Saturday, 4 October 2014

Curl-Crested Manucode: Seen and Heard

Curl-crested Manucode by William Matthew Hart
(1830-1908) for John Gould's Birds of New Guinea
(National Library of Australia)
Saturday 8 February this year saw us on Fergusson Island, one of the D’Entrecasteaux group, to the north-east of the eastern end of Papaua New Guinea. A morning walk had taken us from the point the zodiacs could land to the impressive hot springs. On the way back, a black crow-like bird landed in the top of a tree. The binoculars showed it had what can only be described as an unfortunate hair style. No sooner were we looking at it when it threw its head back, spread its feathers without raising its wings and made an amazing sound. It was a Curl-crested Manucode, a bird-of-paradise endemic to the D’Entrecasteaux and Trobriand Islands. Another, possibly a female, joined it in the same tree. Soon, other males appeared in trees on either side but some distance from the gravel track, each giving their display the full works. They were too far away for me to do anything with the camera I was carrying. However, I was delighted to find that the Curl-crested Manucode is very well shown, along with its song, in the recently completed Birds-of-Paradise Project by the Cornell Laboratory of Ornithology.

Landing on Fergusson Island. Noble Caledonia Caledonian Sky Expedition Team Photograph
The Cornell video shows why the manucodes are famous for the length of their trachaea with loops and curls that travel under the skin the length of the bird several times. The trachea of the Trumpet Manucode, Manucodia keraudrenii, is the most impressive as the following diagrams from Mary Clench’s (1978) paper show:

Other birds that have loud or resonant calls have an elongated trachea and Darren Naish has an excellent blog post (with references to earlier research).

Unfortunately, the Cornell project’s video on this bird includes the tracheal elongation but makes no mention of the effect of a long trachea on respiratory physiology, nor draws attention to what is so special about respiration in birds    compared to mammals. A long trachea means more dead space. The tidal volume—the volume of gas inspired or expired per inhalation—must be greater than in a bird with a short trachea. No matter how efficient the avian lung is at exchanging oxygen and carbon dioxide—and it is really efficient—the volume of air inhaled first into the posterior air sacs before passing through the lungs must be greater. Only birds, with their extensive system of air sacs, can afford such a long trachea (giraffes would be the nearest mammalian equivalent—to see how they do it) since only they can generate such a large tidal volume. But a long trachea must bear a metabolic cost since the fuel to power the muscles that generate the large tidal volume must be supplied continuously. In this respect it is interesting to note that Clench concluded that the increasing complexity of the tracheal loops occurs as the males age (in females the trachea stays relatively simple). So does a more successful male produce a sound more enticing to the female and thus signal the fact that it can afford to invest in a longer trachea, and/or to other males that if you come near me, I am older, fitter and can see you off? In either case the signal would be an ‘honest’ signal of the quality of the male.

Having watched the manucodes plus the other birds for half an hour or so we walked back to the landing where a zodiac soon has us back for what we thought was a well-deserved lunch.

Hot springs, Fergusson Island. Noble Caledonia Caledonian Sky Expedition Team Photograph

Clench, M.H. 1978. Tracheal elongation in birds-of-paradise. Condor 80 423-430