Rainfall and temperature predict bird colouration, study finds

bird colour
An eastern whipbird

An international study led by a Monash University ornithologist has found variation in rainfall and temperature can affect the colours of birds. The study provides a new framework that can be used to understand how future changes in climate will affect animals.

The research, just published in the journal Ecology Letters, examines colour variation across nearly 6000 species of passerines birds, the largest group of birds.

In their global analysis of bird plumage colour variation, the research team observed darker coloured birds occurring in regions with high rainfall and colder temperatures.

“Nearly 200 years ago, Constantin Gloger noticed that animals living in tropical regions tended to be more pigmented,” said lead study author Dr Kaspar Delhey, from the Monash School of Biological Sciences.

“His observations, and that of others, were synthesised into’ Gloger’s rule’, which predicts that animals should be darker in warmer and wetter regions, possibly because darker animals are better camouflaged in shady habitats, such as in tropical rainforests.”

But Gloger’s rule partly conflicts with another rule explaining colour variation in animals: Bogert’s rule, which predicts darker animals in colder regions, because darker colours absorb more solar radiation which helps with thermoregulation.

The global scale of the recent study allowed the researchers not only to comprehensively test both rules, but also to determine why some groups of birds sometimes don’t seem to follow them.

“We found quite a bit of variation across different regions of the world in how closely birds followed the rules” Dr Delhey said. “For example, in South America there seems to be little correlation between coloration and temperature and we wondered why that was so.

“It turns out that both rules can interfere with each other. In particular, precipitation–which has a very strong effect—can overwhelm the weaker temperature effect.”

This happens when climate ranges from cold and dry to hot and-humid. In such places the lightening effect of temperature is swamped by the darkening effect of rainfall.

On the other hand, when climate varies from hot and dry to cold and wet, the rules reinforce each other and their effects become stronger. Thus, the type of climatic gradient determines whether both rules work together or against each other and this can explain why certain groups of animals seemingly fail to follow the rules.

“Understanding how animal form and function is shaped by climate is particularly relevant today, given the ongoing threat of climate change, which predicts future increases in temperature and changes in precipitation patterns,” Dr Delhey added.

“Geographic variation in coloration, such as described by Gloger’s and Bogert’s rules, can therefore provide hints on how animals have adapted to past variation in climate and how they will fare in the future.”

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