Proximity to the equator has increased the color diversity of passerine birds

(ORDO NEWS) — Passerine birds with a wide variety of colors live in the tropics, and as they move towards the poles, their plumage becomes less and less bright.

Ornithologists came to this conclusion after analyzing more than 24,000 museum specimens belonging to 4,527 species.

As noted in an article for the journal Nature Ecology & Evolution, naturalists first noticed this pattern back in the 19th century, but it has only now been confirmed. Most likely, the bright color of tropical birds is associated with a combination of climatic and environmental conditions in their habitats.

As early as the 19th century, naturalists began to notice that tropical animals tend to be more brightly colored and more varied than those living in temperate climates. Since then, many hypotheses have been put forward to explain this pattern.

At the same time, some experts expressed doubts that the coloring of animals really directly depends on the latitude at which they live.

It is only now that scientists have access to accurate information about the coloration and distribution of various species to check which of these points of view is correct has become possible only now.

Proximity to the equator has increased the color diversity of passerine birds 2

A team of ornithologists led by Christopher Cooney (Christopher R. Cooney) from the University of Sheffield decided to find out how latitude affects the color of passerine birds (Passeriformes). This group includes about 60 percent of all modern bird species.

The researchers collected about 140,000 photographs of museum specimens representing 4,527 species of passerines. The analysis included images of males and females, as well as images taken under visible and ultraviolet light and from three different angles: a top view, a bottom view, and a side view.

For each of the more than 24,000 museum specimens, Cooney and colleagues determined the reflectivity of the plumage at 1,500 points. In total, this gave over thirty-six million measurements.

Based on them, the authors built a three-dimensional gradient of all colors and shades in which the studied specimens were painted. To determine the degree of diversity of each species, the researchers assessed how many of the possible colors and shades are present in its plumage.

Comparing data on the coloration of different species of passerine birds with their ranges, Cooney and his co-authors concluded that it really depends on geographic latitude.

The most colorful birds live in the tropics, and as they move towards the poles, their color becomes less and less diverse. This conclusion turned out to be true not only for males, but also for females (p<0.001 in both cases).

The researchers assumed that the pattern they discovered was due to the fact that more species of birds live in the tropics, including multi-colored ones. However, further analysis showed that this was not the case. The authors identified twenty-five percent of the most colorful species and found that most of them inhabit tropical regions.

In an attempt to understand why tropical birds are brighter than those living in temperate climates, Cooney and colleagues tried to identify additional factors that affect the color of birds. It turned out that species with sexual dimorphism, as a rule, have more colorful plumage.

In addition, small birds are usually brighter than large ones. It may be difficult for large species to produce enough carotenoid pigments, or smaller species may need brighter coloration to improve visual communication.

However, neither sexual dimorphism nor body size can explain why birds in the tropics are more colorful. Researchers consider climatic and environmental factors to be more important. For example, the level of variegated plumage of males and females was found to be related to the amount of precipitation and the level of primary production.

Most likely, the fact is that in humid and productive ecosystems, the coloring of birds is not subject to many restrictions that are typical for deserts and northern regions. Species from forest biotopes, which eat a large amount of fruits and nectar, which are sources of carotenoids, also turned out to be brighter.

In regions where many different species of passerines live, on average, more brightly colored birds are found. It would probably be difficult for the birds living here to identify their relatives if they did not have multi-colored plumage.

And, finally, females of migratory passerine species were found to be less colorful than females of sedentary species. This is consistent with the results of a previous study, according to which migratory birds are lighter in color than sedentary ones.

Cooney and his colleagues for the first time managed to confirm the pattern, the existence of which was supposed to exist for more than two centuries. However, the full set of factors due to which it was formed remains unknown.

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