Exploring the unseen

Does the colour of a bird matter for its thermoregulation? Darker colours gain more heat than lighter ones, but with thick plumage and other thermal adaptations, does a bird feel the difference? Maybe not when it comes down to visible colour alone; animal-visible colour makes up only half of the energy reaching the earth’s surface from the sun. The rest of the energy in sunlight falls in wavelengths known as the near-infrared (NIR) range.

Unlike UV and visible colour, NIR wavelengths are invisible to animals and so are free from the selection pressures of signaling and camouflage. Australian birds display variation in the amount feathers reflect NIR light across species and families, indicating a potential adaptive function. And considering the large contribution NIR plays in the energy and heat gain a bird is exposed to, this function may be thermoregulatory. To test this idea, we took spectral measurements (using setup, left) of bird skins preserved in the collections of Museum Victoria, South Australia Museum, and Australian Museum, representing the taxonomic diversity, climate range, and feather colour found in native birds.

Phylogenetic analysis shows a relationship between NIR reflectance and a species’ climate, not seen in UV-visible wavelengths. Our findings will have implications for understanding the adaptations and constraints birds have for their thermal environment and is of particular importance as Australia experiences more frequent and prolonged heat waves and drier periods. How well a bird can protect itself against these events is crucial in predicting the effects of climate change on different species.
An exploration into the structural origins of near-infrared reflectance in white bird feathers is also currently underway and will help us understand the evolution and production costs of this adaptation.

– Elizabeth Newton

Elizabeth completed her Masters with us, exploring the relationship between near-infrared reflectance and thermal environment in birds.