To Be Colorful, or Not to Be… There is no in-between!

By: Felicia Cafua, Maddie Fancher, Sydney Ledoux, and Deirdre Boyer (Stonehill College, BIO323: Evolution, Spring 2021)

One of the most exciting parts of life is the diversity of colors that animals exhibit. When looking at the diversity of an animal’s color, many questions arise: Why are there so many different colors? Wouldn’t this attract more predators? How do these different colors arise? Birds are a fantastic example of the diversity of coloration; even in one bird species, there can be considerable variation between individuals. Parrots, in particular, show a great diversity of coloration and can have very intense plumage coloration. Plumage color is the color of the feathers covering the entire bird collectively. The exact reasoning for such a wide range of plumage colors is not well understood. Here we summarize a recent paper on plumage coloration in parrots that was published in the Journal of Evolutionary Biology by researchers from the Max Planck Institute for Ornithology.

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The authors of this study, scientists Carballo, Delhey, Valcu, and Kempenaers, decided to delve deeper into understanding the diversity of plumage coloration in parrots. Sexual selection is common reasoning for extreme color variation. Part of sexual selection is when one sex has a preference for a particular characteristic of the opposite sex. Sexual dichromatism, or a difference in color between males and females of the same species, is an indicator of sexual selection. There can also be differences in sexual selection depending on if the birds are monogamous or if the females mate with multiple males (extra-pair paternity). Another reasoning for plumage color variation is Gloger’s rule. Gloger’s rule states that darker coloration appears in wet and cold environments and densely forested environments. Lighter colorations are found in dry and warm environments like the deserts. The authors of this study estimated higher sexual dichromatism and diversity of color in males that exhibit dimorphism and breed at higher rates. Their second hypothesis was predicting higher sexual dichromatism and higher diversity of color in smaller males. To test these hypotheses, the scientists used the Handbook of the Birds of the World Alive (HBW Alive) to look at the digital images of 398 species of Psittaciformes, which is the order that includes parrots, cockatoos, and lorikeets. They used statistical analyses to score a random 400 pixels from each of the 12 body patches in the images for their color and scored each species. The overall color, differences between each species’ sexes, how elaborate the coloration was, and the diversity between each color patch was analyzed.

They also used HBW Alive to measure sexual selection and morphology traits, including body mass, wing length, tarsus length, and tail length. The HBW Alive book also gave information on habitat (habitat type, temperature, and average annual precipitation), which the researchers used to score for each species. Finally, they created a phylogeny of all 398 species to compare the variation they found in the coloration to what would be expected based on the relationship between birds.

Figure 7 from the article: “Parrots and cockatoos with more elaborate colours have lower levels of sexual dichromatism. Phylogeny of Psittaciformes depicting a reconstruction of evolutionary changes in male colour elaboration (branch colours, red = high, blue = low) using function contMap in R package ‘phytools’ v.0.6‐99 (Revell, 2012) and levels of sexual dichromatism (bar lengths at the tips). Note how species with low levels of colour elaboration have higher levels of sexual dichromatism. The plot is based on one phylogeny in the sample, but comparative analyses were carried out on 1,000 phylogenetic reconstructions to account for phylogenetic uncertainty. Selected genera have been highlighted and species in illustrations are represented with red bars. Illustrations © Lynx Edicions”

Because birds are able to see in ultraviolet radiation while humans are not, the researchers looked at the direct reflectance measurements of specimens of 51 species to see if what the researchers were measuring from the HBW Alive plates is what the birds are actually seeing. They did not find a large difference between their initial calculations and the reflectance measurements, so they were able to determine that their findings were significant. Still, there was a wide range of correlation scores between the bookplates and the reflectance values. A correlation score closest to one indicates the strongest correlation. The most substantial correlation score was sexual dichromatism of 0.86, followed by color diversity, where males were 0.88 and females were 0.74. The subsequent strongest correlation was the difference in color elaboration between females and males, which was 0.60. Finally, a color elaboration score of 0.53 for males and 0.67 for females showed the weakest correlation out of them all.

Figure 8 from the article: “Variation in colour elaboration and sexual dichromatism in parrots is correlated with body size. Larger parrots have more elaborated colours and lower levels of sexual dichromatism, whilst smaller parrots are less colourful but show higher levels of sexual dichromatism. Resource competition, mutual mate choice, social selection, predation risk, sexual selection on males and extra‐pair paternity are the possible processes that led to the varying patterns of colour elaboration and sexual dichromatism in large and small parrot species. Illustrations © Lynx Edicions”

Through this study, the researchers discovered how plumage coloration is affected. They determined that bird species that are large and have a relatively small clutch size tend to exhibit more intense plumage colors. The researchers also discovered that the bird’s environment impacted the plumage color in the birds. They found that temperature significantly affected the color elaboration in the birds, meaning that birds found in environments with a higher temperature exhibited more blue coloration. The study also showed that birds who lived in areas with more precipitation would display a darker and redder plumage color, supporting Gloger’s rule. Finally, the researchers found that sexual dichromatism was more evident in the regions that are closed and forested, although this effect was small.

This research study allowed others to understand how life-history traits such as predation, the surrounding environment, sexual selection, and environmental variables affect birds’ plumage coloration. The researchers concluded that larger birds were more likely to exhibit more elaborate colors and that smaller birds were more likely to display less complicated colors. Still, the smaller species experienced higher levels of sexual dichromatism. Continued research will explain how plumage color affects bird behaviors, such as defending territories, mate choice, predation risk, reproductive success, and extra-pair paternity.

Article: Carballo, L., K. Delhey, M. Valcu, and B. Kempenaers. 2020. Body size and climate as predictors of plumage coloration and sexual dichromatism in parrots. Journal of Evolutionary Biology, 33(11): 1543–1557.

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