Nervous Systems: Olfaction, Optics, and Hearing

In this section we look at the complex nervous systems of birds. This topic deals with bird intellegence, memory, vision, and hearing.

To start off this topic, lets imagine a hypothetical experiment with a captive dove. The researchers prepare a small apparatus that will present food to the dove if a certain button is pushed. How easily do you hypothisize the bird will learn how to attain the food? Its a classic experiment. To answer this question and to understand bird brains we must learn a bit more about how well the nervous system of birds is wired.

Bird brains

Birds have a three part brain composed of a section for olfaction, optics, and hearing. The relative proportions of each of these sections vary with the ecology of the bird. For example, birds such as vultures and falcons, whom detect low levels of methane gas, have a large olfactory section to their brains.

The cerebrum of the brain is the front part of the brain. It contains the cerebral cortex. In birds, more so than most animals, there seems to be a lot more cerebral tissue.

The hyperstriatum of the brain is a region that is cladisticly new in birds. This section of the brain is associated with intellegence. Smarter birds have a larger hyperstriatum. As a comparison, intellegence in mammal brains is associated with the cerebral cortex.

Birds that sing have a 'song control nuclei' also called the song control region. This region of the brain integrates with the main parts of the brain and ultimately controls the muscles. Migratory birds whose breeding season corresponds with their vocal activity have developmental plasticity in this section of the brain. In other words the song control region grows in the breeding season and shrinks in the off season.

Female birds have song control centers just like male birds do. They can have the wiring, but in general it’s the atrophy in the song-control center that causes them not to sing.

Some birds, like parrots, have the ability to mimic human voices and give the impression of communication. One African gray parrot named Alex is reputed to be able to engage in conversations. While it may be just a form of repeating sounds on cue, the form of communication is fascinating. Alex is even used as a communication tutor for other African gray parrots.

Memory:
Many birds have incredible memories. Nutcrackers, for instance, can catch 2,000 seeds in an area. They then feed on their stashed seeds in the winter. They nearly never make a mistake when it comes to remembering where they stored the seeds

The part of the brain that deals with memory is the hippocampus. The hippocampus is found in the cortex of the brain. Birds that have large spatial memory have a large hippocampus.

Medulla
The medulla is a section of the brain that birds used primarly in hearing. Take owls as a case study. Dark hunting owls who rely on their keen hearing have about 47,000 neurons in the medulla. Light hunting owls who rely on sight only, have about 11,200 cells in the medulla.

Vision in Birds:
As a general rule, birds have incredable visual capabilities. One measure of vision is acuity. Acutiy is the abilty of a bird to resolve fine detail at a distance. Compared to humans, birds have 2.5 to 3 times our acuity at long distance.

Bird eyes are also relatively large. They are on average 15 percent the mass of the bird head. Human eyes are ony 2 percent. The eyes of birds are locate on the side of their heads. Because of their relative positions, head bobbing, a behavior commonly seen in birds, is used to help them with depth perception.

To protect the birds eyes they have a nictitating membrane. These membranes help keep the eyes moist. In many ways it works the same as our eyelids. However, it is important to realize that these membranes are the same as our eyelids. The apparatus is different even though the function is the same. In fact, we have a vestigial nictitating membrane in the side of the head.

Physiological differences in birds also include bony osicles that surround hte eye. These osicles are also found in dinosaurs.

Birds also differ from us in the way that they focus images through the lens of the eye. Instead of fucusing by changing the shape of the lens (as humans do), birds change both the shape of the lense and the shape of the cornea.

Pectin is present in the bird eye. There are about thirty theories that try to explain this. No one is exactly sure what pectin is for. Possibly it is an avian novelty. Avian retinas do not have blood vessels in them like mammalian eyes do. Yet, the pectin is highly vascularized.

Cones
Cones are the daylight receptors. In the human eye there are about 200,000 cone cells per square millimeter. Birds have between 400,000 and 1,000,000 cells per square millimeter.

The fovea is an area of the eye with a considerable density of cone cells. There are four types of cone cells: Near UV, green, yellow and red. Mammals only have three of these cone cell types. The spacing of the four different types of cone cells in birds are of equal distance. Over the top of the cone cells birds have three types of oil droplets (green / yellow / red).

Position
As a general rule, birds have their eyes positioned on the side of their head. This means that at certain distances each eye will see a different image. Some birds can alter this typical patter though.

The yellow billed cuckoo can converge its eyes by moving them side to front. Bitterns have their eyes lower on their head to see bellow them. Binocular vision on in birds occurs when they have the eyes aimed forward. This gives them crossovers in images from both eyes. This cross-over allows them to deal with depth perception.

Fovia
Most birds that have eyes on the side of the head have only one fovea. Fast flying biers have a temporal fovia. This temporal fovia allows them to see light from in front of them through each eye. This in turn helps with depth perception. Birds without this have to turn their head to get the same light in both eyes.

Hearing
While birds can see much better than humans, they can often not hear as well. Part of the reason for this is that birds have less area of cochlea. Birds do have a tympanic membrane close to the surface of the ear. They also have a columella to the inner ear where the cochlea is. This bone puts movement into the fluids in the cochlea which is then detected by hairs in this region.

Hearing in owls:

• Barn owls for example
• Their eyes are at the same level, but the ears and plumage on both sides of the head are unique.
• Spatial asymmetry is very rare in nature. In fact the ear opens at two different heights of both sides of its head.
• Nucleus laminarus detects the differences of arrival of different neurons from the nucleus magnocellaris. Neurons on the same side as the magnocellaris of one side get to the laminarus at different speeds. Thus they can use their asymmetric openings, combined with a calculator in the medulla to get the distance.
Oil-birds can echo-locate in northern south America. They eat fruit.

Mechanoreception
• similar to hair-cells called herps cells.
• Basically the ending of a nerve surrounded by some cup
• As the cup moves (like with a feather), it detects pressure.
• Allows them to know where their feathers are etc.

Air velocity detectors
* Some seabirds are thought to have specialized air-stream pockets in their nasal passages.

By Rob Nelson


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