As far back as the 19th century, biologists pointed out an apparent anomaly in the extra 'colourfulness' of birds and insects in tropical regions.

The reasons for the differences remain unproven, but Alfred Wallace and others suggested that they might be due to the “luxuriant vegetation of the tropics” which acted as a natural camouflage all year round, whereas organisms in the North and South had to adapt their plumage to match the bare trees in winter.

It was also suggested that tropical birds and insects might have more freely available energy to be able to 'indulge' in extra colourfull plumage.

A study published in Nature Ecology & Evolution, April 2022 examined more than 4,500 species, and suggests that previous theories might well have some merit.

We show that male and female birds of tropical passerine species are generally more colourful than their temperate counterparts, both on average and in the extreme. We also show that these geographic gradients can be explained in part by the effects of several latitude-related factors related to classic hypotheses for climatic and ecological determinants of organismal colourfulness."

See : Latitudinal gradients in avian colourfulness

It should be noted, however, that previous studies have suggested that the entire supposition is not scientifically sound.

See : Birds, butterflies and flowers in the tropics are not more colourful than those at higher latitudes, Global Ecology and Biogeography, Volume 24, Issue 12 (sample : 570 species)