Ionce dreamed of a kiss that hadn’t yet happened. I dreamed the angle at which our heads tilted, the fit of my fingers behind her ear, the exact pressure exerted on the lips by this transfer of trust and tenderness.

Sigmund Freud, who catalysed the study of dreams with his foundational 1899 treatise, would have discounted this as a mere chimera of the wishful unconscious. But what we have since discovered about the mind — particularly about the dream-rich sleep state of rapid-eye movement, or REM, unknown in Freud’s day — suggests another possibility for the adaptive function of these parallel lives in the night.

One morning not long after the dream, I watched a young night heron sleep on a naked branch over the pond in New York City’s Brooklyn Bridge Park, head folded into chest, and found myself wondering whether birds dream.

In 1861, just two years after Charles Darwin’s On the Origin of Species, a fossil was discovered in Germany with the tail and jaws of a reptile and the wings and wishbone of a bird, sparking the revelation that birds had evolved from dinosaurs. We have since learned that, although birds and humans haven’t shared a common ancestor in over 300 million years, a bird’s brain is much more similar to ours than to a reptile’s. The neuron density of its forebrain — the region engaged with planning, sensory processing and emotional responses, and on which REM sleep is largely dependent — is comparable to that of primates. At the cellular level, a songbird’s brain has a structure — the dorsal ventricular ridge, or DVR — similar to the mammalian neocortex in function if not shape. (In pigeons and barn owls, the DVR is structured like the human neocortex, with both horizontal and vertical neural circuitry.)

Still, avian brains are capable of feats unima- ginable to us, especially during sleep: many birds sleep with one eye open, even during flight. Migrating species that traverse
immense distances at night — the bar-tailed godwit covers 7,000 miles between Alaska and New Zealand in eight days of continuous flight — engage in unihemispheric sleep, blurring the line between our standard categories of sleep and wakefulness.

But while sleep is an observable physical behaviour, dreaming is an invisible interior 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, or EEG, of electrical activity in the human brain was recorded in 1924, but EEG was not applied to the study of avian sleep until the 21st century, aided by the even more nascent functional magnetic resonance imaging, or fMRI, developed in the 1990s. The two technologies complement each other. In recording the electrical activity of large populations of neurons near the cortical surface, EEG tracks what neurons do more directly. But fMRI can pinpoint the location of brain activity more precisely through oxygen levels in the blood. Scientists have used these technologies together to study the firing patterns of cells during REM sleep in an effort to deduce the content of dreams.

A study of zebra finches, songbirds whose repertoire is learned, mapped parti- cular notes of melodies sung in the daytime to neurons firing in the forebrain. Then, during REM, the neurons fired in a similar order: the birds appeared to be rehearsing the songs in their dreams.

An fMRI study of pigeons found that brain regions tasked with visual processing and spatial navigation were active during REM, as were regions responsible for wing action, even though the birds were stilled with sleep: they appeared to be dreaming of flying. The amygdala — responsible for emotional regulation — was also active during REM, hinting at dreams laced with feeling. My night heron was probably dreaming, too — the folded neck is a classic marker of atonia, the loss of muscle tone characteristic of the REM state.

But the most haunting intimation of the research on avian sleep is that without the dreams of birds, we too might be dreamless. No heron, no kiss.

There are two primary groups of living birds: the flightless Palaeognathae, including the ostrich and the kiwi, which have retained certain ancestral reptilian traits, and Neognathae, comprising all other birds. EEG studies of sleeping ostriches have found REM-like activity in the brainstem — a more ancient part of the brain — while in modern birds, as in mammals, this REM-like activity takes place primarily in the more recently developed forebrain.

Studies of sleeping monotremes — egg-laying mammals like the platypus and echidna, the evolutionary link between us and birds — also reveal REM-like activity in the brainstem, suggesting this was the ancestral crucible of REM before it migrated toward the forebrain.

If so, the bird brain might be where evolution designed dreams — that secret chamber adjacent to our waking consciousness where we continue to work on the problems that occupy our days.

It may be that in REM, this gloaming between waking consciousness and the unconscious, we practise the possible into the real. It may be that the kiss in
my dream was not nocturnal fantasy but, like the heron’s dreams of flying, the practice of possibility. It may be that we evolved to dream ourselves into reality — a laboratory of consciousness that began in the bird brain.

NYTNS