3Aw and https://www.youtube.com/watch?v=l9uoUcW-Yds ). This structure is absent in the kori bustard.

Additionally, males of the Australian bustard inflate air-sacs in the whitish upper neck. The kori bustard likewise inflates the greyish upper neck to show whitish, but in its case the distensions on left and right remain distinct, and the show is accentuated by a dark ‘chin’ (or, more precisely, crook-of-throat).

An adjunct display in the Australian bustard is that of the white feathers of the flank, which are spread to overlap the edge of the folded wing. This too is absent in the kori bustard.

In both species, displaying males raise the tail until its tip touches the neck, enhancing the dark-pale contrast.

However, the erect tail of the Australian bustard looks mainly brown in this display, whereas that of the kori bustard looks mainly white. Furthermore, the Australian bustard keeps its body horizontal while the tail is erect, whereas the kori bustard eventually swings its body into an almost vertical position.

The masculine display of the kori bustard is also more complex in its sequence of different postures, as follows.

First the male erects its tail to show its fluffy white underside, with drooped wings (see https://www.youtube.com/watch?v=KXjppefiMiU and https://www.youtube.com/watch?v=ITijczBTMLc and https://www.youtube.com/watch?v=233NDsLxWE4 ) not seen in the Australian bustard. It then erects its torso while further inflating the upper neck. Finally the tail-tip is lowered to the ground and the display culminates in the male standing vertically to emphasise the fully-ballooned neck at maximum height (see https://www.youtube.com/watch?v=WloXywNfxOE and https://www.youtube.com/watch?v=wC3ADdhCZag) – a posture not seen in the Australian bustard.

The only displays shared by males and females in these bustards seem to be those of the tails in non-sexual contexts. In both sexes while on the ground, certain postures can bluff antagonists by making the whole body seem larger than it really is. The tail is fanned in various ways while the wings are spread to impress rather than to fly.

The kori bustard has been photographed more frequently performing these bluffing displays. Perhaps this is because its habitat differs from that of the Australian bustard. The kori bustard shuns tropical woodlands and prefers the exposure of semi-deserts - where a typically African array of herbivores and predators occurs despite the limitations of seasonal drought.

The difference between the habitats of the two species means that the kori bustard antagonises other large-bodied animals more frequently than does the Australian bustard (e.g. see https://www.traveller24.com/Explore/Bush/watch-secretary-and-kori-bustard-in-bird-brawl-unlike-anything-youve-seen-in-kruger-20190423 and https://www.mikerae.com/birds-living-at-ndutu/kori-bustard-being-badly-hunted-ardeotis-kori/ and https://www.louwphotography.com/Africa/Tanzania/Ngorongoro-Crater/i-Qd5hTGJ/A and scroll in https://douganddaniele.tumblr.com/post/28046046332/safari-101-on-the-wing and 'kori bustard and tawny eagle tangle over termites' in http://www.eyesonafrica.net/updates/2010/upd_jan10-1.htm).

Bustards are furtive compared with storks, cranes and the secretary bird (Sagittarius serpentarius). However, there are times when even females hold their ground while spreading the tail and wings in bluff, whether defensively or offensively; there are threats to eggs and juveniles from trampling herbivores and sundry predators. Spectacular examples are attacks by the martial eagle (Polemaetus bellicosus) (see https://www.facebook.com/biopicsbyjudylehmberg/posts/martial-eagle-killing-a-kori-bustardwe-have-been-lucky-enough-to-spend-a-good-bi/1548749875365018/), a bird-eating raptor with no precise counterpart in Australasia.

When the kori bustard displays in these non-sexual contexts, it fans the tail vertically without spreading its wings (see https://www.youtube.com/watch?v=rB2Fz8kvh6s ) or laterally while spreading its wings (see https://www.alamy.com/kori-bustard-fighting-springbok-image2337624.html ).

The kori bustard sometimes competes with vultures for carrion (https://angama.com/blog-posts/this-week-at-angama/this-week-at-angama-50/). There is even a record of this species performing its bluff while contesting with the southern pale chanting goshawk (Melierax canorus) for prey flushed incidentally by the honey badger (Mellivora capensis). This is a daring example of self-assertion given the reputation of this carnivore (see https://www.sciencephoto.com/media/561323/view/pale-chanting-goshawk-and-kori-bustard and https://fineartamerica.com/featured/kori-bustard-with-pale-chanting-goshawks-tony-camacho.html).

The two subspecies of the kori bustard are widely disjunct within Africa. The following show that the appearance remains similar despite the wide geographical gap:

Southern Africa subspecies, Ardeotis kori kori: http://www.stellenboschbirds.com/gompou.html

East African subspecies, Ardeotis kori struthiunculus: https://www.africanbirdclub.org/afbid/search/birddetails/species/455/4661

SIMILARITY BETWEEN THE AUSTRALIAN AND ASIAN SPECIES

In males, the pale parts of the body tend to be noticeably paler, while the dark parts tend to be noticeably darker, in the great Indian bustard than in the Australian bustard.

The result is that the great Indian bustard has bolder colouration and is the more conspicuous species, whether displaying or not.

However, the females remain so similar that taxonomists might legitimately ‘lump’ the Australian and Indian forms as two subspecies of a single species, were they not so disjunct geographically. This presents a biogeographical puzzle: much similarity has persisted despite the wide sea-barrier of six thousand kilometres.

This close relatedness is particularly revealed by a little-known tract of plumage, as follows.

When the tail is raised in masculine displays, an adjunct whorl of feathers on each side (left and right) - which has yet to be identified anatomically but may correspond to the femoral tract - is also erected in such a way as to cover most of the white under-tail coverts with a brown fan. No such tract of plumage plays any role in the displays of the kori bustard. This categorical difference leaves the Indian and Australian forms in close alliance.

The following photos show masculine display in the Australian bustard. Please note what looks like a brown fan of feathers on the side of the tail: http://www.raywilsonbirdphotography.co.uk/Galleries/Birds/Regions/Australasia/Aus_Australian_Bustard.html and https://www.mindenpictures.com/stock-photo-australian-bustard-ardeotis-australis-male-displaying-victoria-naturephotography-image00415250.html and https://steve-wilson.smugmug.com/Australian-birds/ and https://www.alamy.com/stock-photo-australian-bustard-ardeotis-australis-adult-male-calling-during-courtship-70267954.html and https://www.agefotostock.com/age/en/details-photo/australian-bustard-ardeotis-australis-male-in-courtship-display/U53-601149 and https://www.youtube.com/watch?v=THY4fDU0PPg and https://www.mdahlem.net/birds/7/bustard.php.

Partly in review of the various lines of information presented above, here are photos showing the consecutive stages of masculine display in the kori bustard:

Ardeotis kori struthiunculus, male, in initial stage of sexual display:
https://www.shutterstock.com/es/video/clip-1009864109-kori-bustard-ardeotis-walking-around-on-serengeti

Ardeotis kori struthiunculus, male, with tail fully erected in sexual display:
https://africanbirdclub.org/afbid/search/birddetails/birder/455/23012

Ardeotis kori struthiunculus, male, with air-sacs in the upper neck fully inflated in sexual display:
http://www.stellenboschbirds.com/gompou.html

Ardeotis kori struthiunculus, male, culmination of sexual display with torso fully upright and tail lowered:
https://www.mindenpictures.com/search/preview/kori-bustard-ardeotis-kori-male-displaying-masai-mara-game-reserve-kenya/0_90748197.html

Kori bustard, Ardeotis kori struthiunculus, male, culmination of sexual display with torso fully upright and tail lowered:

Publicado por milewski milewski, 28 de mayo de 2022

Comentarios

Publicado por milewski hace 2 meses (Marca)

The underside of the tail is pure whitish in Ardeotis kori, and I suspect that this also applies to A. arabs (https://en.wikipedia.org/wiki/Arabian_bustard). In these spp., the tail is raised during the male display, and the under-tail coverts are exposed so that their whiteness becomes as conspicuous as possible. The proximal under-tail coverts and feathers of the vent tract, exposed by the raising of the tail in A. kori, are sometimes held relatively tight, and sometimes whorled and spread. The situation is more complicated in Ardeotis australis and Ardeotis nigriceps. In these species the under-tail coverts are likewise whitish, but the pattern is puzzling at the base of the tail. In A. nigriceps and A. australis, the tail is also raised and fluffed in male display, but the tract appears mainly as brown, not white. This is not to say that male display in A. nigriceps and A. australis fails to show a prominent white patch. However, the prominent white patch which is shown in A. nigriceps and A. australis consists of only the more distal of the undertail coverts. Furthermore, even when the tail is inert (as in normal walking and flight), some photos show some dark markings on the ventral surface of the base of the tail. This seems to show the relaxed form of a feather-tract which originates on the dorsolateral base of the tail, left and right. The picture that is emerging, overall, is that the tail shows particular differences in effective colouration between the two ‘superspecies’ of Ardeotis. I have yet to investigate differences between male and female fully, and I may well find that the photographic material is insufficient for this.  
The following shows the plumage tracts of a typical bird. 
The following shows that, in A. kori, the under-tail coverts, vent and dorsolateral base of the tail are all pure whitish. 
A. kori:
The following confirms that the under-tail coverts and vent tract are pure whitish in A. kori. When the tail is erected in the masculine display of A. kori, all of the undertail coverts, plus the vent and even perhaps the femoral tract, prove to have uncomplicated colouration: simply whitish. 
A. kori: 
By contrast, A. nigriceps has the whitish of the under-tail visible, in the masculine display, only for the most distal of the under-tail coverts. The rest of the undertail coverts, plus the vent and possibly the femoral tract, are covered by feathers which are brown like the back of the bird. In nigriceps, the tail does contribute to the display, but it plays second fiddle relative to the oesophageal sac, a feature absent in A. kori. Not only is the tail a less important part of the male display in A. nigriceps than in A. kori, but the tail appears mainly dark in A. nigriceps in this display, whereas it appears mainly pale in A. kori. 
A. nigriceps: 
The following shows again how limited the pale under-tail coverts appear in the erected tail of A. nigriceps in masculine display. Also note that the whole tail is noticeably shorter in A. nigriceps than in A. kori. 
A. nigriceps:
In the following view of A. nigriceps, the erected ‘tails’ seem wholly brown. However, this is partly illusory because, on closer scrutiny, I suspect that the brown whorls/fans originate not strictly on the tail but instead at the dorsolateral base of the tail (left and right). 
A. nigriceps: 
Several points are of interest in the following photo of the masculine display in A. nigriceps. Firstly, the white of the most distal of the undertail coverts is so limited that it forms a mere adjunct to the whitish of the neck. Instead, what the raising of the tail does is more just to expand the brown surface of the body, in profile. Secondly, a noteworthy additional whitish surface has appeared on the flanks ventral to the folded wing, something also seen in australis. I think this consists of the feathers of the anterior flank, fluffed up so that the leading edge of the folded wing is tucked into the whitish plumage. Ardeotis kori has no such aspect to its display; instead it droops its wings, tending to cover the anterior flank with brown.
 A. nigriceps:
The following shows A. australis in similar display, confirming several points for this species. The tail is similar to that of A. nigriceps, above. Furthermore, in this case there is again an additional pale surface, overlapping the ventral edge of the folded wing, in the position of the flank. Note that the tail display produces so little whitish that it seems like a mere afterthought relative to the whitish produced on the neck and flanks of the bird. 
A. australis:
Here is another view of the additional panel of whitish on the anterior flank. There is no analogous panel in the display of A. kori or, presumably, A. arabs. 
A. nigriceps: 
This view of A. nigriceps again shows that the displayed tail is mainly brown, with white restricted to the most distal of the undertail coverts. However, what is remarkable about this is that

Publicado por milewski hace alrededor de 2 meses (Marca)

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