Almut Kelber, Professor at Lund University, takes us on a sweeping tour of the complex array of bird vision – from chickens to owls – to unravel how their ecology affects the way they see the world. This blog is part of our colourful countdown to the holiday season in which we’re celebrating the diversity and beauty of the natural world. Click here to read the rest of the colour countdown series.
Why are some birds, like parrots, so colourful, and other species, such as many birds of prey are not? It all has to do with “seeing and being seen” or “seeing and not being seen”.
So how does the world look through their eyes – would it not be wonderful to see the world through their eyes? These questions have always fascinated me – not only because it would take me into the air, and give me panoramic vision, but also because their world must be so much more colourful than ours. Why is that? Well, birds have an additional colour channel. We humans have an RGB (Red-Green-Blue) system, and the lens in our eye has an ultraviolet filter that helps us protect the sensitive retina from damage. By contrast, birds have RGBU (Red-Green-Blue-Ultraviolet) or RGBV (Red-Green-Blue-Violet). How birds avoid retina damage by ultraviolet light is still a mystery.
The Harris’s hawk to the right has an additional violet channel, but its lens absorbs most ultraviolet, as does our lens, so the difference may not be all that big. The rainbow lorikeet above, however, has the additional ultraviolet channel, and its lens also allows ultraviolet light down to 300 nm to reach its retina and be seen. We know this because researchers have studied the genetic basis of their vision, and we have also measured which light passes the cornea and the lens of many species of birds. For measuring the lenses, we mostly received freshly dead birds that were so severely injured that they could not be saved from a bird rescue station. We measured how much of the ultraviolet and visible light passed through their cornea, their lenses and the entire eye ball and reached the retina. We found major differences in that passerines and parrots have very ultraviolet-transparent eyes, while the lenses of most birds of prey, mallards, or swifts absorb almost all ultraviolet light.
Now how can we know what they see? We have to ask them, and we do that by giving them a task and then a food reward if they can solve it. We do this again and again, making the task more difficult, and curious as they are, birds like this. We have trained chickens, and asked them to choose between two little food containers with food crumbs. We have made the colours more and more similar and thus, could find out how small colour differences they could discern. What we have learned is that they can differentiate between colours just as well as we can. From this we can extrapolate that they can see so many more colours, because they can also perceive the ultraviolet part of the spectrum.
Can we somehow illustrate, visualise bird vision? There are wonderful ways to illustrate how mammals such as dogs and horses see colour, with only two channels. Can we do the same for birds? Well, it is easy to show how the rainbow lorikeet would look if birds had two colour channels only:
Below you see it with an RB, a, RG and a GB system.
We already know how it looks with our RGB eyes. And then we have a problem: with our RGB eyes, we simply cannot see the ultraviolet channel. But just imagine that the light blue feathers on the head are not really light blue, but have an alltogether different colour, blue-ultraviolet. And maybe the yellow feathers are in fact yellow-ultraviolet. I have tried, and have never been able to see it really, maybe you can.
I will stick to the owls, they are the big exception among birds, and have lost the ultraviolet channel in the retina. They can still perceive much of the ultraviolet light as the lens allows it to pass, but they see it just like a brighter blue. They will not miss it as an extra colour, because they are quite dull in colour themselves, and also mostly active in dim light. For owls, the little ultraviolet that they see makes their nocturnal world a little bit brighter, but not more colourful.
For most diurnal birds of prey, it is different, they may still see a more colourful world, the large eagles and vultures also see much more detail, but their lens absorbs almost as much of the ultraviolet as ours.
So while we can get a better understanding of the visual world of birds, exactly how they see, may stay a mystery even in the future. We can still admire their colours with our human eyes – not so bad, compared to most mammals such as dogs, cats and horses.