Saturday, 9 February 2013

Bird Vision (1) - "The Impressionist Osprey"

With the ending of the First World War, exhibitions of art began again in Paris - and none was more eagerly awaited than a new collection of works by the acknowledged master of the impressionist style: Claude Monet.

The subject was no surprise: “Les Nymphéas” - water lilies, which the great man had rendered many times. But the canvases were huge – up to twenty feet long – and when the paintings[1] were revealed, there was consternation among the self-appointed experts:

“Name of a holy dog, what is this that it is?” they muttered (in French) behind their hands, “Has le pauvre Claude totally lost the plot, malheureusement?”

The lily flowers were pink, not white as they should have been. In place of Monet's subtle graduations of hue and tone, the new pictures were a riot of purple and blue, with shimmering traceries of indigo and blinding white in the backgrounds. Many were shocked, but some of Monet's fellow artists wondered if their leader had abandoned his exploration of colour, and was now trying to say something about the very nature of light itself. 

They may have been right...

Humans have trichromatic vision: specialist cone-shaped cells in the retina of the eye respond to different wavelengths of light. The widest response is to red and green, with a narrower response in blue, peaking at around 450nm. Perceived response to very short wavelengths (below 400nm) is almost non-existent in people with normal colour vision, because a pigment in the lens of the eye filters out this “near ultra-violet” light.

Birds are different. 

Many have a fourth type of cone receptor with a peak response around 380nm. (It varies between species.) This means that birds really can see reflected or transmitted ultra-violet light, whereas we only notice materials which are fluorescent under UV – a different phenomenon that can be observed (when sober) in most night clubs. Birds use their enhanced colour vision in many ways: some have plumage which shows clear patterning or bands of UV reflectance, invisible to us. Kestrels can spot the urine trails left by rodents. Hummingbirds use UV patterns to identify their favourite flowers. And there's more.

The ultra-violet components in sunlight transmit more effectively through water than longer wavelengths, and have a different angle of scattering from the surface. The ability to see UV could give ospreys a hunting advantage, because they may be able to use their short-wavelength vision to pick out targets with varying luminance in water at some depth. This attribute of vision, called “contrast sensitivity” would be more important to an osprey than to us, because our food is found in trees. (or in Tescos.) Interestingly, it has also been discovered that the absorbance of UV by water is related to the amount of dissolved oxygen in it.[2] This raises the possibility that hunting ospreys – and many other bird species – may be able to assess the quality of a potential feeding site while flying over it, because the water surface would actually be a different “colour” to them!

Claude Monet would have understood that. During the war, he had discovered that his sight was deteriorating, and underwent surgery for cataracts in both eyes. With his  “UV filter” removed, he saw the world of colour in a different way, and developed a new mode of painting to interpret it.

We can never know exactly how birds perceive their environment, but the idea of an osprey hunting through that ever-changing impressionist landscape of pastel shades and iridescent water is an attractive one. I like it.

[1] Now on permanent display in two purpose-built galleries at the Musée de l'Orangerie in Paris.
[2] "Estimation of biochemical oxygen demand based on dissolved organic carbon, UV absorption and fluorescence measurements." Kwang et al, J.Chem, v2013

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