Spectacular plumage color like that seen in many parrot species is usually associated sexual selection, which raises the question: are parrots genuinely monogamous? Or are other factors influencing their dazzling plumage colors?
People love parrots, at least partly because of their often spectacular plumage colors.
“The colors that parrots display are impressively beautiful and rare when compared with other birds”, said Luisana Carballo, who is the lead author of a recent study of parrot plumage color and a doctoral candidate at the Max Planck Institute for Ornithology.
In fact, parrots are one of the most colorful groups of birds, but the reasons for this are unclear. Is it because most parrot species nest in tree hollows, where they are safer from predators than if they nested in open cup nests or on the ground? Or perhaps it’s because parrots are the only birds that can synthesize their own red, orange and yellow plumage pigments (known as psittacofulvins; more here), allowing them to display more intense colors than other birds that must obtain the carotenoid pigments that produce red plumage colors from their diet (m0re here)?
“Plumage colouration is normally attributed as a consequence of sexual selection, however parrots are considered to be mainly monogamous”, said Ms Carballo in email. Monogamous birds typically do not show strong sexual selection so males and females tend not to be brightly colored, and they often have the same plumage colors and patterns.
“Thus, I was very curious to explore whether parrots were genuinely monogamous or whether there were other factors that could cause the striking colouration parrots display.”
To better understand plumage color in parrots, Ms Carballo made a series of predictions based on a variety of life history traits. She predicted fewer color differences between males and females (sexual dichromatism) but higher color diversity and color elaboration in larger parrots that lay smaller clutches. She also predicted there would be more sexual dichromatism and more diverse plumage color in males of smaller parrot species that lay larger clutches and breed at higher densities (i.e. are gregarious). Ms Carballo also predicted there may be an effect of climate on plumage color such that species with darker plumage colors should inhabit wetter and colder environments or denser forests rather than open habitat types.
A simple hypothesis that was hugely difficult to test
To learn more, Ms Carballo and her collaborators conducted a comparative survey that tested her predictions about parrot plumage color and a variety of life history traits, such as body size, and environment.
The hypotheses that Ms Carballo were testing were straightforward, but actually collecting the data needed for the analyses was quite challenging. Her original plan was to analyze colors in photographs of live parrots, but Ms Carballo wasn’t able to accomplish that.
“In theory everything sounded great when I was preparing myself to go to [Loro Parque in] Tenerife to work with the animals, but in practice, it was a mess!” Ms Carballo said in email.
“Between me and a field assistant, Alessandra Battistotti, we really tried to take these photos but we failed terribly. Parrots are extremely strong and they really don’t like to be held, so these combined with us being careful trying to not get badly bitten made it very hard for us to estimate colors from photos. We got bitten anyway but that’s how we learned to hold them properly and to not hurt them… and on one occasion trying to take these photos, one of the animals was moving so much that it escaped from our hands and we, together with all the animal keepers in the breeding station, spent quite some time looking for that guy!”
Mangled and crushed fingers combined with elusive parrot escapees effectively ended that plan. Ms Carballo and her collaborators went back to the drawing board and decided to instead analyze photographs of museum specimens until they realized they didn’t have access to such a large collection of samples. (There are at least 398 extant species of parrots.)
So they devised yet another plan.
“We decided to use book plates to be able to include all parrots and because there were previous studies showing that this technique was quite accurate”, Ms Carballo explained in email.
Book plates? How scientifically rigorous are those? Apparently, the plates depicting parrots featured in the most recent books are quite accurate, as Ms Carballo and her collaborators discovered when they checked the color measurements themselves.
“We had available colour estimates from museum specimens for a subset of the species, and we used these to evaluate how accurate the book plates measures were”, Ms Carballo explained in email. “Generally we found a good correlation, thus we considered that book plates provide a good representation of plumage colour.”
The researchers compiled digital images of color plates of both sexes for each of the parrot species illustrated in the reference series, Handbook of the Birds of the World Alive (HBW Alive, del Hoyo et al., 2017). Ms Carballo and her collaborators then delineated 12 body patches (nape, crown, forehead, throat, upper breast, lower breast, shoulder, secondary coverts, primary coverts, secondaries, primaries and tail) for each sex and extracted red, green, and blue (RGB) color values from 400 randomly chosen pixels in each patch.
Bigger parrots are more colorful
For all 12 body patches, Ms Carballo analyzed and computed three color variables: color tone (or color lightness; black = 0, white = 100, Figure 1a), color intensity between green (low values; Figure 1b) and red (high values; Figure 1b) and (3) color intensity between blue (low values; Figure 1c) and yellow (high values; Figure 1c).
They also estimated the color elaboration score (Figure 1d) by computing the average distance between the previously estimated two color values (Figure 1a,b,c) for each species. They classified highly elaborate colors (red, blue and yellow) as those that differed more from the average color (greenish-brown) across all the parrots.
Sexual dichromatism was more common in smaller parrots
Ms Carballo estimated the body size of males and females from three body measurements: wing, tarsus (leg bone) and tail length, and estimated the average body size for each species by calculating the average male and female body sizes. Although body mass did correlate strongly with the other three measures, Ms Carballo specifically excluded it from all her analyses because this trait is more condition-dependent than the other size measures that she used and because the sample size for this trait was small, which decreased the statistical power of her analyses.
Sexual differences in color were computed in two ways: sexual dichromatism (Figure 2a) estimates absolute color differences between male and female parrots irrespective of which sex is more ornamented, whereas color elaboration (Figure 2b) indicates whether males or females of each species have more elaborate colors. According to this measure, positive values reflect species where males have more diverse colors than females. In species where males and females have different colors but with the same level of elaboration, such as for Eclectus parrots, this index has a low score.
Using this method, Ms Carballo and her collaborators found that sexual dichromatism was negatively correlated to body size, as they predicted, indicating that smaller shorter-lived parrot species are more likely to show visual differences between male and female plumage colors or patterns. This suggests that female mate choice acts more strongly on males in smaller parrot species, whereas large parrot species where both sexes typically have the same elaborate plumage colors and patterns appear to rely on mutual mate choice.
Additionally, Ms Carballo and her collaborators found that sexual dichromatism was more pronounced in parrots living in closed or forested habitats. Perhaps parrots inhabiting dark forests rely upon bright colors to enhance a male’s overall conspicuousness to females?
Ms Carballo and her collaborators also measured a variety of other life history traits, such as clutch size and gregariousness, to test whether any of these may affect plumage color in parrots. None of them did. But Ms Carballo and her collaborators found that an important component of plumage color and variation in parrots can be explained simply by shared evolutionary history (Figure 7, way up at the top).
Climactic effects on plumage color
Ms Carballo and her collaborators found that environmental effects were largely restricted to two climactic variables: temperature and precipitation. Basically, parrots that live in warmer and wetter environments are darker (as predicted by Gloger’s rule, which specifically proposes that birds and mammals should have darker colors in humid and warm environments), whilst parrots living in hotter climates show more blue. There are two plausible explanations for this: (1) it is easier for parrots with dark plumage colors to remain camouflaged from predators in darker habitats, or (2) feather damage caused by feather-degrading microbes that live in wetter environments is mitigated by dark pigments.
The researchers found no effect of habitat type on color, so they concluded that the parasite-resistance hypothesis is the more plausible underlying explanation. Further, Ms Carballo and her collaborators found that parrots also have redder plumage in wetter environments. Red plumage in parrots is created by psittacofulvins, a group of pigments that are unique to parrots, which have previously been shown to provide some protection against feather-degrading bacteria (more here). Taken together, these findings bolster the idea that specific plumage colors that make feathers stronger in parrot species living in wetter environments are the most likely explanation.
Large parrots are generally the most colorful parrots
“I was happily surprised with the general result that larger parrots are more colourful yet have less colour dimorphism”, Ms Carballo explained in email, but added: “Our results summarise a general trend among parrots, but the effects we found didn’t not explain 100% of the variation.”
She also noted that whilst parrots are mainly monogamous, similar trends in color ornamentation were also found in an earlier study of their closest relatives, the passerines (ref), although those trends in plumage color are not driven by monogamy.
Parrots are one of the most endangered groups of birds on the planet, mainly due to poaching for the pet trade and habitat destruction. But there is little research about them and their lives, so this comparative survey is an important development because it suggests five predictions that parrot researchers can test in future field studies:
- In larger, colorful species, both males and females defend scarce nest cavities and plumage colors may play an important role in mediating these aggressive interactions. Conversely, in smaller species, competition for cavities should be weaker and not necessarily associated with plumage colors, especially female colors
- Mutual mate choice based on plumage color should be more common in large parrots
- Large parrots should experience lower predation risk
- Sex differences in the variance in reproductive success should be size-dependent. In smaller species, male variance should be larger than female variance, whereas larger species should have little difference between males and females
- Extra-pair paternity may be the mechanism allowing higher male variance in reproductive success in spite of social monogamy, and thus, higher levels of extra-pair paternity should be expected in smaller parrots
“I believe every study conducted about them is a little contribution to help us understand them better and to evaluate how we can protect them”, Ms Carballo said in email. “I think everyone will agree that we should do as much as we can to better enjoy these gorgeous animals that have such incredible charisma.”
Luisana Carballo, Kaspar Delhey, Mihai Valcu, and Bart Kempenaers (2020). Body size and climate as predictors of plumage colouration and sexual dichromatism in parrots, Journal of Evolutionary Biology | doi:10.1111/jeb.13690