How Amazon parrots spread across the Greater Antilles

Mitochondrial DNA points to a stepping-stone scenario from Jamaica to Puerto Rico.

Islands have often been described as “natural laboratories” for evolutionary biologists. But before evolution can start tinkering with the available genetic variation of newly founded island populations, organisms first need to reach and spread across these archipelagos. Reconstructing the chain of colonization of an island group is no easy exercise. A nice example of this challenge concerns Amazon Parrots on the Greater Antilles (an archipelago in the Caribbean Sea including Cuba, Hispaniola, Puerto Rico, Jamaica, and the Cayman Islands). This collection of islands houses five species of Amazon Parrot:

  • The Cuban Amazon (Amazona leucocephala) on Cuba
  • The Black-billed Amazon (Amazona agilis) on Jamaica
  • The Yellow-billed Amazon (Amazona collaria) on Jamaica
  • The Hispaniolan Amazon (Amazona ventralis) on Hispaniola
  • The Puerto Rican Amazon (Amazona vittata) on Puerto Rico

Ornithologists have been speculating about the evolutionary history of these parrots. Patricia Ottens-Wainright and her colleagues suggested that there may have been two independent dispersal events to the Greater Antilles. David Lack, however, argued for a single colonization event based on the morphological similarities between the Cuban Amazon, the Hispaniolan Amazon and the Puerto Rican Amazon. A recent study in the journal Genes attempted to resolve this debate by sequencing the complete mitochondrial genomes of all five species.

A Phylogeny of Parrots

First, Sofiia Kolchanova and her colleagues reconstructed the phylogenetic relationships among the parrots on the Greater Antilles and their South and Central American cousins. These analyses revealed that the five species from the Greater Antilles form a distinct cluster that is sister to the White-fronted Amazon (Amazona albifrons), a species from Central America. This finding suggests that the Greater Antilles were colonized only once, arriving on Jamaica. But how did these parrots subsequently spread across the islands? To answer this question, the researchers explored several biogeographic models with the R-package BioGeoBEARS.

Phylogenetic relationships among Amazona parrots based on the whole mitochondrial genome. The five species from the Greater Antilles are most closely related to the White-fronted Amazon (Amazona albifrons) from Central America. From: Kolchanova et al. (2021).

Stepping Stones

All biogeographic models indicated that the ancestor of Amazon Parrots on the Greater Antilles reached Jamaica about 3.5 million years ago. This date coincides with a period of low sea levels when birds could easily reach the islands from the Central American mainland. About 3.1 million years ago, the founding population on Jamaica split into two species: the Black-billed Amazon and the ancestor of the four other species. However, it is unclear what happened next. Some models indicated that the speciation event happened on the same island (i.e. sympatric speciation), giving rise to the Yellow-billed Amazon. Roughly 1.4 million years ago, part of this population colonized Cuba. However, other models pointed to an allopatric speciation event after dispersal to Cuba (or some islands that have now disappeared). Later on, some of these Cuban birds returned to Jamaica and evolved into the Yellow-billed Amazon. More detailed analyses – with nuclear genetic data – are needed to discriminate between these scenarios.

The remainder of the evolutionary story of these parrots is more clear. The researchers write that “most models agree that once the parrots reached Cuba, they have continued to disperse to Hispaniola and then to Puerto Rico in a stepping stone fashion.” The colonization of Hispaniola occurred ca. 760,000 years ago while Puerto Rico was reached about 690,000 years ago. However, it is important to keep in mind that this scenario, and the accompanying dates, is solely based on mitochondrial DNA. Analyses of the whole nuclear genome might tell a slightly different story. Although it seems almost certain that the Greater Antilles were colonized once by these parrots. David Lack was right.

An overview of the eight biogeographical models tested in this study. Each small circle corresponds to a model and the colors indicate the most likely island where the parrots resided. From: Kolchanova et al. (2021).

References

Kolchanova, S., Komissarov, A., Kliver, S., Mazo-Vargas, A., Afanador, Y., Velez-Valentín, J., … & Oleksyk, T. K. (2021). Molecular phylogeny and evolution of Amazon parrots in the Greater Antilles. Genes12(4), 608.

Featured image: Cuban Amazon (Amazona leucocephala) © Laura Gooch | Wikimedia Commons

Why are some parrots more colorful than others?

It turns out that size matters.

The colors of parrots never disappoint. Think of the red, yellow and blue plumage of the scarlet macaw (Ara macao), or the bright pink crest of the Major Mitchell’s Cockatoo (Cacatua leadbeateri). A quick overview of the ca. 400 species of parrots (order Psittaciformes) reveals a huge variety of color combinations. What explains this wealth of colors? And why are some species more colorful than others? One important factor is that parrots can synthesize their own pigments for the colors red and yellow (i.e. psittacofulvins), while other bird species need to extract such pigments from their diet (mostly carotenoids). This might allow parrots to deposit more of these pigments in their feathers. A second reason relates to the breeding behavior of parrots. Most species make their nests in cavities where they are safe from predators. There is no need for camouflage, so parrots can develop colorful plumage patterns.

These are just two possible explanations for the colorfulness of parrots. A recent study in the Journal of Evolutionary Biology dug a little deeper and uncovered several other factors that contribute to the amazing variety of colors in parrots.

Big vs. Small

Luisana Carballo and her colleagues quantified the color patterns of 398 parrot species using the Handbook of Birds of the World. Their analyses showed that phylogeny explained a large part of the variation. It seems that birds of a feather flock together on the evolutionary tree. After controlling for the shared evolutionary history, the remaining factors explained up to 15 percent of the variation in colors. Let’s have a look at the main patterns.

First, larger parrot species tend to have more elaborate colors. This finding probably relates to the lower predation pressure on larger species. Most predators are no threat to large parrots, so they can afford to draw the attention with their colorful plumage. In addition, larger parrot species form long-lasting pair bonds which involves mutual mate choice. Both partners need to be satisfied with their choice if they are going to spend a lifetime together (which can be up to 100 years…). So, we can expect both sexes to be equally ornamented due to this mutual mate choice, similar to other tropical species (see for example this study).

Interestingly, although smaller species sport less elaborate colors, they show larger color differences between males and females (i.e. higher levels of sexual dichromatism). These patterns suggest stronger sexual selection in the smaller species. This idea is supported by a study on sperm morphology that reported a positive correlation between the length of sperm cells (a proxy for the strength of sperm competition) and the degree of sexual dichromatism in parrots. High levels of sperm competition also suggest that females will mate with multiple partners, but the frequency of these extra-pair copulations remains to be determined.

An overview of the data on color patterns in parrots. The figures illustrate the variation in (a) dark vs. light, (b) green vs. red, and (c) blue vs. yellow. Figure (d) show the spread of colors for males and females. From: Carballo et al. (2020) Journal of Evolutionary Biology.

Gloger’s Rule

Another peculiar pattern is that parrots tend to be darker in wetter areas, a phenomenon known as Gloger’s Rule (which I also covered in this blog post on gulls). The exact mechanisms behind this biogeographical rule remain unclear. It could be that darker colors are more suitable for camouflage in wetter environments where more vegetation results in lower light conditions. However, this hypothesis conflicts with the elaborate colors of larger parrot species that do not rely on camouflage. A second possibility concerns degradation of feathers by bacteria. Wet conditions are conducive for the growth of feather-degrading bacteria. The deposition of the dark pigment melanin protects the plumage from these feather-munching menaces. In addition, parrots are redder in wetter environments. The red pigment psittacofulvin might also provide more protection against these bacteria.

Parrots and cockatoos with more elaborate colous have lower levels of sexual dichromatism. The colors of the phylogeny (from blue to red) indicate the degree of color elaboration, while the bars on the outside correspond to the level of sexual dichromatism. From: Carballo et al. (2020) Journal of Evolutionary Biology.

Unweaving the Rainbow

This study nicely illustrates the complex interplay of evolutionary and ecological forces in shaping the wonderful colors of parrots. It reminds me of a conversation between physicist Richard Feynman and an artist. The artist argued that science takes away the beauty of nature. Science is “unweaving the rainbow” as John Keats put it. Feynman disagrees and explains how science only add to the beauty, using the example of a flower. The same reasoning applies to the amazing wealth of colors in parrots.

I can appreciate the beauty of a flower. At the same time, I see much more about the flower than he sees. I could imagine the cells in there, the complicated actions inside, which also have a beauty. I mean it’s not just beauty at this dimension, at one centimeter; there’s also beauty at smaller dimensions, the inner structure, also the processes. The fact that the colors in the flower evolved in order to attract insects to pollinate it is interesting; it means that insects can see the color. It adds a question: does this aesthetic sense also exist in the lower forms? Why is it aesthetic? All kinds of interesting questions which the science knowledge only adds to the excitement, the mystery and the awe of a flower. It only adds. I don’t understand how it subtracts.

References

Carballo, L., Delhey, K., Valcu, M., & Kempenaers, B. (2020). Body size and climate as predictors of plumage colouration and sexual dichromatism in parrots. Journal of Evolutionary Biology33(11), 1543-1557.

Featured image: Scarlet Macaw (Ara macao) © Travis Isaacs | Wikimedia Commons