Have you ever noticed how many white bird species, such as most gulls and geese, have black wing feathers? This is because black and brown colours are the result of melanins that are incorporated into the feather structure while it is growing. Melanins strengthen feathers and reduce wear, especially in birds that fly long distances or that live in marine environments filled with abrasive sand and salt.

But birds use their feathers for a variety of purposes. Probably the most familiar function is visual communication. The sex and age of an individual is often revealed by the intensity, quality, hue and pattern of its plumage colours.

The brightest plumage colours are provided by carotenoid-based pigments, which are red, orange and yellow. But birds do not synthesize their own carotenoids; instead, these pigments are co-opted from their diet and are placed into growing feathers. Thus, carotenoid-based feather colours can provide visual information about the state of a particular individual's health and the quality of its diet.

This is the reason that many captive flamingos are white while wild flamingos are a brilliant pink: they obtain carotenoid-based pigments from their diet of algae and invertebrates and place these pigments into their skin, feathers and even into the keratin sheath covering their beaks. The pinkest flamingos are those that consumed the best diet and are the healthiest.

But among birds, the parrots are unique: their bright reds, oranges and yellows are not derived from dietary carotenoids. Unlike any other group of birds, parrots synthesize their own red, orange and yellow pigments, which were named "psittacofulvins" in honor of their avian creators [DOI: 10.1016/j.cbpc.2004.03.011]. Interestingly, these lipid-soluble pigments are found nowhere else; not in other birds, not in plants, nor even in plankton.

In addition to their unique pigments, most parrots are monochromatic: males, females and even juvenile birds look very much the same and the brightness of their plumage doesn't vary, regardless of diet.

Taken together, these factors suggest that their unique feather pigments serve more functions than just visual communication in parrots.

This is where Edward H. Burtt, Jr., a professor in the Zoology Department at Ohio Wesleyan University in Delaware, Ohio, comes into the story. A few years ago, he noticed that a variety feather-consuming microbes, including Bacillus licheniformis, Bacillus pumilus and other Bacillus species, are present on feathers, particularly among birds that live in salty or humid habitats. So of course, he wanted to know why those bacteria were there.

Dr Burtt's investigations showed these bacteria were eating feathers, and he also found that melanin-containing feathers are more resistant to bacterial degradation than those without melanins. Might the recently identified psittacofulvins serve a similar function in parrots, which primarily dwell in steamy tropical jungles? A tantalizing hint was provided by another researcher's observation that green feathers from Blue-crowned Parakeets, Aratinga aduticaudata, were unusually resistant to bacterial degradation [Grande et al].

To test the potential degradation-resistant properties of parrot feathers, Dr Burtt's team, which included several undergraduate honors students, first classified colourful flight feathers (rectrices and remiges) from 13 parrot species into six general colour categories: blue, green, red, yellow, black and white. The team placed differently-colored feathers into a bacterial medium containing Bacillus licheniformis, a bacteria that enzymatically degrades feathers. They measured the increased concentration of enzymatic by-products created as the bacteria broke down the feathers and compared this rate of accumulation between feathers of different colors (Figure 1):

As you can see, they found that feather colour significantly affected the bacterial degradation rate: White feathers degraded more rapidly than black, blue, green and red feathers. This is consistent with their hypothesis that colourful psittacofulvin pigments reduce microbial damage to parrot feathers.

Based on those data, it is reasonable to predict that green feathers -- which contain both psittacofulvins and melanins -- would be more resistant to bacterial degradation than feathers containing only melanins (blue, black) or only red psittacofulvins. But this is not what the team found.

The reason lies in the biochemistry of these pigments: green feathers only contain yellow psittacofulvins -- not red. If you look closely at the above figure, you'll see that feathers containing yellow psittacofulvins degraded nearly as rapidly as did white feathers, which do not contain any pigments at all.

Biochemical analysis of yellow psittacofulvin molecules found that they are formed by small carbon chains with few double-bonds, while red psittacofulvins are longer carbon chains with more double-bonds. In short, red psittacofulvin molecules are bigger than yellow psittacofulvins and have stronger bonds, so therefore, they should be more difficult for bacteria to break down.

So the team tested this hypothesis by comparing the rate of feather degradation to the amount of red or yellow psittacofulvins that the feathers contained, and found a direct relationship (figure 2):

So simply stated, the presence of psittacofulvins alone is not enough to resist bacterial damage: the feathers have to contain red psittacofulvins.

Even though this isn't an earth-shaking finding, it is interesting to me, an evolutionary biologist studies and lives with parrots, and who finds avian color to be source of endless fascination. This sweet little study serves as a reminder that there can be (and probably are) several evolutionary functions for specific characters, all exerting their individual influences simultaneously. Even the tiniest of influences, such as biochemistry, can have some stunning evolutionary consequences, as exemplified by the featured photograph of a Green-winged Macaw, Ara chloroptera.

Sources:

Burtt, E., Schroeder, M., Smith, L., Sroka, J., & McGraw, K. (2010). Colourful parrot feathers resist bacterial degradation. Biology Letters DOI: 10.1098/rsbl.2010.0716

McGraw, K.J., & Nogare, M.C. (2004). Carotenoid pigments and the selectivity of psittacofulvin-based coloration systems in parrots. Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, 138 (3), 229-233 DOI: 10.1016/j.cbpc.2004.03.011

Grande, J. M., Negro, J. J. & Torres, M. J. (2004). The evolution of bird plumage colouration: a role for feather-degrading bacteria? Ardeola 51 (2), 375–383.