The World of UV Vision, Seen Through the Eyes of Birds

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The World of UV Vision, Seen Through the Eyes of Birds
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Welcome to the second post of a four part series looking into the wonderful ultraviolet world of various species across the planet; from animals and birds to insects and fish. In this post I find out how having the ability to see into the UV spectrum affects the way birds interact with the world, and try to illustrate how they may visually perceive the world. I also explore how the famous artist Claude Monet was said to have developed ‘bird vision.’

It took us a long time to discover just how sophisticated the visual colour systems of birds are. As humans, our trichromatic vision means we have photo-pigments which are sensitive to three peak light wavelengths: red, blue and green.

But if it was a competition, birds would have us well and truly beaten. Many of them are tetrachromats or even pentachromats, meaning they have photo-pigments which are sensitive to four or five peak wavelengths. Some of the photo-pigments are sensitive well into the UV range; largely hidden for us

humans.

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Unlike in animals – whose UV sensitivity varied greatly within the UV spectrum – it seems that birds can be broadly separated into two groups: Those with violet vision (between 405-420nm on the light spectrum) such as pigeons, waterfowl and woodpeckers. And those that see some way into the ultraviolet spectrum (between 355 – 380nm) such as Blackbirds, Common Kestrels and European Starlings (shown in the diagram above).

Experiments to determine just how different birds saw the world involved sequencing the DNA of over 40 bird species. Scientists extracted DNA from blood samples, muscle tissue and the bases of the feather quills and reconstructed the proteins that made Up the light-sensitive pigments in the birds’ eyes.

It was a single gene mutation in the DNA which revealed what vision category (violet or ultraviolet) each bird type fell into.

Violet-vision bird species and Ultraviolet-vision bird species.

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It is unclear why certain species of bird only had violet-sensitive photo-pigments, whilst others evolved the ability to see in the UV spectrum. Throughout their evolution some species even switched their sensitivity. It’s thought that the specialisation of colour vision had several advantages:

1. Choice of mate.

The ability to attract mates while still evading predators could be one reason. According to a Live Science article, ‘a bird with ultraviolet-sensitive vision might have spectacularly bright plumage in order to impress a female, but that same plumage might appear dull to predator birds that see only in the violet range.’

There are some species where males and females appear similar but light Up spectacularly when seen through UV cameras, such as starlings. With UV vision, the difference is of course very apparent to the birds themselves.

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L: How humans see Starlings. R: How Starlings may see each other – with bolder markings and more colour.

A compelling reason behind the theory that UV vision helps find a mate was the discovery that the famously colourful bills of Atlantic Puffins fluoresce spectacularly under UV light. Also known as ‘Sea Parrots’ and ‘Clowns of Sea’, Puffins actually alter their appearance. Little known is the fact that their beaks and contrasting plumage only last for the duration of the breeding season, whereupon they return to more muted colours and shed their beaks. The Puffin’s bills are key to signalling sexual prowess to other puffins.

L: Puffin during non-breeding season. M: Puffin during breeding season with colourful bill. R: Puffin bill under UV light.

2. Orientation when migrating.

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According to Wikipedia the perception of magnetic fields by migratory birds could be light dependent. Having UV vision may offer some advantage since red light (at the opposite end of our visual spectrum) has been shown to interfere with the magnetic compass in birds.

3. Foraging for food.

Ultraviolet light might also provide higher contrast that makes finding food easier. Since green leaves and foliage absorb UV light, it makes the UV reflective fruits much more conspicuous.

Ultraviolet sensitive vision may be useful for detecting food by either reflecting, absorbing or scattering UV light in relation to its background.4. Looking out for friends and foe.

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Having that extra photo-receptor or two could help give birds the edge when it come to spotting predators, especially those who don’t share their UV sensitive vision. On the flipside, one bird species was found to feed its young based on how much UV the chicks reflected. The larger and healthier the chicks, the more UV they reflected. Chicks that reflected less were given special attention at feeding time.

So what does the world look like to a bird?

An article on the Phys Org Website summarised exactly why this is so hard to recreate for us humans. Firstly, our understanding of bird vision still has a long way to go as ‘most studies are based on the hypothesis that human colour vision can be used to assess what birds see.’ There are several differences between human and avian perceptions of colour, which show that certain shades that can be seen clearly by birds are not perceived at all by humans

‘It’s a bit like a colour blind person describing the colours of clothes – it’s often quite accurate but sometimes it can go badly wrong.’ It is as hard for us to imagine how birds see colour and the world at large due to it being fundamentally outside of our experience.

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However… for the nearest approximation of what birds may see, why not take a look at the artwork created by Claude Monet.

How would it affect us if we had ‘bird vision’?

According to the Color Uncovered app produced for the Exploratorium of San Francisco, a science, art, and perception museum, the story of Monet may provide a clue as to how our feathered friends see the world.

In his later life Claude Monet developed cataracts which left him struggling to see, let alone pursue his painting. At aged 82 he decided to have the lens of his left eye completely removed which enabled him to not only see clearly again, but to see colours he had never seen before. Without his cornea, a natural filter barrier for UV, Monet began to see and paint in ultraviolet. His famous paintings of white water lilies were tinted blue as he captured the UV light reflected off the petals.

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In fact many of his paintings were blue toned due to his newly acquired super vision. It’s not the same as an extra photo-receptor in the UV range, but it does give us insight as to how seeing into the UV spectrum may appear and alter colours.

Another approach for understanding how birds may see each other is to take a look at pictures of Cockatiels and Budgies under UV light. There are numerous amazing photos taken by proud owners revealing hidden markings. A few have even hazarded a guess as to how these creatures may look to each other.

L: Human view of Cockatiel. M: Cockatiel under UV light. R: Simulated bird view with VU light.

Birds are not the only creatures to see into the ultraviolet spectrum. Our planet is populated with countless species of insects, birds and fish all with some degree of UV vision.

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Why not check out my other posts in this series to see what life is like through their eyes:

‘The Amazing World Of UV Vision, As Seen Through The Eyes Of ….Animals’

‘The Amazing World Of UV Vision, As Seen Through The Eyes Of ….Insects’

‘The Amazing World Of UV Vision, As Seen Through The Eyes Of ….Fish’

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