The iridescent feathers of birds are one of nature’s greatest marvels. The dazzling, shimmering colours on certain birds play an essential role in visual communication between individuals, including the sunbird species. New research on these tiny birds demonstrates that this beauty comes at its own cost, in the form of increased heat absorption.
There are essentially two different types of bird colouration: pigment colours and structural colours. Black, brown and grey colours are created by melanin, produced by the bird. The warm colours like yellows and pinks come from a family of chemical compounds known as carotenoids, found in the bird’s diet. Bright reds and green both come from specialised pigments unique to certain bird families. The turacos, for example, have both a vibrant red pigment called turacin and a rich green pigment known as turacoverdin. Parrots also have a particular group of pigments: psittacofulvin pigments.
On the opposite side of the spectrum, so to speak, blue colours and iridescence are structural colours created by light refraction caused by microstructures in the feathers. Keratin protein and specialised organelles called melanosomes in the feathers. They act as a kind of prism, scattering light into different wavelengths and producing an array of colours. The colours observed depend on the angle of the light and the position of the observer. Whether as a result of pigments or structural design, these colours communicate the physical fitness of the individual – a brightly coloured bird is likely healthy and strong and, therefore, a suitable potential mate.
Sunbirds are tiny nectar-feeding birds found across Africa, Asia and Australia. Their feathers are decorated in a bright array of different colours that use both pigments and iridescence. The males of all sunbird species are always more brightly coloured than their female counterparts, many of which are drab in comparison.
Researchers wanted to test how this iridescence affects the thermoregulation of the birds. In order to accomplish this, they exposed 15 different sunbird specimens to a lamp that mimics sunlight. They then measured the temperatures of the feathers and the underlying skin. The results showed that iridescent feathers heated up over 10˚C more than those with yellow/red pigments, while the skin underlying the iridescent feathers was 5-8˚C warmer than that beneath pigmented feathers. Scientists attribute this to a combination of melanin pigments and the arrangement and shape of the melanosomes that store these pigments. Thus, iridescent feathers likely heat up even more than black feathers.
Male sunbirds use their iridescence during courtship displays, which suggests a trade-off between sexual selection and thermoregulation. However, a significant limitation of this study is that it was not conducted on living birds in realistic situations and thus cannot account for active thermoregulation measures. From a simple behavioural point of view, sunbirds spend little time in one place in direct sunlight. Instead, they flit from flower to flower, often in dense vegetation. Birds can also pant to lose heat. Given that iridescence has evolved independently in multiple different species, the cost of this increased heating is unlikely too high.
Why then is this research necessary? It provides another important facet in evaluating the foreseeable effects of rising temperatures due to climate change. Although often underreported, birds are already severely affected by climate change, and extreme temperature events have resulted in several mass die-offs across the globe. The hotter it gets, the more energy sunbirds will have to invest in thermoregulation. This could reduce feeding times on hot days and, in turn, increase competition with each other and other bird species during the cooler hours.
The full article can be accessed through a paywall here: “Enhanced photothermal absorption in iridescent feathers”, Rogalla, S., et al., (2021), Journal of the Royal Society Interface
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