In Brief:

• Brain plasticity: The brains of canary females maintain the ability to sing throughout their lives, even though the birds do not usually sing, shining light on how neural circuits can stay dormant yet functional.

• Advanced techniques: Using a combination of advanced microscopy and spatial gene expression patterns, researchers found that a brain region important for singing (the “HVC”) does not to physically grow for birds to produce songs. Instead, under the influence of hormones, existing neurons change their behavior, becoming more or less active, strengthening or weakening their connections, and altering their gene expression patterns.

The human brain, once thought to lose much of its flexibility after childhood, continues remodeling itself throughout life – recovering from injuries, learning new skills, and adapting to challenges – a remarkable capacity for change called neuroplasticity. Songbirds show similar neuroplasticity on a seasonal basis, with changes in their brain enabling elaborate songs during breeding seasons, and to recover full vocal repertoires even after years of reduced singing. This raises a fundamental question: how does the brain preserve these complex abilities when they are not being actively used?

A new study from the Max Planck Institute for Biological Intelligence tackles this by focusing on female canaries, which do not normally sing but nevertheless contain the full neural machinery for song. Like humans have specialized brain regions for language, songbirds evolved these specialized regions for song – and singing ability in canaries is partly inherited and partly learned through practice. By giving females testosterone – a hormone that rises in males during breeding season – the team could watch what happens over several weeks as this dormant singing ability switches on and the females fine-tuned their newly found voices.

Using traditional imaging methods, scientists had previously observed the HVC – a key brain region for song – appearing much larger in the images of singing as compared to non-singing canaries, e.g. during breeding season. However, by tracking individual brain cells in the HVC, a combination of advanced microscopy and studies of gene expression revealed something different. Neurons didn’t move apart or multiply as it might have seemed – instead, they were becoming more active strengthening connections, ramping up activity, and shifting gene expression, and easier to detect, creating an illusionary growth in the anatomical images.

“Rather than spacing out to expand the brain region, gene mapping showed that testosterone orchestrates these changes throughout the HVC without changing its size”, says Shouwen Ma, researcher of the Department of Behavioural Neurobiology at the Max Planck Institute for Biological Intelligence in Seewiesen. “Crucially, this means that the HVC maintains its size and neural architecture even when not in use, allowing birds to regain complex singing abilities even after years without singing. What is so exciting for me about this discovery is that it’s a great illustration of how the brain doesn’t need to rebuild structures from scratch: The neural machinery stays ready, just waiting to be switched on and fine-tuned.”

The capacity for song proved remarkably persistent – the researchers even induced singing in seven-year-old females, well beyond their typical lifespan in the wild.

“This project has been a significant and cumulative effort for the department. We have refined genomic tools to assess the molecular basis of singing behavior of canaries at high resolution. This ranges from genome sequencing to sophisticated bioinformatic methods that visualize gene expression to reveal brain architecture. This diversification of knowledge lays the foundation for new research areas, such as understanding how species cope with different hormonal systems,” says Carolina Frankl-Vilches, a researcher in the Department of Behavioural Neurobiology.

“By studying how individual cells change their behavior, we’re uncovering fundamental principles of brain plasticity and how brains retain capacity for innovative changes throughout life,” says Manfred Gahr, head of the department. “Understanding these mechanisms – how hormones trigger changes in brain cells, how neural circuits preserve their structure and flexibility – could shed light on the broader principles in brain plasticity and therefore broader questions such as how aging brains maintain their capacity to adjust and better understand the mechanisms to promote recovery after stroke or injury.”

Shouwen Ma, Carolina Frankl-Vilches, Manfred Gahr; Invariant HVC size in female canaries singing under testosterone: Unlocking function through neural differentiation, not growth. PNAS, online 20 October 2025 Link

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The pupil not only regulates the amount of light entering the eye, but also reflects the state of the brain: when we are aroused or focused, the pupils dilate. The pupil even changes as mammals sleep, constricting during deep non-REM sleep and dilating in REM sleep. Now, for the first time, researchers at the Max Planck Institute for Ornithology and the Neurosciences Research Center in Lyon have studied pupil behavior in birds. Surprisingly, the exact opposite happens here: During arousal and REM sleep, the pupils become smaller, while they enlarge during non-REM sleep. The unexpected pupillary behavior of birds provides a new window into the sleeping brain.

The pupil literally plays a central role in vision: it is the opening in the middle of the eye that regulates how much light falls on the retina. If it is dark, the pupil becomes large and lets a lot of light through – if it is bright, it becomes small and prevents us from being blinded. However, it not only regulates the incidence of light, but also reflects emotions and the state of the brain. If we are excited or concentrated, for example, the pupils enlarge – without us being able to influence it.

In birds, pupil sizes have never been systematically studied. Yet, rapid pupil constrictions are a well-known feature of bird body language among parrot owners. A team led by Gianina Ungurean from Niels Rattenborg’s research group at the Max Planck Institute for Ornithology in Seewiesen now studied pupil behavior in pigeons. Surprisingly, the pupils of male pigeons became smaller during courtship – in contrast to mammals, in which the pupils dilate during arousal.

Recently, it was shown in mice that pupil size also changes during sleep: In deep non-REM sleep with quiet behavior, pupils constrict; in more active REM sleep, they can slowly dilate. The team therefore next examined the pupil behavior of sleeping birds. The transparent eyelids of pigeons and a special camera system allowed the scientists to record pupil sizes even with the birds’ eyes closed and in the dark. Again, the birds’ pupils did the opposite to mammals: they dilated during non-REM sleep, but during REM sleep, the pupils rapidly constricted over 1,000 times a night, a phenomenon the researchers called rapid iris movements.

The researchers were able to observe the pigeons’ pupil behavior also in a cuckoo species, suggesting that it is a general trait among birds. “These results showed us that basic behaviors such as pupil size can be completely opposite in different animal groups”, explains Niels Rattenborg, leader of the study. “Examining why this difference exists might cause us to reconsider our understanding of pupillary behavior, in general”

In birds and mammals, pupil size is regulated by different types of muscles in the iris. Mammals use smooth muscles, which cannot be controlled voluntarily. Birds, on the other hand, regulate the pupil by striated muscles, like those that move our arms or legs, for example. The researchers were able to confirm this structural aspect as a possible cause for the different pupil behavior: If they used a drug to silence receptors that activate striated muscle cells, the pigeons no longer constricted their pupils during sleep.

“Pupil constrictions in sleeping birds probably fulfill important functions,” Gianina Ungurean explains. “The constriction could exercise the muscle’s fine motor skills. But perhaps it’s also a sign that brain regions activated during aroused states are active again during REM sleep, possibly when memories are replayed.” In this regard, pupil constrictions during REM sleep could help to determine when and how the sleeping brain processes memories.

The fact that birds regulate their pupil size by striated muscles, and that these are usually controlled voluntarily, opens up an exciting question: Can birds change the size of their pupils intentionally? Future studies will now investigate this question and the pupils’ role in communication between birds.

Budapest pigeon:

Video recording of a Budapest pigeon transitioning from non-REM to REM sleep with rapid iris movements (RIMs).

Guira cuckoo:

Video recording of a guira cuckoo transitioning from a period with dilated pupils associated with behavioral quiescence (presumed non-REM sleep to a period with rapid iris movements (RIMs) associated with active behaviours (presumed REM sleep).

Gianina Ungurean, Dolores Martinez-Gonzalez, Bertrand Massot, Paul-Antoine Libourel, Niels Christian Rattenborg. (2021). Pupillary behavior during wakefulness, non-REM sleep, and REM sleep in birds is opposite that of mammals. Current Biology 31, 1–7.
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Benichov JI, Vallentin D. (2020). Inhibition within a premotor circuit controls the timing of vocal turn-taking in zebra finches. Nat Commun. 11(1):221.
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Zebra finches can take turns when exchanging short calls with one another, much like humans do during conversations. Researchers from the Max Planck Institute for Ornithology have now shed light on the underlying neural dynamics in a songbird brain area called HVC, or the so called “song control centre”. The researchers identified neurons involved in initiating a call as well as another class of neurons that inhibits neighboring cells shortly before a bird calls. The importance of these cells for vocal turn-taking was further supported by the observation that specific disruptions to the signaling of either type of neuron resulted in distinct changes in call timing.

Imagine if every time you started a conversation, the other person would speak as soon as you began talking. Not only would this be considered rude, it would be difficult to hear each other and the interaction would not get very far. In our everyday conversations we rely on our ability to take turns and to avoid speaking at the same time. We do this so effortlessly that we often take it for granted. This feat involves withholding or delaying our responses when we hear another person speaking and initiating our speech once they’ve stopped. In other words, what you hear does not only influence what you say but also when you say it.

Other animals perform similar back-and-forth vocal exchanges. However, the underlying neural mechanisms of this vocal turn taking were so far unknown. Therefore, Jonathan Benichov and Daniela Vallentin from the Max Planck Institute for Ornithology started a study with zebra finches, small Australian songbirds, to find out how they coordinate their simple short calls during interactions. It has been shown that a forebrain cortical region called HVC is centrally important for song learning and production. “We thought that this specialized area in the zebra finch brain is a good place to look for an explanation of how vocal turn-taking is achieved”, says Jonathan Benichov, first author of the study. This area is known to act like a metronome to control the tempo of the elaborate courtship song that males learn from a tutor as they grow up. Benichov and Vallentin found that neurons in this area are also important for controlling the timing of the simpler short calls used in ordinary social interactions. In this case the area can act like a traffic light that determines when to call and when to wait.

The researchers recorded from individual nerve cells in this brain area using electrodes that are 1/50th the width of a human hair. This is done with a lightweight electronic device that the bird can wear on its head while it moves around, sings, and interacts with another bird or with recordings of another bird’s calls. With this, they found that some brain cells in HVC were active a few milliseconds before a bird actually calls, hinting that these neurons are involved in initiating a call. They also found similar cells that had their activity turned down right before a bird’s call. “This led us to look at another type of cell that is known to quiet down neighboring cells”, explains Daniela Vallentin, head of the research group Neural circuits for vocal communication. These activity-suppressing cells turned out to be active before the cells that cause the bird to call, suggesting that they control when this call initiating signal occurs.

To test this directly, the researchers chemically blocked the action of these inhibitory cells and found that the call-initiating cells produced stronger and earlier signals. Moreover, the birds responded much faster to a partner and were more likely to interrupt the calls they heard.

Both humans and songbirds have specialized neural pathways which link hearing areas to vocal areas. Perhaps there are also parallels across species with respect to how we avoid interrupting each other.

(Sabine Spehn)

Original Publication

 Benichov JI, Vallentin D. (2020) Inhibition within a premotor circuit controls the timing of vocal turn-taking in zebra finches. Nat Commun. 11(1):221.
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SIGNIFICANCE STATEMENT: Most social-living vertebrates produce large numbers of calls per day, and the calls have prominent roles in social interactions. Here, we show neuronal mechanisms that are active during call-based vocal communication of zebra finches, a highly social songbird species. HVC, a forebrain nucleus known for its importance in control of learned vocalizations of songbirds, displays predictive activity that may enable the male to adjust his own calling pattern to produce very fast sequences of male female call exchanges.

Ma S, Ter Maat A, Gahr M. (2020). Neurotelemetry Reveals Putative Predictive Activity in HVC during Call-Based Vocal Communications in Zebra Finches. J Neurosci. 40(32):6219-6227.
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Under the framework of a new Marie Skłodowska Curie Innovative Training Network, an international team of scientists, including Max Planck Research Group Leader, Julijana Gjorgjieva (Max Planck Institute for Brain Research), will investigate how the highly advanced networks in biological systems, from molecules to the brain, are built and how they process information and perform computations. The European Union granted nearly 3.5 million euros for the project, called Smart Network Structures, or SmartNets.

Biological systems are organized into networks at every level, critically determined by its structure. This structure leads to behavior that can only be understood by analyzing the whole network in relation to its constituent parts. With the EU grant, five universities will work together with six partner organizations to understand network computations.

The relation between network structure and information processing is essential at every scale: from molecules and genes to large neural networks, such as the brain. On every level, nodes form complex networks underlying the most essential functions of the functioning of the brain, body, and society. Only recently, with high-throughput techniques, we have begun to collect the vast amounts of data needed to study the structure and functioning of these networks. However, analyzing these data is still a challenge and the nature of complex network processes is still poorly understood.

In order to compare networks, simulated or physical ones, or healthy versus diseased ones, tools are needed across three dimensions: structure, activity, and information processing. The novelty of this project lies in the combination of existing and the development of new techniques, from different research domains, and across the three analysis dimensions. By transcending specific data sets or domains, the researchers expect to open up new insights into the properties, behavior, and dynamic evolution of biological networks. The grant will largely be used to train the data scientists of the future. They will be able to analyze biological networks across levels, domains, and types.

Julijana Gjorgjieva is one of the scientists driving this collaborative effort. “Our work in this project will aim to identify how network structures emerge and change during postnatal development in model organisms establishing mature network connectivity and computations”, Gjorgjieva says.

Learn more:

https://fleurzeldenrust.nl/smartnets/

https://www.ru.nl/donders/@1249307/getting-know-architecture-brain/?reload=true

https://brain.mpg.de/research/computation-in-neural-circuits-group.html

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When a male or female white-browed sparrow-weaver begins its song, its partner joins in at a certain time. They duet with each other by singing in turn and precisely in tune. A team led by researchers from the Max Planck Institute for Ornithology in Seewiesen used mobile transmitters to simultaneously record neural and acoustic signals from pairs of birds singing duets in their natural habitat. They found that the nerve cell activity in the brain of the singing bird changes and synchronizes with its partner when the partner begins to sing. The brains of both animals then essentially function as one, which leads to the perfect duet.

White-browed sparrow-weavers (Plocepasser mahali) live together in small groups in trees in southern and eastern Africa. Each bird has a roosting nest with an entrance and an exit. The dominant pair will have a breeding nest which is easily recognisable by the fact that one passage is closed to prevent eggs from falling out. In addition to the dominant pair, there are up to eight other birds in the group that help build nests and raise the young. All group members defend their territory against rival groups through duets of the dominant pair and choruses together with the helpers.

White-browed sparrow-weavers are one of the few bird species that sing in duet. It was assumed that some cognitive coordination between individuals was required to synchronise the syllables in the duet, however the underlying neuronal mechanisms of such coordination were unknown.

Miniature transmitters enable recording under natural conditions
“White-browed sparrow-weavers cannot develop their complex social structure in the laboratory. We were therefore only able to investigate the mechanisms of the duet singing in the natural habitat of the birds”, says Cornelia Voigt, one of the three lead authors of the study. Because of this, researchers and technicians at the Max Planck Institute for Ornithology in Seewiesen developed mobile microphone transmitters to record the singing in the wild. These weigh only 0.6 g and were attached to the bird like a backpack.
With another newly developed transmitter, weighing only 1 g, the scientists could also make a synchronous record of the brain activity in the birds while they were singing in their natural environment. An antenna placed near the birds’ tree recorded up to eight of these signals in parallel. With the help of an external sound card and a laptop, the singing and the brain signals were synchronously recorded with millisecond precision. “The technology we have developed must withstand the extreme conditions of the Kalahari Savannah in northern South Africa”, says Susanne Hoffmann, a scientist in the Department of Behavioural Neurobiology. “The electronics for recording the signals were stored in a car. During the day, it got so hot that the laptop almost began to glow. But the recordings all worked well, even when the birds and their transmitters were caught in one of the few downpours”.

Brain activity of the duetting birds synchronizes
Lisa Trost, also a scientist in the department, says: “Fortunately, the procedure for fixing the implants for neuronal measurements on the heads of the birds did not take long. After complete recovery, the respective bird was quickly returned to the group and did not lose its social status. All birds sang in the tree immediately after their return”. The researchers recorded almost 650 duets. In many cases, the males began with the song and the partner joined in after some introductory syllables. The syllables between the duetting pair followed each other without delay and in perfect coordination. The coordination was so precise that analysis showed only a 0.25s delay between the duetting partners’ singing bouts.

The singing of songbirds is controlled by a network of brain nuclei, the vocal control system. In one of these nuclei, the HVC, the call of the partner bird triggers a change in neuronal activity in the bird that began singing. This, in turn, affects its own singing. The result is a precise synchronization of the brain activity of both birds. “The rhythmic duet of the individuals is achieved through sensory information that comes from the partner”, says Manfred Gahr, who led the study. The brains of the partners form a network that functions like an extended circuit to organize the temporal pattern for the duet. The researchers suspect that similar mechanisms are also responsible for coordinating movement during social interactions in humans (e.g. dancing with a partner).
“Until now, this kind of study has only been performed in the laboratory. Measuring the activity of nerve cells in the field using wireless transmitters is much less stressful for the birds,” says Susanne Hoffmann. “We hope this study has laid the foundation for the further development of neuroethology”.

Hoffmann S, Trost L, Voigt C, Leitner S, Lemazina A, Sagunsky H, Abels M, Kollmansperger S, Maat AT, Gahr M. (2019) Duets recorded in the wild reveal that interindividually coordinated motor control enables cooperative behavior. Nat Communications, 10, 2577.
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Read More Here

Hoffmann S, Trost L, Voigt C, Leitner S, Lemazina A, Sagunsky H, Abels M, Kollmansperger S, Maat AT, Gahr M. (2019) Duets recorded in the wild reveal that interindividually coordinated motor control enables cooperative behavior. Nat Communications, 10, 2577.
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Rattenborg, N.C. et al. Evidence that birds sleep in mid-flight.
Nat. Commun. 7:12468 doi: 10.1038/ncomms12468 (2016).
DOI:http://dx.doi.org/10.1038/ncomms12468

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For the first time, researchers have discovered that birds can sleep in flight. Together with an international team of colleagues, Niels Rattenborg from the Max Planck Institute for Ornithology in Seewiesen measured the brain activity of frigatebirds and found that they sleep in flight with either one cerebral hemisphere at a time or both hemispheres simultaneously. Despite being able to engage in all types of sleep in flight, the birds slept less than an hour a day, a mere fraction of the time spent sleeping on land. How frigatebirds are able to perform adaptively on such little sleep remains a mystery.

It is known that some swifts, songbirds, sandpipers, and seabirds fly non-stop for several days, weeks, or months as they traverse the globe. Given the adverse effect sleep loss has on performance, it is commonly assumed that these birds must fulfill their daily need for sleep on the wing.

Half-awake or fully awake in flight?

How might a bird sleep in flight without colliding with obstacles or falling from the sky? One solution would be to only switch off half of the brain at a time, as Rattenborg showed in mallard ducks sleeping in a dangerous situation on land. When sleeping at the edge of a group, mallards keep one cerebral hemisphere awake and the corresponding eye open and directed away from the other birds, toward a potential threat. Based on these findings and the fact that dolphins can swim while sleeping unihemispherically, it is commonly assumed that birds also rely on this sort of autopilot to navigate and maintain aerodynamic control during flight.

However, it is also possible that birds evolved a way to cheat on sleep. The sleep researcher’s and colleagues’ recent discovery that male pectoral sandpipers competing for females can perform adaptively for several weeks despite sleeping very little raised the possibility that birds simply forgo sleep altogether in flight. Consequently, evidence of continuous flight is not by default evidence of sleep in flight: Without directly measuring a bird’s brain state, previous claims that birds sleep in flight remain mere speculation.

Flight data recorder catches birds napping on the wing

To actually determine whether and how birds sleep in flight, the researchers needed to record the changes in brain activity and behavior that distinguish wakefulness from the two types of sleep found in birds: slow wave sleep (SWS) and rapid eye movement (REM) sleep. Niels Rattenborg teamed up with Alexei Vyssotski (University of Zurich and Swiss Federal Institute of Technology, ETH) who developed a small device to measure electroencephalographic changes in brain activity and head movements in flying birds.

In collaboration with the Galápagos National Park and Sebastian Cruz, an Ecuadorian seabird biologist, the team focused on great frigatebirds nesting on the Galápagos Island. Frigatebirds are large seabirds that spend weeks flying non-stop over the ocean in search of flying fish and squid driven to the surface by predatory fish and cetaceans. The researchers temporarily attached the small “flight data recorder” to the head of nesting female frigatebirds. The birds then carried the recorder during non-stop foraging flights lasting up to ten days and 3000 kilometers. During this time, the recorder registered the EEG activity of both brain hemispheres and movements of the head, while a GPS device on the birds’ back tracked their position and altitude. After the birds were back on land and had had some time to recover, they were re-caught and the equipment was removed. Bryson Voirin, a post-doc and co-first author on the paper with Rattenborg observed that, “Like many other animals in the Galápagos Islands, the frigatebirds were remarkably calm and would even sleep as I approached to catch them for the second time.”

The flight data recorder revealed that frigatebirds sleep in both expected and unexpected ways during flight. During the day the birds stayed awake actively searching for foraging opportunities. As the sun set, the awake EEG pattern switched to a SWS pattern for periods lasting up to several minutes while the birds were soaring. Surprisingly, SWS could occur in one hemisphere at a time or both hemispheres together. The presence of such bihemispheric sleep indicates that unihemispheric sleep is not required to maintain aerodynamic control. Nonetheless, when compared to sleep on land, SWS was more often unihemispheric in flight. By carefully examining the movements of the frigatebirds, the researchers discovered clues to why they sleep unihemispherically in flight. When the birds circled on rising air currents the hemisphere connected to the eye facing the direction of the turn was typically awake while the other was asleep, suggesting that the birds were watching where they were going. “The frigatebirds may be keeping an eye out for other birds to prevent collisions much like ducks keep an eye out for predators”, says Rattenborg.

In addition to engaging in both types of SWS in flight, on rare occasions, bouts of SWS were interrupted by brief episodes of REM sleep. Although this finding may seem remarkable to scientists who study sleep in mammals, based on Rattenborg’s experience with birds, he was not that surprised. In contrast to mammals, wherein episodes of REM sleep are long and accompanied by a complete loss of muscle tone, REM sleep episodes only last several seconds in birds. In addition, although a reduction in muscle tone can cause the head to drop during avian REM sleep, birds are able to stand (even on one leg) during this state. Similarly, when frigatebirds entered REM sleep their head dropped momentarily, but their flight pattern remained unchanged.

Ecological demands require full attention 24/7 at sea

Perhaps the greatest surprise was that despite being able to engage in all types of sleep on the wing, on average frigatebirds slept only 42 minutes per day. In contrast, when back on land they slept for over twelve hours per day. In addition, episodes of sleep were longer and deeper on land. Collectively, this suggests that frigatebirds are actually sleep deprived in flight. “Why they sleep so little in flight, even at night when they rarely forage, remains unclear”, says Rattenborg. As previous studies have shown that frigatebirds follow ocean eddies predictive of good foraging conditions throughout the day and night, perhaps this is what they are up to. Interestingly, the low amount of sleep in flight suggests that this task requires more attention than that afforded by sleeping with one half of the brain at a time. As such, frigatebirds face ecological demands for full attention 24/7 while at sea.

In the long term, Rattenborg hopes to determine how frigatebirds are able to sustain adaptive performance on such little sleep. People will fall asleep driving a car after losing just a few hours of sleep, even when fully aware of the dangers and struggling to keep themselves awake. “Why we, and many other animals, suffer dramatically from sleep loss whereas some birds are able to perform adaptively on far less sleep remains a mystery”, notes Rattenborg. Reconciling the findings from frigatebirds with the wealth of evidence underscoring the importance of sleep in other animals may provide new perspectives on our understanding of sleep and the consequences of its loss.

Original Publication:
Rattenborg, N.C. et al. Evidence that birds sleep in mid-flight.
Nat. Commun. 7:12468 doi: 10.1038/ncomms12468 (2016).
http://www.nature.com/articles/ncomms12468

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