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It would seem that if black/dark eye stripes in fact do *function *to enhance vision (which I do not think the study demonstrates), the selection pressure for this trait to evolve might actually be greater for a bird, such as the masked shrike whose hunting efficiency seems not only affected by its ability to *detect* prey (e.g., low glare in high light-intensity environments) but also by its ability to *avoid detection by the prey*(e.g., small vertebrates and large invertebrates that head for cover when they perceive a threat). These factors could combine to make it very beneficial for a masked shrike to be able to deal with intense and direct light. In contrast, a forest bird like an American Redstart might be highly affected by glare when hunting for flies, but, arguably, their prey are more abundant and perhaps less likely to detect and avoid predation (?). Furthermore, a forest species, like redstarts, that forage in and under canopies where light intensity is reduced relative to open areas, would create less obvious shadows so the pressure to hunt into the sun and deal with high intensity light might be reduced and other factors might be at play. I agree with others here who suggested there are likely several factors influencing the cost/benefits of dark feathers or masks around the eye and that likely different mechanisms are involved for different species. I would also add in the competing possibilities of phylogenetic constraint (e.g., species 'b' might merely have a mask if it recently evolved from masked species 'a'), sexual selection by female choice (e.g., females choose males based on certain properties of masks...shape, size, visible/UV light reflectance/refractance), dark colouration on head and back to avoid detection and predation by aerial predators... Thanks for the interesting discussion and perspectives on this Steve and Lance. Cheers, James. On Sat, Mar 17, 2012 at 3:24 AM, Stephen R. Shaw <srshaw@dal.ca> wrote: > 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<http://www.newscientist.com/article/dn21591-zoologger-unmasking-the-zorro-of-the-avian-world.html> >> >> (Sent from Flipboard) >> >> >> Richard Stern >> Sent from my iPad >> > > > -- James Churchill Kentville, Nova Scotia Email: jameslchurchill@gmail.com Phone (h): (902) 681-2374 Skype: jameslchurchill --14dae934036b67530a04bbb85af7 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable hi folks,=A0=20 <div>=A0</div> <div>This was an interesting article and definitely generated a lot of good= discussion.<br> </div> <div>Unfortunately, I agree: to me, the results of the study (presented in = the Behavioral Ecology article) did not demonstrate what the authors claime= d=A0they did:</div> <div> <div>=A0</div> <div>It does look like=A0Shrikes with white masks=A0hunted at a different a= ngle to the sun, and had lower hunting efficiency than unpainted and contro= l shrikes. However, this only demonstrates an effect of white paint=A0aroun= d the eyes on orientation angle during hunting and on hunting success. In o= ther words, this does not necessarily provide any information about the <i>= function </i>of the black mask - it just suggests that having white (paint)= around the eyes=A0might be less conducive to hunting vs having a black mas= k.</div> </div> <div>=A0</div> <div>The authors state in the discussion "<font size=3D"1"> </font><i>= we demonstrate that the facial mask of the Masked Shrike serves the purpose= of allowing the bird to face into the sun, enabling it to identify prey by= its larger shadow cast toward the perched shrike, and enabling the shrike = to approach the prey without alerting it to the impending attack</i>"<= /div> <div>=A0</div> <div>From my reading of=A0the article,=A0there was no measurement of=A0prey= shadows,=A0shadow detection by shrikes, nor=A0the effect of an approaching= shrike on prey behaviour. Similarly the results suggest that unpainted and= control Shrikes did not in fact "face into the sun" (which would= be around 0 degrees in Figure 1), but rather adopted an "oblique"= ; mean angle of orientation relative to the sun (generally between 45 and 9= 0 deg to the right of the sun which seems interesting in itself - why this = angle, and why is it biased towards having the left eye facing directly int= o the sun and the right eye not in the sun at all, or being in it very litt= le,=A0since it is on the opposite side of the head? Try it out!). This=A0ad= opted hunting orientation (45-90 deg relative to sun) would still result in= a shadow being cast behind them rather than in front of them - that part s= eems supported.</div> <div>=A0</div> <div>It would seem that if black/dark <span style=3D"color:rgb(0,0,0)">eye = stripes in fact do </span><i style=3D"color:rgb(0,0,0)">function </i><span = style=3D"color:rgb(0,0,0)">to enhance visi</span>on (which I do not think t= he study demonstrates), the selection pressure for this trait to evolve mig= ht actually be greater for a bird, such as the masked shrike whose hunting = efficiency seems not only affected by its ability to <i>detect</i>=A0prey (= e.g., low glare in high light-intensity environments) but also by its abili= ty to <i>avoid detection by the prey</i> (e.g., small vertebrates and large= invertebrates that head for cover when they perceive a threat). These fact= ors could combine to make it very beneficial for a masked shrike to be able= to deal with intense and direct light. In contrast, a forest bird like an = American Redstart might be highly affected by glare when hunting for flies,= but, arguably,=A0their prey are more abundant and perhaps less likely to d= etect and avoid predation (?). Furthermore, a forest species, like redstart= s, that forage in and under canopies where light intensity is reduced relat= ive to open areas, would create less obvious shadows so the pressure to hun= t into the sun and deal with high intensity light might be reduced and othe= r factors might be at play.<br> <br></div> <div>I agree with others here who suggested there are likely several factor= s influencing the cost/benefits of dark feathers or masks around the eye an= d that likely different mechanisms are involved for different species. I wo= uld also add in the competing possibilities of phylogenetic constraint (e.g= ., species 'b' might merely have a mask if it recently evolved from= masked species 'a'), sexual selection by female choice (e.g., fema= les choose males based on certain properties of masks...shape, size, visibl= e/UV light reflectance/refractance), dark colouration on head and back to a= void detection and predation by aerial predators...<br> </div> <div>=A0<font face=3D"AdvPSA33E" size=3D"1"><font face=3D"AdvPSA33E" size= =3D"1">=A0</font></font></div> <div>Thanks for the interesting discussion and perspectives on this Steve a= nd Lance. <br>Cheers, <br>James. <br></div> <div><br><br><br> <div class=3D"gmail_quote">On Sat, Mar 17, 2012 at 3:24 AM, Stephen R. Shaw= <span dir=3D"ltr"><<a href=3D"mailto:srshaw@dal.ca" target=3D"_blank">s= rshaw@dal.ca</a>></span> wrote:<br> <blockquote style=3D"BORDER-LEFT:#ccc 1px solid;MARGIN:0px 0px 0px 0.8ex;PA= DDING-LEFT:1ex" class=3D"gmail_quote">I had a look at this and while it'= ;s interesting as Richard says, it seems iffy, perhaps more on account of t= he treatment by the blogger than of the original authors. =A0The first thre= e 'straw man' ideas offered as to what function the bird's dark= eye-stripe fulfills are pretty unconvincing from the start.<br> <br>This bird, a masked shrike, must either be somebody's favorite or j= ust one that was readily available for study, because it doesn't have a= particularly prominent eye-stripe. =A0You could pick better birds, over he= re at least, for such a study (swallow family, two shrikes, both waxwings, = black-throated gray warblers), that have much more completely dark eye-surr= ounding feathers. =A0The idea eventually advanced in principle sounds plaus= ible, that the dark feathers in the eye stripe help reduce the sun's re= flection into the eye. =A0This would reduce glare and so increase the visua= l contrast available to the shrike hunting insects, when perched facing int= o the sun and looking downwards. =A0If that's the case, though, wouldn&= #39;t you'd expect that the feathers ABOVE the eye would be black to re= duce the sun's reflection, as in chickadees, or do the facial bones und= er the eye jut out a lot so you'd put the black pigmentation there too = (as do AFL-ers)? =A0Just having a dark strip running horizontally through t= he eye would not seem to be particularly useful for all this.<br> <br>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-re= flective} may not be. This could easily have made the reflections much larg= er than those that would arise from having normal light-coloured feathers i= n that location -- not a fair test of the idea, unless they measured local = light reflectance and controlled for this (not reported in the blog). =A0Bl= ack 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. =A0Perhaps some of the birders here might know, if lo= cal bleaching has ever been used earlier to mark individual dark birds for = identification.<br> <br>The glaring omission at least in the blog is that many birds have feath= er patterns that are known or believed to have species- and/or sex-recognit= ion functions. That this has nothing to do with enhancing visual contrast s= eems obvious where there are pronounced sex-differences, with completely bl= ack headed or black-faced males at least in the breeding season, e.g. ameri= can redstart, both orioles, bay-breasted warbler. =A0If the major advantage= is to possess anti-sun-reflection eye surrounds for insect-hunting, you= 9;d think that both sexes would have developed it and also retained it year= round. Not so.<br> <br>Browsing through the warbler pictures in Peterson, an even more frequen= t feather pattern on the head is to have a supercilium/eyebrow that is whit= e or light coloured. =A0What could that do for vision? -- it would seems li= ke a really bad idea, based on this report, but it is very common. =A0A dev= elopmental hangover from the ancestral warbler (bottom of the barrel idea)?= =A0Has anyone in the bird world asked this question?<br> =A0Steve, Halifax<br>=A0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<u></u>~~~~~~~~~~~~= =20 <div> <div><br><br>Quoting Richard <<a href=3D"mailto:sternrichard@gmail.com" = target=3D"_blank">sternrichard@gmail.com</a>>:<br> <blockquote style=3D"BORDER-LEFT:#ccc 1px solid;MARGIN:0px 0px 0px 0.8ex;PA= DDING-LEFT:1ex" class=3D"gmail_quote">Interesting bird related article from= the New Scientist<br><br>Zoologger: Unmasking the Zorro of the avian world= <br> <a href=3D"http://www.newscientist.com/article/dn21591-zoologger-unmasking-= the-zorro-of-the-avian-world.html" target=3D"_blank">http://www.newscientis= t.com/<u></u>article/dn21591-zoologger-<u></u>unmasking-the-zorro-of-the-<u= ></u>avian-world.html</a><br> <br>(Sent from Flipboard)<br><br><br>Richard Stern<br>Sent from my iPad<br>= </blockquote><br><br></div></div></blockquote></div><br><br clear=3D"all"> <div><br></div>-- <br>James Churchill<br>Kentville, Nova Scotia<br>Email: <= a href=3D"mailto:jameslchurchill@gmail.com" target=3D"_blank">jameslchurchi= ll@gmail.com</a><br>Phone (h): <a href=3D"tel:%28902%29%20681-2374" value= =3D"+19026812374" target=3D"_blank">(902) 681-2374</a><br> Skype: jameslchurchill<br><br><br><br></div> --14dae934036b67530a04bbb85af7--
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