What's behind the color and pattern of bird feathers?

September 26, 2018 by Matt Miles, Phys.org report

While it may be true, as the old adage goes, that 'birds of a feather flock together,' what is less certain is how the feathers on those birds come to have their distinct patterns and colorations. Current data suggest that patterns of stripes and spots on animals' fur or feathers are formed through an open-ended or stochastic process during the embryonic formation of skin, at which time a dynamic—such as Turing's reaction-diffusion—is at play. On the other hand, owing to their specific orientation and periodictity, and the highly reproducible nature of these patterns within species, it is thought that other factors may also be at work in the development of patterning and coloration.

To better understand the phenomenon of feather patterning and coloration, a group of French researchers led by Nicolas Haupaix undertook a study of galliform birds. In looking at an initial group of 10 species, the researchers sought to deterimine whether or not early developmental landmarks play a role in establishing spatial reference for periodic patterns in feathering. They identifed two main types of feathers among these species: entirely black (eumelanic) and yellow (a black base and tip, with a central yellow [pheomelanic] band in between). Henceforth, they were able to establish a common stripe sequence emanating from the dorsal midline: a central black stripe, sometimes containing a few yellow feathers, flanked on either side by a yellow stripe, each of which was bordered by another black stripe, which in turn was borderd by another yellow stripe.

From this, the researchers observed that an increase in dorsum size correlated with an increase in stripes, suggesting that color pattern variation is not necessarily the result of scaling. In fact, in measuring absolute distances between groups of stripes, they were able to infer that stripe position is determined early, before the skin expands. The researchers also found, by plotting feather tracts in relation to stripe boundaries, "that color boundaries are highly reproducible within species," but that these boundaries vary between species in relation to stripe width and stripe shape, suggesting that the variations in width and shape are due to local effects present during feather tract formation.

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