Budgie Vision (4)

I would like to add something to the budgerigar vision series: the simple fact that budgies have monocular vision. Basically, this means they can use each eye independently!

Because of monocular vision, budgerigars have a wide field of view. Photo © nkj.ru

By using their eyes this way, their field of view is increased, as opposed by binocular vision where you use both your eyes at the same time. In case you tried to use both your eyes separately, you will probably know that humans have binocular vision ;)

A budgerigar's eyes are positioned on opposite sides of their head, giving it the ability to see two objects at once - or two predators, coming from different angles. This is a big advantage in the wild.

Other posts in this series:
Budgie Vision (3)
Budgie Vision (2)
Budgie Vision (1)

Budgie Vision (3)

Magnetic Field

Migratory birds use the Earth's magnetic field for orientation. Some studies have suggested that the magnetic field is light dependant and birds are able to "see" it. During their migration through Australia, budgerigars are able to fly miles and miles in the correct direction, as they always seem to go "straight" to a waterhole or a food source. Budgerigars are probably able to see the magnetic field as well, but this has not been proven yet. Or do they use another technique we don't know about?

Image © cairns.ru

Edge detection

A lot of scientific effort has been devoted to studying how migratory birds navigate over long distances, but relatively little is known about how birds detect edges and avoid obstacles during landing.
A study in Australia has has investigated landing in budgerigars, by training them to fly from a perch to a feeder, and video-filming their landings.  

Image © greencheek on Deviantart

Description of the experiment:

"The feeder was placed on a grey disc that produced a contrasting edge against a uniformly blue background. We found that the budgies tended to land at the edge of the disc and walk to the feeder, even though the feeder was in the middle of the disc. This suggests that the birds were using the visual contrast at the boundary of the disc to target their landings.

When the grey level of the disc was varied systematically, whilst keeping the blue background constant, there was one intermediate grey level at which the budgerigar's preference for the disc boundary disappeared. The budgerigars then landed randomly all over the test surface. Even though this disc is (for humans) clearly distinguishable from the blue background, it offers very little contrast against the background, in the red and green regions of the spectrum.

In short:

Budgies rely on visual features in the environment, such as contrasting edges, to determine where they will land. Colours don't play a big part in this. The key to a successful landing is a clear contrast between the boundary (the edge) and the rest of the landing platform. That is how they do it! If the contrast is unclear, budgies may detect too many "edges" and not immediately find a perfect landing spot.

Copyright Bhagavatula P, Claudianos C, Ibbotson M, Srinivasan M (2009). "Edge Detection in Landing Budgerigars (Melopsittacus undulatus)"

Budgie Vision (2)

Light perception

Budgies are able to see ultraviolet light, which is involved in courtship. The plumage patterns in ultraviolet are invisible to the human eye, unless you place your budgie under a UV lamp.

Image © Mark Neville

A UV receptor also may give wild budgerigars an advantage in foraging food. Although they feed mainly on grass seeds, they also eat fruit. The waxy surfaces of many small fruits and berries reflect UV light that might advertise their presence. A quick and easy meal for a budgie!

Flicker tresholds

Budgies can resolve rapid movement much better than humans, for whom flickering at a rate greater than 50 Hz appears as continuous movement. Thus, humans are unable to distinguish individual flashes of a fluorescent light bulb oscillating at 60 Hz, but no more than that. Budgies have flicker tresholds of more than a 100 Hz!

Image © volnistij-gorod.ru

Here's another breathtaking fact: budgies are able to detect slow moving objects. The movement of the sun and the constellations across the sky is imperceptible to humans. We are only capable of saying that the sun has gone a bit down or up, compared to half an hour or an hour ago. Budgerigars can actually detect this movement while it is happening! This amazing ability allows budgies to properly orient themselves in the wild.

Budgie Vision (1)

Vision is the most important sense for budgerigars. Good eyesight is essential for safe flight.

Budgies have got their eyes on the side of their heads, which provides them a wide visual field, useful for detecting predators. Birds that have eyes on the front of their heads, such as owls and eagles, have binocular vision, which is in turn useful for estimating distances while for example hunting.



Images © Wikipedia

While humans have only two eyelids, budgerigars have a third concealed eyelid that is called the nictitating membrane. This thin and transparent "eyelid" sweeps across the eye like a windscreen wiper, acting as a lubricating duct - equivalent to our tear duct. The nictitating membrane also covers the eye and acts as a kind of contact lens in many aquatic birds when they dive underwater. When your budgie is asleep, the lower eyelid rises to cover the eye completely.

If you look hard enough, you will be able to see the nictitating membrane on the picture below. It's quite hard to see in general, as your budgie often moves it with lightning speed. I have seen it in one of my budgies, Wit Zingbeest. Back then, I didn't know why he suddenly covered his eye with a transparent layer. I thought he was tired of me talking to him, but still showing his eye somehow to indicate that he was listening :-)

Image © Birds Online