The closer to the equator, the more colorful the bird world - and this is what natural scientists like Alexander von Humboldt or Russel Wallace once reported

completely fascinated by their travels far from our temperate latitudes.

In "Nature Ecology & Evolution", British and Hungarian researchers now describe in a recently published study how such personal impressions and observations can be quantified with the help of modern technology and AI and how old museum collections can be used for this purpose.

Sonya Kastilan

Editor in the "Science" department of the Frankfurter Allgemeine Sunday newspaper.

  • Follow I follow

A team led by evolutionary biologist Gavin Thomas from the University of Sheffield studied the skins of 4,527 bird species from the order Passeriformes, which are archived at the Natural History Museum in Tring, Hertfordshire, north-west of London and represent around 75 percent of this group who count among others the songbirds.

Each plumage - from both males and females of a species - was photographed under normal lighting conditions as well as under ultraviolet radiation in order to then evaluate 1500 pixels and create a multidimensional color scale.

"In our analysis, we tried to consider how birds would perceive the feathers, which have a more complex color perception than humans," explains lead author Christopher Cooney in an interview.

With the collections at the museum in Tring, he and his colleagues had access to a real treasure of ornithology, because there are stored a total of more than a million specimens (including 750,000 skins, some more than 250 years old) of around 9000 bird species.

It is the world's largest single archive and includes 95 percent of the avifauna.

"This allowed us to look at the variation globally, not just regionally, and draw conclusions about ecology and biodiversity."

Computer scientists developed a mathematical approximation of the visual spectrum of the animals, and geographical gradients actually resulted from the data collected over several years: the colorfulness of the birds correlated with the degree of latitude, i.e. their habitat.

It is in the humid tropics that evolution has produced the most colorful birds, particularly in dense forest areas, which are denser and therefore darker, and in ecological niches rich in fruit and nectar.

Which is probably related, because attractive plumage costs energy, and European songbirds often have to work hard to earn it.

However, there are also quite colorful birds here, and Cooney cites the blue tit as an example,

In addition to a general trend, Cooney and his colleagues discovered a connection between color and body size, which is still controversial, so smaller birds are usually more colorful than larger ones.

"Not all experts are convinced yet, but we found clear evidence for this, even if the mechanisms behind it have not yet been precisely understood." There are several possible explanations.

For example, it could simply be due to the fact that large birds cannot take in as many color substances with their food, such as carotenoids, to store enough pigments in red, orange or yellow for a trendy bright plumage.

The specific interaction or the habitat could also be a reason that smaller chirps are more colorful, just to attract the attention of conspecifics in their environment.

Not only was the plumage used during the breeding season, the respective nesting areas also served to localize a species, which was particularly important for migratory birds that commute between Europe and Africa, for example.

In her case, it was also found that the females in particular were strikingly colorless: "Maybe because they don't need to waste energy on it," explains the British biologist.

When they reach the breeding grounds, they have a choice;

the males who arrived earlier would then have already found their territories and have to compete for the favor of the females.

All in all, the play of colors is mainly based on pigments, for which the food must be rich in carotenoids, but iridescent blue or green is produced with the help of physical tricks and this requires special feather structures: "These fine structures have arisen several times in the evolution of birds and found in different groups, such as hummingbirds and African sunbirds, which look similar but aren't closely related," Cooney explains.

Her plumage could sparkle in every imaginable color, a real spectacle for the eye.

Whether visual splendor in turn replaces the acoustically effective singing talent or vice versa, as has long been suspected, is unclear and would have to be researched more closely.

What seems to apply to the tropical bird world, Cooney would like to transfer to other groups of animals in further investigations and have it confirmed: "The colorfulness of the tropics was also observed in plants or insects, and what is better suited for a photo analysis than butterflies?" After all, these are known to be colorful and almost two-dimensional.

As invertebrates, they offer an interesting contrast to birds to better understand the mechanisms and drivers of evolution;

their colors also take on other tasks, such as warning of poison, and form complicated patterns.

After all, Europe would have nice exceptions to the rule with peacocks and swallowtails if a similar gradient were to emerge as for the birds.

Your play of colors exists here, far from the tropics.