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A bird's eye view

August 27, 2023
Charlotte Bartleet-Cross

A bird's eye view

The majesty of different sizes, shapes, and colours that our feathery friends come in is a delight to many. A YouGov poll found that 63% of UK adults watched birds and listened to their repertoire of song to soothe anxiety during the Covid-19 pandemic. This renewed interest in birds and their ecology poses an interesting question; why are some birds brightly coloured and others dull? And is this related to how birds see the world?

Birds are tetrachromatic; which means that they possess at least four types of ‘cone’ photoreceptors in their eyes. These include three that we humans have (cone cells that detect blue, green and red light), and a fourth which allows them to see in the tantalizing world of violet and ultraviolet (UV) light. They also have special retinal oils, which allows them to filter out certain wavelengths of light and stops their eyes being damaged by UV.

But how does this benefit our bird species?

As we know, birds are excellent at navigating through dense bushes and trees, and the use of UV allows them to see more distinctly between leaves and dense foliage. This can be seen more clearly in this image, taken from a Nature publication from 2019. Their UV vision is thought to help them find berries, fruits, insects, and spiders on the underside of leaves by creating a contrast between different hues. This allows bird species to forage efficiently by the quick identification of food sources from further away and enables them to build an enhanced 3D map of their environment.

How this difference in their vision relates to plumage colouration, however, is slightly more complex. Female birds tend to have the upper hand in being able to select the best mate; termed ‘sexual selection’.  This means males must put on a show for their genes to be passed on to the next generation. This usually results in males showcasing their amazing dances, birdsong and displaying their best breeding plumage to impress the ladies.

The use of intense feather colouration for breeding season is costly for individuals, and so is an honest signal to females that males are good ‘quality’. However, not all songbirds are so bright, so are we missing out on the secrets of the ‘little brown job’?

One US study would suggest that we are!

Over 90% of songbird species that looked identical in both sexes to the human eye, looked vastly different under UV with UV-reflectance found in different patches in males and females. Research that showcases that your little brown job doesn’t look so dull and beauty truly is in the eyes of the beholder!

The SBS Team

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