Chris Packham, in BBC’s Springwatch Guide to Sea Birds, discussed the tubular extensions to the nostrils of petrels (Procellariiformes). He repeated the common explanation that they serve to remove secretion from the salt glands, adding that otherwise the salt solution would fall on the feathers. They certainly do act as conduits for salt-gland secretion. Shortly after Knut Schmidt-Nielsen discovered salt glands, he wrote an article in 1959 for Scientific American illustrated by a photograph showing salt gland secretion being blown from the tubular nostrils of a petrel.
The late Jim Linzell and I repeated this explanation and reproduced the photograph in our 1975 monograph, Salt Glands in Birds and Reptiles. However, I recall that we both had qualms on whether removal of salt-gland secretion was a real explanation for the evolution of tube noses. Of course, the secretion will come out that way and blowing the secretion out would keep the air channels clear (some species have separate channels in the nostrils for air and secretion). However, other marine and potentially marine birds that have operative salt glands manage perfectly well without tubular extensions. Because we had no other explanation for the adaptation we concluded:
Continuous flight may hamper the flow of fluid from the nostrils because of the current of air over this region and the tubular extensions through which the fluid can be blown by a forced expiration could well act in the way Schmidt-Nielsen suggested.
We did not know then that sea birds have remarkable powers of olfaction and that they use it to detect their prey in the open ocean.
It has been suggested that the tubes serve to direct the current of air to the olfactory epithelium. But is there something special about the tubular arrangement in addition? Does it help in some way to find the direction the odour of prey is coming from while birds fly a zig-zag pattern towards the source? Has anybody looked at the patterns of air flow in the nostrils of birds with and without tubular extensions?
 |
You can see the tubular extensions in this young Murphy's Petrel
(Pterodroma ultima). I photographed this chick on Ducie Atoll in the
Pitcairn Islands on 24 October 2010 |
So, if I were revising Salt Glands in Birds and Reptiles today, I would point out that salt-gland secretion is blown from the tubular extension to the nares but add that the morphological adaptation is more likely to reflect the survival advantage of an improved efficiency of olfaction rather than of extra-renal excretion.
Schmidt-Nielsen, K. (1959) Salt glands. Scientific American 200, 109-116.
Peaker, M. & Linzell, J.L. (1975). Salt Glands in Birds and Reptiles. Cambridge. Cambridge University Press.
Remarkably, I find our monograph was re-published as a paperback by CUP in 2009 (ISBN:9780521112031) and is available from their website:
http://www.cambridge.org/gb/knowledge/isbn/item5706871/?site_locale=en_GB
