(writing in progress)

The Australian bustard (Ardeotis australis) is noteworthy because it is the largest species of flying bird in Australia. The mature male can reach about 14 kg, which is severalfold heavier than most species of eagles.

It also has a remarkable display in the male during the breeding season, in which the appearance is transformed by the inflation of several air sacs and the ‘flagging’ of several aspects of the plumage. During the early settlement of Australia, the Australian bustard was widespread and common enough to have been a sought-after item of food. Its flesh is tasty and not as tough as might be expected for a bird which spends its life walking as it forages.

However, it is only when one thinks laterally that the puzzling aspects of the Australian bustard emerge.

Firstly, it is a surprising fact that similar bustards (formerly regarded as conspecific) occur also in India, Arabia, and widely in Africa. This defies the usual pattern in which the Australian avifauna (other than aquatic or wetland-associated forms, plus certain raptors) is rather distinctive owing to the isolation of this continent. In the case of the largest bustards it is as if God said ‘the isolation of Australia be damned, I’m going to put the same bustard down on the African continent, the Indian subcontinent, and the Australian continent’.

Secondly, one of the most interesting and overlooked aspects of the Australian bustard (and its close relatives) is the extreme sexual dimorphism. Why so dimorphic in body size, without much difference in body shape? (For example, neither ostriches nor emus are particularly sexually dimorphic in body size).

And thirdly, associated with the great sexual dimorphism is the secretiveness of the female, in contrast to the conspicuousness of the male. Most photos of large bustards show the mature male, which tends to walk in the open. The female, being smaller, tends to hide (crouching down when disturbed), and i
I get the impression that it seldom photographed). It is as if the female is not even missed by photographers; nobody seems to notice that they are not spotting or photographing the female, and so certain surprising aspects of sexual dimorphism remain overlooked.

It occurs to me that, once one focuses on the female for a change, these large bustards may perhaps qualify for a certain kind of ‘record’ among birds.Who can think of a larger species of diurnal bird in which the colouration of the adult female is cryptic/disruptive? The general pattern among birds is that thoroughly inconspicuous (cryptic and/or disruptive) colouration tends to be restricted to nocturnal forms and, among diurnal forms, certain rather small birds of extremely open environments, such as larks, which can remain stationary and thus blend in to the surroundings. The large bustards, particularly in the form of the female, seem to be exceptional in this regard, not so?

One incidental point worth mentioning here is that large bustards lack a ‘blaze’ display on the tail when flying, in contrast to a general pattern among flying birds. The tail is inconspicuously marked and lacks dark or pale accentuation, so that when the tail is spread during take-off it does not become particularly conspicuous as in so many familiar birds including the domestic pigeon.

If I am right, then what would emerge is a realisation that the largest bustards are not not only the largest flying birds (as referring to the mature male). They are also the largest camouflage-coloured diurnal birds (as referring to the female). There is minimal sexual dimorphism in the colouration of female and male in normal postures, but the plumage and air-sacs are configured in such a way that the male – which tends to be more conspicuous anyway because of its large size – can transform itself into a virtual living ‘flag’ when displaying sexually. This ambivalence in large bustards, between remarkable inconspicuousness (female) and remarkable conspicuousness (displaying male) is, to my mind, one of the overlooked aspects of the biology of the Australian bustard.

I would like to know more about the subtle, oft-overlooked aspects of sexual difference in plumage between mature female and mature male in the Australian bustard, as well as its close relatives (Ardeotis kori, A. nigriceps, A. arabs). Most writers seem to have assumed that the sexes have similar colouration in their normal (non-diplaying) postures and actions, but I suspect that there are significant differences, along the lines I refer to in the captions below.

The following shows what I take to be adult female A. australis, and if I’m right then this is one of the few good photos of the female of the largest species of bustards. Note the relative downplaying of the dark markings, particularly at the base of the neck, and note the lack of tonal (pale/dark) contrast between wing-coverts and lower flanks/drumstick plumage. The ‘flank-panel’ (which actually occurs on the wing-coverts of the folded wing) is inconspicuous here owing to its ‘speckling’, whereas in mature males (particularly in the Indian species A. nigriceps) this panel can form a conspicuous dark band. The neck is not pale enough in this female to be conspicuous, and the dark marking on the crown also seems relatively subdued. I ask the question: is this the largest species of camouflage-marked diurnal bird in existence?

The following two photos show the mature male A. australis in breeding display. Not only are the markings somewhat bolder than in the female, but the inflation of several air-sacs, plus postural erections, transform what is otherwise a rather inconspicuously-coloured bird into a ‘living flag’.

(writing in progress)

Publicado por milewski milewski, 29 de mayo de 2022


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