From gaudy and colorful to monochromatic and simple: as is well known, there is a wide range in the characteristics of the plumage in the bird world. An analysis of 4,500 bird species in the world now reveals principles of the global spread of color traits. For the first time, a widespread assumption was confirmed on a scientific basis: Birds that live in the tropical regions of the world have more colorful plumage on average than birds that live in the temperate zones at higher latitudes. The blaze of color gradually decreases the further one moves away from the equator. The scientists also report on the possible reasons for the detected gradient.
People who travel to the tropics from Europe and other temperate regions often report the strikingly large blaze of color found in nature in the warm climes. Among other things, the great naturalist Alexander von Humboldt raved about how strikingly colorful the flora and fauna appear in the tropics, which also includes numerous colorful bird species. The subjective impressions led to the assumption that the birds in the area of the equator are generally more colorful than species that live in the high latitudes. So far, however, this has not been considered scientifically proven, as there have been no systematic and globally comprehensive studies on it. It seemed possible that the particularly “colorful” impression was due to the greater overall biodiversity in the tropics and not to a statistically tangible basic principle.
Analytical look at the worldwide “bird fashion”
In order to provide more clarity, the scientists working with Christopher Cooney from the University of Sheffield have created a broad database: They created digital photos of the females and males of more than 4500 species of passerine birds (Passeriformes) from all over the world. It is by far the largest group of birds, which also includes all songbirds – from the sparrow to the bird of paradise. The images were automatically evaluated using image processing techniques and machine learning methods to capture and characterize the color features. The researchers also developed an evaluation system for the study that is based on so-called “colour loci scores”. These color values result from the number of differently colored feather areas and their contrast. For example, among tropical birds, the colorful seven-colored tangar has a high value, while only dark-feathered tropical birds have a very low value. For us, birds such as the colorful goldfinch are at the top, while sparrows and blackbirds are at the lower end of the scale.
By linking them to the respective distribution areas and through their statistical evaluations, the scientists were now able to confirm scientifically for the first time: Passerines that live in the tropical regions of the world actually tend to be more colorful than birds that occur in the temperate zones of the high latitudes. In concrete terms, this means that the number of different color areas in the plumage is highest at the equator and decreases towards the north and south depending on the latitude. This principle can be found in both male and female animals, which can differ significantly in appearance in many species, the researchers report.
What is behind the gradient?
It remains unclear which effects exactly lead to the global pattern in the spread of the joy of colour. According to the scientists, however, fundamental aspects are emerging that seem to play a role. Thus, the pronounced peak in color in the tropical zones can be explained in part by latitude-associated gradients in climatic conditions and ecological properties. Because, as the results show in detail, the color variety tends to be higher in bird species that live in closed forest habitats, which are particularly typical of many tropical areas. Bird species that are more likely to be found in open grassland, on the other hand, are more often simply colored.
In detail, it was also shown that those bird species that feed on fruit and flower nectar are also more colorful. Thus, the need for visual cue for communication in cluttered habitats, as well as the ability to ingest color-forming compounds with food, is thought to be associated with the pattern of distribution of color complexity in birds. “But many other potentially important factors remain to be explored,” write Cooney and his colleagues.
Finally, they take a look beyond the world of birds. A possible latitude gradient in color was not only postulated for these animals: early naturalists such as Alexander von Humboldt generally noticed the striking colorfulness of many tropical taxa, including plants, insects and aquatic animals. “While our results provide clear evidence for a latitudinal gradient in color in passerine birds, it remains to be clarified to what extent other creatures follow the possible rule that life in the tropics is generally more colorful than in the temperate zones,” concludes the scientists.
Source: Nature Ecology & Evolution, doi: 10.1038/s41559-022-01714-1