Birds produce most of their sounds with their syrinx, the sound producing organ sited where the windpipe divides into two. What is less known is that there are secondary acoustic structures that modify the sounds produced by the syrinx – whether to spread, amplify or reverberate. One of these is the vocal sac, prominent and exaggerated in some species.
According to Dantzker & Bradbury (2006), the bare vocal or gular sacs seen in the North American grouse and the Neotropical cotingas are inflated only in acoustic display. As most of these sacs are brightly coloured, they are probably also involved in visual signaling. The pan-tropical frigatebirds (below left) and two storks, the Old World Marabou (Leptoptilos crumeniferus) (below right) and New World Jabiru (Jabiru mycteria), also inflate their necks and vocalise, but not always at the same time.
In the above species, the sacs are often held fully inflated for lengthy periods in a strictly visual display and only used occasionally in sound production.
Three other groups have vocal sacs that are equally impressive but not devoid of feathers. Perhaps the most striking is the kakapo (Strigops habroptilus), an endangered flightless parrot from New Zealand that seems to inflate its whole body when booming. Many medium to large bustards, like the Kori Bustard (Ardeotis kori) (below left) inflate sacs that are often covered in elaborate feathering; and some but not all inflating bustard species vocalise while inflated (Collar, 1996; Dantzker & Bradbury, 2006).
According to Johnsgard (1983), certain calls among yearling crowned cranes involve the inflation of the gular sac. This is thought to serve as a resonator that may provide increased carrying power. In the Australian Crane (Grus rubicundus), the gular sac of the male is inflated during display and possibly helps to resonate low-frequency sounds. The Grey Crowned Crane (Balearica gegulorum) of Africa is shown above (right).
Strangely, there is no mention of barbets having gular sacs, not even in the most recent monographs of these birds. It is now obvious that the Blue-eared Barbet’s black sac plays a role in vocalisation, possibly also in fruit storage. And according to Adrian, other species of barbets also have these sacs. Obviously, there is much to be learnt about gular sacs and barbets. Happily, bird photographers like Adrian are currently at the forefront of this investigation.
YC Wee & Adrian Lim
1. Collar, N. J. (1996). Family Otididae (Bustards). Pp. 240-275 in: del Hoyo, J., Elliott, A. & Sargatal, J. eds. Handbook of the birds of the world. Vol. 3. Hoatzin to Auks. Barcelona: Lynx Editions.
2. Dantzker, M. S. & Bradbury, J. W. (2006). Vocal sacs and their role in avian acoustic display. Acta Zoologica Sinica (Suppl.) 52:486-488.
3. Johnsgard, P. J. (1983). The cranes of the world. Bloomington: Indiana University Press.
4. Short, L. L. & Horne, J. F. M. (2001). Toucans, barbets and honeyguides: Ramphastidaer, Capitonidae and Indicatoridae. Oxford University Press.
5. 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.
Image of barbet by Adrian Lim, others by YC.
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.
This post is a cooperative effort between www.naturepixels.org and BESG to bring the study of bird behaviour through photography to a wider audience.