Written by Ben Coley
For the majority of the animal kingdom, by far the most important sense is smell. Even in humans, this sense exhibits some of the strongest memories and there are numerous examples of humans unconsciously being affected by pheromones, hormones and the information contained within. There is one group of animals, however, that have little use for smell as a rule: birds.
The software and hardware necessary to facilitate and interpret smell comes at a price. Complex organs used to produce and interpret these messages mean taking up vital space and ultimately weight. For birds, where most rely on aerodynamics and weight-saving adaptations for survival, these additional requirements make no sense. Instead of smell, birds have evolved some of the most advanced eyes and the accompanying visual capabilities known in the natural world. Some vultures, for example, are reputed to be able to read the equivalent of a newspaper from one kilometre up!
If one looks at a standard bird, its eyes do not necessarily seem overly large to us, but strip away the feathers, eyelids and skin and one quickly sees that the head basically houses the eyes and little else, including the brain! The term ‘bird brain’ then is particularly apt when describing a person of lesser intelligence.
It is not only the sheer size and magnitude of the eye that sets birds apart from other animals. They possess a whole variety of adaptations that allow them to exploit their acuity to levels well beyond our own.
Human eyes contain around 200,000/mm² of photosensitive cells used for interpreting information. Many raptors possess in excess of a million of these cells, and can therefore see far more detail that we can ever dream of. It also allows them to be far more specific in their colour recognition. We would find it very difficult to see the difference between different shades of the same colour, but to birds, the distinction would be no problem at all. This plays an important role in key areas such as prey identification, species recognition and mate selection.
In most eyes, there are three main colour receptive cells – red, blue and green – much like those used to formulate the image you see on your computer screen or television. Many birds, however, possess a fourth group of cells that allows them to see in the ultraviolet spectrum. This means that the range of colours and hues visible to them is far greater.
Here are some examples of how birds benefit from this:
• Many fruits exhibit waxy coatings which make finding food for birds easier in thick undergrowth.
• A large proportion of flowers glow under UV light, forming directional cues as to where to find the sweet nectar.
• Some birds are able to watch for urine trails of their prey just like black lights are used in forensic investigations to identify body fluids invisible to our naked eye.
• Bird plumage also reacts differently to UV light and therefore makes finding a mate easier.
• Seeing in the UV spectrum can also aid in navigation, especially for those birds who migrate.
The fovea is the spot at the back of the retina upon which an image is focused by the lens. Not only does a bird’s fovea have more visual cells, but in some species a second fovea is present. This means that a bird can focus on objects close to them and far away simultaneously! Birds of prey can hone in on prey spotted in the distance whilst safely manoeuvring through vegetation without losing focus on its quarry.
The pecten oculi is a comb-like organ found in birds’ eyes that supplies additional blood flow to the retina. More oxygenated blood means more efficiency. The pecten also concentrates the blood vessels in one place, reducing the number of vessels on the surface of the retina itself which can cause visual distortion. Make a small circle with your thumb and index finger and hold it close to your eye. Then look at a bright light source (avoid looking at the sun, please) and jiggle your hand so that the image you see quickly flips between light and dark. You will soon see lines in your vision, and these are the blood vessels in your eye.
For a bird that is negotiating thick vegetation while flying through light and shadow at high speed, any interference in its visual acuity could end up with it crashing unceremoniously into a tree! The pecten oculi negates this distortion and allows for high speed pursuit and manoeuvrability in heavily vegetated habitats.
Birds’ eyes contain special oil droplets that not only act as a sunscreen to protect the eyes from UV rays, but also help filter certain colours depending on its lifestyle. Different colour oils filter out certain colours of light, making the eyes more sensitive to particular colours. Seed eaters for example might have an oil to enhance reds, oranges and yellows, whilst a fish-eating bird would have oils to increase their ability to see greens and blues.
Flight is a wonderful adaptation, but it comes with pitfalls. Flying through vegetation or urban environments at high speeds can be a perilous exercise. It requires constant tiny adjustments and continuous reassessments of stimuli to ensure safe passage and success. These are just some of a plethora of adaptations that have allowed birds to become a highly successful class of animals.