In the 19th century, when the German naturalist Ludwig Edinger conducted the first anatomical studies of avian brains and discovered the absence of the neocortex—the most evolutionarily nascent outer layer of the brain responsible for complex cognition and creative problem solving—he dismissed birds as little more than Cartesian reflex puppets. This view was strengthened in the 20th century by the deviation, led by BF Skinner and his pigeons, to behaviorism—a school of thought that viewed behavior as a Rube Goldberg stimulus-response machine governed by reflexes, indifferent to internal mental states and the emotional response.
In 1861, just two years after Darwin published On the Origin of Species, a fossil with the tail and jaws of a reptile and the wings and pincers of a bird was discovered in Germany, sparking the revelation that birds had evolved from dinosaurs. We have since learned that, although birds and humans have not shared a common ancestor for more than 300 million years, a bird’s brain is much more like ours than a reptile’s. The neuron density of his forebrain – the area involved with planning, sensory processing and emotional responses, and on which REM sleep largely depends – is comparable to that of primates. At the cellular level, a songbird’s brain has a structure, the dorsoventral ridge, similar in function to, if not more than, the mammalian neocortex. (In pigeons and barn owls, the DVR is structured like the human neocortex, with horizontal and vertical neural circuitry.)
But bird brains are also profoundly different, capable of feats unimaginable to us, especially during sleep: Many birds sleep with one eye open, even in flight. Migratory species that travel vast distances at night, such as the bar-tailed godwit, which covers the 7,000 miles between Alaska and New Zealand in eight days of continuous flight, engage in monocular sleep, blurring the line between the typical categories of sleep and wakefulness.
But while sleep is an outwardly observable physical behavior, dreaming is an invisible inner experience as mysterious as love—a mystery to which science has brought brain imaging technology to illuminate the inner landscape of the sleeping bird’s mind.
The first electroencephalogram of electrical activity in the human brain was recorded in 1924, but EEG was applied to the study of bird sleep well into the 21st century, aided by the still nascent functional magnetic resonance imaging, developed in the 1990s. The two technologies complement each other. . By recording the electrical activity of large populations of neurons near the surface of the cortex, EEG tracks what the neurons are doing more directly. But fM.RI can pinpoint the location of brain activity more precisely through blood oxygen levels. Scientists have used these technologies together to study the firing patterns of cells during REM sleep in an attempt to infer the content of dreams.