Coos and Snooze: Scientists Decode Pigeons’ Dreams

The boundless and bizarre content of human dreams has attracted the interest of biological researchers for decades. A new study by German scientists shows that pigeons dream too. The team used a technique called functional magnetic resonance imaging (fMRI) to look at bird brain areas that lit up during sleep. The imaging data suggested that they may be dreaming of flight.

The study was published in Nature Communications.

Napping and flapping

The refreshed feeling that we have upon waking up after a good night’s sleep comes from a set of biological changes in our brains that occur while we rest. Mammalian sleep has multiple phases, including rapid eye movement (REM) and non-REM sleep. The brain switches modes when we enter REM sleep by increasing activity. This heightened state might explain why our dreams during this period are full of vivid emotions and experiences.

In non-REM sleep, the brain takes things down a notch, lowering its metabolism and taking the chance to empty itself of waste products through its glymphatic system, where cerebrospinal fluid (CSF) in the brain is washed out through compartments surrounding the brain called ventricles.

Whether all these processes also occur in birds has remained unclear. Part of the reason to think they might is due to sleep’s ancient origins. “The last common evolutionary ancestor of birds and mammals dates back about 315 million years, to the early days of land vertebrates,” says Professor Onur Güntürkün, head of the Biopsychology Department at Ruhr University Bochum and a study co-author.

“Yet the sleep patterns in birds are remarkably similar to those in mammals, including both REM and non-REM phases.”

To dive deeper into bird dreams, the scientists raised a cohort of 15 pigeons who had been trained to sleep comfortably while being monitored with infrared video cameras and fMRI.

Dove dreams

This system allowed the researchers to track the birds’ eye movements to assess sleep state.  “We were able to observe whether one or both eyes were open or closed, and to track eye movements and changes in pupil size through the pigeons’ transparent eyelids during sleep,” explains Mehdi Behroozi, another paper author and researcher at Ruhr University Bochum. They were able to pair this data with fMRI data indicating brain activity and CSF flow.     

“During REM sleep, we observed strong activity in brain regions responsible for visual processing, including in those areas that analyze the movement of a pigeon’s surroundings during flight,” says Behroozi. The team also noticed activity in the areas that process signals from the body, especially from the wings. “Based on these observations, we think that birds, just like humans, dream during REM sleep, and might be experiencing flight in their dreams,” Behroozi adds.

Interestingly, the amygdala – the emotional processing center of the brain – was also seen to light up while the birds were in REM sleep, which indicates that the birds may, like humans, be feeling emotions during these dream bouts. This hypothesis is reinforced by the rapid contraction of the birds' pupils during REM sleep, a behavior noted during courtship or aggressive behaviors when awake. This data was only obtained after building off of previous studies, which showed that bird eye movements are informative but opposite to that of mammals during sleep.

Inside a bird brain

One key difference between sleeping bird and human brains was that, while our CSF flow peaks during non-REM sleep but remains relatively high during other phases, bird brains dramatically cut off the CSF taps during REM sleep. This is a finding that has not been previously recorded in sleep say the authors.

“We think that the increased flood of blood into the brain during REM sleep, which supports the elevated brain activity, might block the CSF from moving from the ventricles into the brain,” explains Niels Rattenborg, head of the Avian Sleep Group at the Max Planck Institute. “This suggests that REM sleep and its functions might come at the expense of waste removal from the brain.”

This finding was surprising to the team because avian brains are even more densely packed with neurons than mammalian brains – when do they get the chance to clear out? The birds went through more frequent and shorter REM/non-REM cycles than humans. Rattenborg’s colleague Gianina Ungurean suggests that these quick switches may have a deeper benefit to brain clearance:

“At the onset of REM sleep, the influx of blood increases vessel diameter. This might force CSF that entered the space surrounding the vessels during non-REM sleep to flow into the brain tissue, and enhance the outflow of fluids carrying waste products.”

Dream messengers could advance research

To take this study to the next step – accurately detailing exactly what pigeons dream about – the team hopes to teach their birds some Inception-level recall. “We hope to train birds to report if and what they just saw upon awakening from REM sleep. That would be an essential step towards establishing whether they dream,” says Ungurean.

Reference: Ungurean G, Behroozi M, Böger L, et al. Wide-spread brain activation and reduced CSF flow during avian REM sleep. Nat Commun. 2023;14(1):3259. doi:10.1038/s41467-023-38669-1

This article is a rework of a press release issued by the Max Planck Institutes. Material has been edited for length and content.