A field guide to mesozoic birds and other winged dinosaurs

carotenoid granules look the same as melanin granules (melanosomes), and
unlike melanin, carotenoids cannot be distinguished by shape. According
to Li & al. (2009), special chemical tests could be run to determine if a melanosome
is really a carotenoid, and what color it was. The chemical-based
analysis of feather color patterns conducted by Wogelius & al. (2011) brings
us another step closer to being able to identify cartenoid-based coloration
in fossil birds.
Even if we could currently test for the presence of carotenoids, it is uncertain
whether or not most Mesozoic birds would have been able to use
them as feather pigment the way modern birds do. The biological ability
to sequester carotenoids in the feathers appears to be absent from birds of
the subgroups Palaeognathae (the ostriches, emu, kiwi, tinamou, etc.) and
Galloanserae (including ducks, geese, pheasants, etc.). While these birds
can and do use carotenoids to color the skin of the feet, face, or bill, they
seem to lack the chemical pathways necessary to transfer carotenoids into
the feathers (Hill 2010). Unless this represents two successive evolutionary
reversals, it is probable that colorful, carotenoid-pigmented feathers are
unique to the modern bird group Neoaves, in which case almost all Mesozoic
birds could not have had bright yellow, orange, red, or green feathers,
but would have been limited to iridescence and striking contrast to create
effective visual displays. Artists should keep in mind that adding orange,
yellow or green feathers, or red, orange or yellow beaks or skin, implies that
a bird is eating a diet containing carotenoids, and that even this may not be
reasonable for the feathers of non-avian birds.
While more rare than melanin and carotenoid pigmentation, porphyrin
pigments provide another method of producing color in birds. Porphyrins
are perhaps most famous for lending blood its red color and leaves
their green (both heme and chlorophyl are varieties of porphyrin), but it
can also color feathers, adding browns and reds as well as green, though
green is only found in the specialized turacoverdin variety found in Turacos.
Interestingly, porphyrins may play a role in temperature regulation. In
addition to insulating eggs (see below), they are mainly found in the downy
feathers of nocturnal birds like owls, and those that are active at cold temperatures.
Another reason porphyrins are found mainly in non-pennaceous
feathers is that the compound makes feathers more labile, and so it would
be detrimental to employ it in feathers which must stand up to mechanical
strain and to the elements. This is opposite to the effect of melanins, which
add strength and are often found at the tips of wing feathers where stresses
are high.
Porphyrins are often responsible for producing the blue of American
Robin eggs, and most other egg coloration. In fact, some researchers
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