A question: What combinations give out blues, greens and
yellows in bird feathers?
Yellow is carotenoids. Bright red is also achieved this way (think Gouldian
Finch, which is famously polymorphic, with a red, black or yellow face).
Essentially requires frugivory or granivory. Some exceptions exist, such as
many nectarivores and a few insectivores. Thus, its occurrence is
phylogenetically rather restricted, e.g. among Passeri (advanced songbirds) it
is only really common among Passeroidea and Paroidea (the others are generally
rather specialized insectivores and few of them are anything other than
brown-grey or black-and-white, with perhaps some iridescence).
Green may be due to pigments (turacoverdin istthe most famous), which would be porphyrin derivates.
Usually however it is a combination of yellow pigment + "blue" structure. You can see
this nicely in parrots (if you have a skin specimen at your hands): the exact hue of green shifts
according to the angle you view the plumage, and if you view it at a very small angle, the feathers
appear essentially yellow (since the light goes a long way through the yellow outer layers of the
barbs and is reflected off the "blue" core). Every time I handle parrot specimens, I am
floored by this effect; it is extremely beautiful and it's one of those things that get more and
more beautiful the more you know about it.
Blue is always structural color. There are two types of structural color - simple
scattering (the same phenomenon that causes the sky to be blue) and iridescence.
Iridescence is a thin-layer phenomenon, just like the iridescence of an oil film on a
puddle. Both are ultimately caused by melanin, not stored in granules like in black/brown
but arranged to very thin sheets or a "foam".
See here for some more details:
As a source, you can use: Gill, Frank (2007): Ornithology (3rd ed.): pp.94-100.
Regarding the new paper, I can only say: fantastic! To be able to distinguish
fossil eu- and phaeomelanins, I would not have thought this possible! But it
obviously is. And with the Messel Buprestidae retaining structural color even
as fossils, and carotenoids probably leaving chemical traces, the coloration of
any feather color should be technically possible as long as the fossil is
It may be worthwhile to study Shenzhouraptor/Jeholornis next. One specimen has
seed fossils in its stomach. These have only been identified to a form genus,
but seem to be the endocarp of a drupe. This would mean that its food did quite
likely contain a source of carotenoids (the exo/mesocarp). It is probably
futile to search for carotinoid signatures in a predatory taxon's fossil
feathers (we don't even know if they leave distinct traces at all, but it's
quite likely) - but in a frugivore, yellow to red or even green plumage is
almost to be expected.
Blue, however, is very rare up to and including "higher waterbirds" (the ?clade
encompassing tubenoses and storks). Iridescent coloration is also generally restricted to
display feathers in these. The same would hold true for green, though we don't know
whether green has not been achieved via pigments in some extinct lineage. Turacos prove
it can happen, and uroporphyrins are readily available. You need to evolve the correct
enzymes to reconstruct them from being feces-brown to turaco-green though, and that's why
green-pigmented feathers are only found in Musophagidae today.
Theoretically, structural coloration may of course have been achieved independently
multiple times, but its lack - especially blue - in the more basal Neornithes suggests
that in the Mesozoic, structural coloration was probably all but restricted to more or
less iridescent black. Though the descendants of an arboricolous frugivore, if they have
shifted to some other diet, may of course have as much blue in their plumage as is
advantageous (Kingfishers come to mind - their ancestors, judging from their place in the
roller-woodpecker-passerine-?parrot clade, lived in trees and ate invertebrates and
fruit, and were most likely green). A blue _Iberomesornis_ as in "Walking with
dinosaurs" is very implausible - it is much more likely to have been flamingo-pink,
and even that's not very likely.
(I pondered an interesting question recently: many hummingbirds feed on plants whose nectar has elevated
carotinoid content. But as it seems, they metabolize the carotinoids as radical/reactive oxygen
"catchers". At least this is what it seems like, since hummingbird "feces" (which is
actually rather liquid) is apparently not colored, and "red" hummingbird feathers have
phaeomelanin, i.e. they are rusty-red not bright-red. So the carotinoids must be metabolized somehow, and a
hummer's metabolism is bound to yield large amounts of radicals/ROS, to detoxify which carotinoids lend
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