In the earlier post on the courtship behaviour of the Blue-eared Barbet (Megalaima australis) by Adrian Lim a.k.a wmw998, there was a mention of a prominent black throat pouch that the male displayed when making its mating call (below).
The male was described as puffing and blowing to expand his throat pouch. As the pouch expanded, it pushed aside the black feathers that make up the black upper breast band, exposing a smooth, rounded, black sac.
Adrian is of the view that “the sac is only a tool for making the call, I doubt it is for attracting the female. If you look at the shots carefully, you will notice that the breast of the male bird sinks in whenever the sac inflates or puffs up.”
Dr Geoffrey Davison was consulted and responded: “The throat of most birds bulges a bit when they call, but this looks rather extreme. I would have guessed that its crop is stuffed with food, and then when it is calling as well the upper part of the breast would swell even more.
“I have watched other species of barbets calling, and have not seen such an extreme swelling. On the photo it looks as though part of the patch is bare skin, and black – or is this perhaps a patch of feathers that have got wet and sticky from its food? If it is bare black skin then this does imply a signal function. The bases of the feathers within this patch are black.“
On seeing an enlarged image of the pouch, Geoff added: “Quite dramatic, isn’t it? The black skin is very clear in your tweaked version of the photo. I’m not familiar with the literature on anatomy, but many fruit-eating birds are able to store quantities of food in a gular pouch, for later regurgitation. It’s the equivalent of the macaques’ cheek pouches, though a different part of the anatomy (lower down in the oesophagus). I would be inclined to avoid the word ‘sac’ for such a structure.
“Presumably a female barbet would be able to distinguish at a glance a male who has a supply of fruit ready (bigger black pouch = more food), and more inclined to allow copulation. I remember seeing something recently… about male birds rewarding the female who allows copulation by giving her fruit after she has submitted, rather than using fruit to tempt her beforehand.
“The other possibility is that this is a hollow structure, part of the air sac system, used as a resonance chamber to enhance sound production as it calls. These are not mutually exclusive possibilities (there could be both a resonating chamber in the air sac system and a pouch in the oesophagus) but not quite so easy to visualise how the two would work together.”
Morten Strange had this to say: “Bizarre image… This bird is not calling, the gular pouch seems to be stuffed to the brim with fruits…” while Wang Luan Keng suggested a ventriloquism function.
Yes, the pouch stores fruits, plenty of fruits. This species of barbet apparently has to offer a fruit for each act of copulation and usually a series occur one after another.
According to the literature, the skin of the throat or the neck on many non-passerine birds is bare, loose and distensible. In many instances it forms a pouch, especially in fruit-eating birds like hornbills (Kinnarid & O’Brien, 2007). The pouch comes in useful in the transporation of fruits to the nest to feed the chcks and/or mate.
In pelicans the pouch is for catching and holding fish for the young birds. That of the male Great Bustard (Otis tarda) is inflated in display (Garrod, 1874). In male frigatebirds (Fregata spp.) (above left), Marabou Stork (Leptoptilus crumeniferus) (above right), among others, similar pouches are inflated and displayed in courtship or social displays.
In grouse and Painted-Snipe (Rostratula benghalensis), the pouches increase the vocalisations by enlarging the sound resonating chamber (Stettenheim, 2000). Morten also pointed out that the Chestnut-winged Babbler (Stachyris erythroptera) produces low-pitched notes by inflating its neck, barring two patches of skin. The puffed-out neck-skin is a conspicuous blue or violet. His image is published in Collar & Robson (2007).
Image of barbet by Adrian Lim, those of frigatebird and stork by YC.
1. Collar, N. J. & Robson, C. (2007). Family Timaliidae (Babblers). Pp. 70-291 in: del Hoyo, J., Elliott, A. & Sargatal, J. eds. Handbook of the birds of the world. Vol. 12. Picathartes to Tits and Chikadees. Barcelona: Lynx Editions.
2. Garrod, A. H. (1874). On the “showing-off” of the Australian bustard (Eupodotis australis). Proc. Zool. Soc. London 1874, No. 31:471-473.
3. Kinnarid, M. F. & O’Brien, T. G. (2007). The ecology and conservation of Asian hornbills: Farmers of the forest. University of Chicago Press.
4. Short, L. L. & Horne, J. F. M. (2002). Family Capitonidae (Barbets). Pp. 140-219 in: del Hoyo, J., Elliott, A. & Sargatal, J. eds. Handbook of the birds of the world. Vol. 7. Jacamars to Woodpeckers. Barcelona: Lynx Editions.
5. Stettenheim, P. S. (2000). The Integumentary morphology of modern birds – An overview. Amer. Zoologist 40:461-477.
An account of this barbet’s gular sac has now been published: Lim, A. T. H., L. K. Wang & Y. C. Wee, 2009. The Blue-eared Barbet Megalaima australis and its gular sac. BirdingASIA 11: 98-101.
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