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.
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.
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.
Vision in Birds:
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.
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.
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).
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.
Hearing in owls:
• Barn owls for example
Air velocity detectors
By Rob Nelson