[NatureNS] Article: Zoologger: Unmasking the Zorro of the

Date: Sat, 17 Mar 2012 03:24:11 -0300
From: "Stephen R. Shaw" <srshaw@Dal.Ca>
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I had a look at this and while it's interesting as Richard says, it  
seems iffy, perhaps more on account of the treatment by the blogger  
than of the original authors.  The first three 'straw man' ideas  
offered as to what function the bird's dark eye-stripe fulfills are  
pretty unconvincing from the start.

This bird, a masked shrike, must either be somebody's favorite or just  
one that was readily available for study, because it doesn't have a  
particularly prominent eye-stripe.  You could pick better birds, over  
here at least, for such a study (swallow family, two shrikes, both  
waxwings, black-throated gray warblers), that have much more  
completely dark eye-surrounding feathers.  The idea eventually  
advanced in principle sounds plausible, that the dark feathers in the  
eye stripe help reduce the sun's reflection into the eye.  This would  
reduce glare and so increase the visual contrast available to the  
shrike hunting insects, when perched facing into the sun and looking  
downwards.  If that's the case, though, wouldn't you'd expect that the  
feathers ABOVE the eye would be black to reduce the sun's reflection,  
as in chickadees, or do the facial bones under the eye jut out a lot  
so you'd put the black pigmentation there too (as do AFL-ers)?  Just  
having a dark strip running horizontally through the eye would not  
seem to be particularly useful for all this.

Their test of painting the black feathers white to see if this worsens  
hunting skill (apparently so) is in the right direction, but using  
gouache paint that is known to contain white pigments (making it  
unusually white-reflective} may not be. This could easily have made  
the reflections much larger than those that would arise from having  
normal light-coloured feathers in that location -- not a fair test of  
the idea, unless they measured local light reflectance and controlled  
for this (not reported in the blog).  Black feather pigments seem to  
be melanins, and I'm not sure if you could bleach these out locally,  
to retain the feathers' reflective textures but turn them whiter.   
Perhaps some of the birders here might know, if local bleaching has  
ever been used earlier to mark individual dark birds for identification.

The glaring omission at least in the blog is that many birds have  
feather patterns that are known or believed to have species- and/or  
sex-recognition functions. That this has nothing to do with enhancing  
visual contrast seems obvious where there are pronounced  
sex-differences, with completely black headed or black-faced males at  
least in the breeding season, e.g. american redstart, both orioles,  
bay-breasted warbler.  If the major advantage is to possess  
anti-sun-reflection eye surrounds for insect-hunting, you'd think that  
both sexes would have developed it and also retained it year round.  
Not so.

Browsing through the warbler pictures in Peterson, an even more  
frequent feather pattern on the head is to have a supercilium/eyebrow  
that is white or light coloured.  What could that do for vision? -- it  
would seems like a really bad idea, based on this report, but it is  
very common.  A developmental hangover from the ancestral warbler  
(bottom of the barrel idea)?  Has anyone in the bird world asked this  
question?
  Steve, Halifax
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Quoting Richard <sternrichard@gmail.com>:
> Interesting bird related article from the New Scientist
>
> Zoologger: Unmasking the Zorro of the avian world
> http://www.newscientist.com/article/dn21591-zoologger-unmasking-the-zorro-of-the-avian-world.html
>
> (Sent from Flipboard)
>
>
> Richard Stern
> Sent from my iPad


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