A Bird's Brain And Nerves
Birds have a similar basic plan to their nervous system as the rest of the vertebrates. The central nervous system is made up of a brain, a spinal chord and nerves. The brain is situated inside the skull and connected to one end of the spinal chord which as its name suggests runs down the centre of the spinal column. The nerves are bundles of neurones - a neurone is a single cell which transmits a micro-electrical pulse from one end of itself to the other. Neurones come in two sorts, sensory and motor. Sensory neurones are little alarms triggered by a variety of sense organs; eyes, ears, bristles, etc. They carry the messages to the brain that the bird uses to build up and maintain its image of the world. Motor neurones transmit messages the other way - from the brain to the muscles.
The brain of a bird weighs about 10 times as much as a brain of a reptile of the same weight, but slightly less than that of a mammal of the same weight. However, there is considerable variation between birds of similar size. For birds weighing in at around 85g, brain weight varies from 0.73g for a Quail to 2.7g for a Great Spotted Woodpecker. There is therefore quite a range in the intelligence of birds, with game birds at the bottom of the list and Woodpeckers, Owls and Parrots at the top.
A bird's brain is different to a mammalian brain in that the complex folds found in the cerebral cortex of mammals are missing and the cerebral cortex itself is much smaller proportionally than in mammals. Instead the corpora striata, a more basic part of the cerebral hemispheres is proportionally larger and better developed. It is this portion of a bird's brain which is used to control instinctive behaviour - feeding, flying, reproduction etc. The mid-brain is also well developed as this is the part of the brain primarily concerned with sight, while the olfactory lobes are reduced as would be expected given that bird's in general have little use of the sense of smell.
The bird's skull is mostly occupied by eyes and the brain has to make do with what space it can find in a rather narrow cranium.
The brain contacts most of the body through the spinal column/chord with which it forms the central nervous system. Birds normally have 38 pairs of spinal nerves radiating out to the body along the spinal chord, a number of these are grouped in small bundles called plexi, (i.e.) the brachial plexus, which act as regional headquarters maintaining and controlling some actions with minimal input from the brain.
Birds also have what is called an autonomic nervous system, which as in mammals and reptiles controls such essential actions as heartbeat, breathing and digestion. This can be divided into two sections; the sympathetic nervous system and the parasympathetic system.
The sympathetic nervous system works in harmony with the endocrine system and the release of adrenaline and noradrenaline to stimulate a rapid response to danger. This is often called the 'fight or flight' reflex as it determines when a bird decides to make a rapid exit from the awareness of a predator.
The parasympathetic system is made up of a series of groups of ganglia situated near various important organs such as the heart, lungs and digestive organs. These it controls and regulates with only occasional input from the brain.