Gouldian Finch

A lot of genetic coordination and several different cellular processes go into the Gouldian finch’s extravagant coloration. To start, their six colors come from a number of different sources. For example, finches acquire their red and yellow colors from their diet, which consists of grass seeds, particularly sorghum. These foods contain red-hued carotenoid pigments that are absorbed by the bird’s fatty tissues and expressed in its feathers. The blues we see are structural colors, which are expressed when light is reflected through nanostructures within the feather. Greens and purples are a combination of blue structural colors and carotenoid pigments. Finally, black is the result of melanin pigment, produced by the skin cells as the feathers grow. Whether or not melanin is produced is determined by a recessive sex-linked gene. When present, melanin completely masks the carotenoid pigments.

Coloration provides a tremendous amount of information within a Gouldian finch population and plays an important role in mate selection and reproductive success.

Male and female Gouldian finches share the same distribution of colors but males are, as is common in the animal kingdom, brighter than females. The brightness of a bird’s coloration can provide a powerful visual indication of its fitness, and in particular, its ability to obtain food. Birds that have the richest diets tend to have more vibrant plumage and will therefore be more attractive to potential mates.

Interestingly, head color is the most important indicator of all and is linked closely to behavior: Red-headed finches are the most competitive and aggressive of all the morphs, usually winning access to the best nesting sites—hollows in eucalyptus trees—and food sources. The trade-off for this advantageous characteristic is a shorter life-span and decreased parental investment; red-headed males are less helpful in raising young because they spend more time fighting.

Gouldian finches also follow an assortative mating pattern, meaning that they prefer to mate with finches that look like them. This isn’t just aesthetic preference. Studies show that females in incompatible pairings tend to produce higher levels of stress hormones. As a result, they have smaller broods with fewer females, the sex most adversely affected by a poor match (in line with a genetic observation known as Haldane’s rule). Although mating with like individuals tends to improve breeding success, it also negatively affects genetic diversity within a population by limiting gene flow. This in turn may lead to reproductive isolation and a loss of rare morphs.

In most organisms, breeding between different morphs doesn’t have the same sort of negative impact on reproductive success as breeding between species. The fact that breeding between two morphs has such a powerful effect suggests that E. gouldiae population segments are in the process of diverging.

There are a couple hypotheses as to what scientists are currently observing in Gouldian finch behavior. First, it’s possible that the three different morphs are a result of vicariant evolution, a speciation tale as old as time that involves the separation—usually by environmental factors—of a population and its subsequent evolutionary branches. The morphs would have evolved into different subspecies in isolation, but rejoined into one breeding population after another geographic shift. This type of transitional phase usually has one of several results: the two species will interbreed freely without incompatibility and merge back into a single species; one of the two species will go extinct due to incompatibility; or they will develop pre-mating isolation behavior (such as assortative mating) and evolve into separate species. For Gouldian finches, the latter two possibilities seem the most likely.

A second hypothesis points toward sympatric speciation, in which a new species evolves from an ancestral species through a genetic mutation. Interestingly, the ratio of Gouldian finch morphs across Australia has stayed relatively constant across time and geography to the present day, with black morphs comprising 70% of the total population, red at 30%, and yellow morphs making up less than one percent. If the morphs were separated geographically at one point, one would expect a higher percentage of one morph or another to be found in certain geographic regions. Gouldian finches are long-distance travelers, however, making this even distribution plausible.

Although scientists don’t know exactly what caused such incompatibility within Gouldian finch populations, it’s very likely that the existence of all three morphs is a mere blip in this species’ long evolutionary history. What we’re witnessing now is likely just a particularly fascinating stage of speciation in progress.