Monday, 21 September 2009

Silver-winged Sooty Shearwater

I wasn't really ready for this post so soon after the last crop of Sooty Shearwater images but sometimes chance deals you a hand that you can't ignore. Saturday was mostly a bright day with good sunshine. Whilst out on the Northern Experience pelagic and in a lull I was reviewing images on the camera screen and came across a shot of a Sooty with what looked like silvery white secondaries. A few frames later same bird no silvery white secondaries. Puzzled I began to show MK and we both had shots of same bird, recognized by the two white scapulars with and without shiny silvery secondaries. We discussed this briefly concluding it was perhaps a trick of the light. I made a mental note to read the literature and see if the feathers were metameric (looked different colours in different light). Issue parked move on.
Tonight a post appeared in my reader from the excellent 10000birds entitled 'Explanation For Silvery Wings' which briefly summarised a news story from the BBC here.


Essentially they report for the first time the existence of a structural mechanism of feathers different from iridescence that makes plumage conspicuous. By using electron and light microscopy, they have shown that the mechanism consists of special lengthened and twisted distal barbules that are very susceptible to damage. The dorsal side of these barbules is translucent, which creates a distinctive sheen colouration to feathers that otherwise would be dark. When distal sheen barbules are broken, the black proximal barbules are exposed, thus generating a conspicuous difference between abraded and non-abraded areas. Total and ultraviolet reflectance of sheen (non-abraded) areas are strikingly higher than in abraded areas. They propose that this mechanism represents a case of convergent evolution in species that are limited in developing colourful plumage patches.
Lo and behold it seems that our 'silver-winged' Sooty Shearwaters could be another example of a dark species with the specialised barbule mechanism discovered by a group of scientists based in Spain and Canada led by Dr Ismael Galvan of the National Museum of Natural Sciences in Madrid. Given that Sooty Shearwater easily falls into the category of a large dark feathered, open habitat species it would seem entirely reasonable to conclude that this may be the case.

The researchers suspect that dark coloured birds evolved this silver sheen as a way to show off, while maintaining the benefits of dark coloured feathers.
Dark feathers contain the pigment melanin, which protects feathers against the damaging effects of abrasion and UV radiation.

So there you have it you get it all from this blog, investigative journalism, witty repartee and now I can add cutting edge science to the list. And all for free....



5 comments:

Mike said...

You offer quite a value proposition here, Alan!

forestal said...

Very nice post

Dan

Martin Kitching said...

A very quick search of Lammergeier on Google Images, and of my own archive of Sooty Shearwater photos reveals an important difference between the two species; Lammergeier wings seen from above are either both dark or both silvery, Sooties either appear all dark or have silvery secondaries on the distal wing only. We know from Saturday that the same bird can show silvery secondaries one second, and dark secondaries the next (dependent on wing angle) so it may be due to 'sheen' feathers although it could equally be a 'trick of the light'.

Pale panels on the distal upperwing of e.g. Little Shearwater are already well described in the literature and I've found images of Bullers Shearwater and Manx Shearwater with obvious bright panels on the distal wing as well.

What we really need are lts of images of seabirds in flight - where the direction of sunlight is known. Now there's a project for next year (although I should be out taking seabird shots over the next couple of weeks...).

Alan Tilmouth said...

I can't find too many top down view images of Lammergeier in the first 15 pages of Google, granted there are a couple that show silvery portions of both wings. There is one top down view on Birdguides at http://www.birdguides.com/species/species.asp?sp=30056

That shows some of the wing on both wings silvery but other areas dark.
Anyway I've emailed the Sooty images to the Spanish team and enquired if Sooty was one of the species skins they examined.
Interesting suggestion though.

Martin Kitching said...

I found 2 images in the first 10 pages of Google that I felt showed the areas of interest. One silvery on both wings, one dark.

Galvan's research points towards cranes, vultures, pelicans as having this structural modification to the feathers. Sooty Shearwaters are burrow nesting so, if pairing/display takes place on/around the nesting islands then they'll be losing the sheen (if they have this feather modification) just at time when they would most need it.

I suspect we'll find a preponderance of seabirds with pale panels on the distal wing though, and a lack of other birds showing a similar pattern. That could well be down to the chance of the dorsal surface of a flying bird being photographed (or well observed)...