Signal evolution and morphological complexity in hummingbirds (Aves: Trochilidae)

Understanding how animal signals are produced is critical for
understanding their evolution because complexity and modularity in the
underlying morphology can affect evolutionary patterns. Hummingbird
feathers show some of the brightest and most iridescent colors in nature.
These are produced by optically complex stacks of hollow, platelet-shaped
organelles called melanosomes. Neither how these morphologies produce
colors nor their evolution has been systematically studied. We first used
nanoscale morphological measurements and optical modeling to identify the
physical basis of color production in 34 hummingbird species. We found
that, in general, the melanosome stacks function as multilayer reflectors,
with platelet thickness and air space size explaining variation in hue
(color) and saturation (color purity). Additionally, light rays reflected
from the outer keratin surface interact with those reflected by small,
superficial melanosomes to cause secondary reflectance peaks, primarily in
short (blue) wavelengths. We then compared variation of both the
morphological components and the colors they produce. The outer keratin
cortex evolves independently and is more variable than other morphological
traits, possibly due to functional constraints on melanosome packing.
Intriguingly, shorter wavelength colors evolve faster than longer
wavelength colors, perhaps due to developmental processes that enables
greater lability of the shapes of small melanosomes. Together,
these data indicate that increased structural complexity of feather
tissues is associated with greater variation in morphology and iridescent
coloration.