On a Sunday afternoon in April, the main minibus terminal in Sukabumi, Indonesia, looked sleepy from the outside. But in an open space round the back, hundreds of men were gathered. Amid chatter and cigarette smoke, the air buzzed with excitement, for one of the region's biggest bird-singing competitions was set to begin, and a motorbike was among the prizes.
As the day progressed, dozens of songbirds were brought out for their 10-minute rounds, from tiny garden sunbirds and grey-cheeked bulbuls to larger oriental magpie-robins and orange-headed thrushes. Then the emcee announced the main event — the singing contest among the highly popular, strikingly handsome white-rumped shamas — and a hush fell over the crowd.
The shamas' owners murmured final words of encouragement and stepped away from their cages. Judges swept in with clipboards, assessing each bird for its song, ability to hold a steady tune, volume and showmanship. Soon it was down to a final two birds ... and then "Baby White" was crowned the winner amid cheers from the crowd.
Indonesians have a long-standing culture of keeping birds as pets, and songbirds are especially popular, prized by collectors for their melodious singing and colorful plumage. "I keep songbirds as a hobby, to relieve stress and also gain a bit of money," explained Harry Gunawan, a 78-year-old businessman and owner of 39 shamas, including the multiple prizewinning Baby White, while waiting for his new motorbike.
Gunawan's shamas are among an estimated 66 million to 84 million caged birds that are kept across Java, the island where 56 percent of Indonesia's population lives and one in three households owns birds. These include more than 3 million white-rumped shamas and 2 million oriental magpie-robins. Wild birds are believed to be better songsters; hence, many are trapped in forests then crammed into tiny crates, drainpipes and even plastic bottles, destined for pet markets in Jakarta, Surabaya and other big cities. Birds that survive the journey — estimates of mortality rates range from 30 to 80 percent — will spend the rest of their lives confined to cages.
This birdkeeping hobby, paired with the growing popularity of singing competitions, has already driven some species to extinction and many species — also threatened with habitat loss — are on the brink, a phenomenon dubbed "the Asian songbird crisis."
"The silent forest is really happening," says Agung Nur Haq, who is in charge of conservation at the Wak Gatak Songbird Rescue and Rehabilitation Center near Pontianak, West Kalimantan.
Experts warn that action must be taken quickly, or the consequences may be dire and irreversible. If nothing is done and populations can't recuperate, says Alexander Lees, a conservation biologist at Manchester Metropolitan University in the United Kingdom, we may experience an "empty forest syndrome," whereby forests appear intact and lush but are devoid of animals.
The illegal bird trade is thriving elsewhere in southeast Asia, including Vietnam, Thailand, Malaysia and Singapore. But "Indonesia's songbird trade is one of the world's most prolific," says Chris Shepherd, a wildlife trade expert at the Center for Biological Diversity in Canada. "It's terrifying."
The biodiversity-rich nation is home to about 1,800 bird species — more than double the number found in the United States. Of those, one in five have been spotted in pet markets, including protected, endangered and endemic species (ones found nowhere else). These include the endangered black-winged myna and the Javan green magpie, of which fewer than 250 and 100 wild individuals, respectively, remain.
Estimates suggest that up to 30 percent of Indonesia’s population, some 90 million people, keeps between 164 million and 187 million wild-caught songbirds.
"There may be more birds behind bars than left in the wild," wrote Lees in an overview of the songbird crisis in Current Biology.
The fondness of Indonesians for caged birds stems from a centuries-old belief among the Javanese, the country's largest ethnic group, that a man is considered successful if he possesses five elements: a wife, a house, a vehicle, a ceremonial dagger and a bird. The bird symbolizes that he is in touch with his softer side, and can indulge in both work and leisure. In a 2025 study, researchers found that a bird's singing ability was the biggest driver of demand — after that, people preferred rare, endemic or unusual birds that flaunt one's status.
Singing competitions emerged in the 1970s as another reason to keep songbirds. These local or regional events are typically held monthly or weekly, involving up to 1,000 birds.
"The owners go home with, at the very least, tons of prestige, but more often lots of prize money, sometimes up to 10 years' salary," says acoustic biologist Benjamin Mirin, founder of the Creative Conservation Lab with Cornell University. Competitors also stand to win exclusive trophies, goats, motorcycles and even cars.
Mirin, who has been studying the songbird trade since 2018, says the events can be life-changing. "They're so popular and financially beneficial that they're accelerating the poaching of birds to the point where now the forests are falling silent."
Unfortunately, wild-caught birds are perceived to have better song quality and a wider singing repertoire than their captive-bred counterparts. As a result, competitions "have been driving the demand in certain species," says Serene Chng, a wildlife trade researcher at TRAFFIC, a nonprofit focused on reducing harmful and illegal trafficking of plants and animals.
An expert group formed by the International Union for Conservation of Nature has identified 52 species that are most impacted by trade in Asia. These include the straw-headed bulbul (known for its lovely duets) and the Bali starling (with its stark white plumage and electric-blue eye ring). Although Indonesian law forbids the capture and trade of more than 500 bird species, many of them songbirds, enforcement is weak, thanks to poor resources, corruption and other factors. Consequently, several species, including the Javan pied starling, are now locally extinct.
It's hard to predict what will happen to ecosystems if songbird species are wiped out en masse in Indonesia. Birds play important ecological roles: pollinating plants, dispersing seeds and helping with insect control. The Pacific island of Guam offers a cautionary tale: It lost nearly all of its songbirds after the brown tree snake, which preyed on the birds and their eggs, was accidentally introduced after World War II. Guam's forests shifted to what Lees has described as "a nightmarish alternative state as a spider-dominated ecosystem." It was, he says, "pretty catastrophic."
Indonesia is much larger and far less isolated, so "your starting conditions are not as bad," says Lees. "But if we don't recuperate some of those populations, then we can expect maybe similar sorts of downstream impacts."
Government agencies have had some success in tracking down wild-captured birds but are reluctant to confiscate them because they lack a place to house them. This dilemma in West Kalimantan province, a hotspot for illegal bird trade activity, led to the establishment of the Wak Gatak sanctuary by the nonprofit group Planet Indonesia.
One of its key aims is to provide suitable facilities to encourage more frequent confiscations and rescues by authorities with Indonesia's natural resources and conservation agency, BKSDA, while ensuring the welfare of confiscated songbirds, says Abrar Ahmad, a technical advisor for terrestrial conservation at Wak Gatak.
The center, which sits on a leafy plot near the city of Pontianak, is worlds apart from the dark, cramped conditions birds endure when smuggled aboard a truck or ship. On a morning in April, visitors turn off a sleepy road and drive past the gates down a short muddy track. Dragonflies flit lazily through tall grass and a smattering of coconut, banana and durian trees sway in the distance. The facilities are modest: a small office block with light-filled aviaries.
Animals often arrive at Wak Gatak in an alarming state: Malnourished, with missing feathers, many appear fatigued or withdrawn, says Happy Ferdiansyah, Wak Gatak's head veterinarian. Some birds have wounds on their upper beaks from gnawing on cage bars, while others — especially if they're more territorial — suffer leg fractures from fighting with other birds in close quarters.
"A lot of them, 70 to 80 percent, die within the first two weeks. Their condition is very poor," says Ferdiansyah.
To avoid causing the birds further stress, his team conducts only a visual inspection of the new arrivals. Those that are obviously ill are transferred to the on-site clinic for treatment. The rest are transferred to cages in quarantine.
The birds receive vitamins, nourishment (an assortment of fruits, insects, sugar syrup and so on), in addition to enjoying more space — one or two individuals typically share an oven-size cage. They remain in quarantine for at least 14 days, a critical time period when diseases such as avian influenza and Newcastle disease tend to manifest, says conservation director Haq. To test for these and other issues, Ferdiansyah and his fellow vets analyze blood and stool samples, in addition to carrying out rapid antigen tests.
Once their health has been given the all-clear, the birds are moved to rehabilitation aviaries on the opposite side of the premises. These enclosures are much more luxurious: bathroom-size, dotted with green plants, with generous views of the surroundings and sky.
Here, stimulation is key, as is getting the birds comfortable with flying again. "The aviaries have different perches and plants for the birds to hop around or to hide in the bush," says Ferdiansyah. "We also do food enrichment like giving them live insects or putting the food in different places."
Only some birds are fit for release. Those that appear habituated to people are not good candidates; they may continue to approach humans and might teach non-natural calls to wild birds or just fail to communicate with them. The center, for example, currently houses four locally endangered and protected common hill mynas — glossy black starlings with heavy orange bills that are renowned for their intelligence and mimicry. When I visited, the birds happily wolf-whistled, chuckled and offered the traditional Islamic greeting "As-salamu alaykum," among smatterings of Indonesian chatter. These mynas have been at Wak Gatak for nearly a year and will remain there for the foreseeable future.
But for birds that are suitable to release, the center works with BKSDA to identify possible sites. Ideally, these would be forest lands in good condition, with abundant food and water, minimal human activity and few natural predators like civets and snakes, says Ferdiansyah. A local community that's supportive of conservation is a plus — increasing positive outcomes, such as survival and reproduction, by 10 percent, according to a 2023 study that analyzed 305 wildlife restoration projects in 80 countries.
Once a site is determined and permits are secured, Ferdiansyah and his team transport the birds — as they did at the end of April, taking 130 birds to a nature reserve seven hours north of Pontianak. At such sites, the team carries out what’s known as a soft release, placing the birds in a large makeshift cage that allows them to acclimate to their new surroundings. After four to 10 days, the team opens the cage, allowing the birds to fly away when ready. Team members stay on for up to two weeks, tracking the birds to make sure they're settling in problem-free.
It's rough, sweaty work, says Ferdiansyah, but highly rewarding. The center has so far helped 348 songbirds return to the wild. The April release is their largest to date, and included some crimson sunbirds and greater green leafbirds, the latter of which is endangered. Another 705 birds were seized in December from a ship at the local port in Pontianak. Within hours, most of the birds had died. But 36 are still recovering at Wak Gatak and 22 have been rehabilitated and released.
Nearly 3,000 birds from 45 species have passed through the center's doors since it opened in 2022. It’s proof that such a rehabilitation facility — the only one in Indonesia for the foreseeable future — can make an impact.
But given how many songbirds are in peril, its work is a drop in the ocean.
To slow the crisis, experts say it must be fought on multiple fronts, but above all, at its root: demand. It requires "thinking about how we can shift attitudes and behaviors to reduce pressure on wild populations," says Lees, who explores such measures in a look at the state of the world’s birds in the 2022 Annual Review of Environment and Resources. In 2023, the Wak Gatak team carried out a campaign across some of the city's billboards, discouraging people from joining songbird competitions. And in the coming months, they'll host a series of behavior-change workshops in two towns close to forests where birds are commonly poached and competitions are growing in number.
Shifting the needle will not be easy. Indonesia's songbird hobbyist groups are powerful — in 2018, they successfully petitioned to have five species downlisted from the national list of protected birds. Plus, government officials often take part in competitions themselves. In 2018, for example, then-president Joko Widodo presided over the President's Cup, one of Indonesia's most prestigious songbird competitions, even entering his own white-rumped shama.
Yet change is possible: Ferdiansyah himself kept four songbirds as a kid because they "sounded very nice" and he thought he could make them happy. But when he entered veterinary college, he realized the error of his ways and set the birds free.
This article originally appeared in Knowable Magazine, a nonprofit publication dedicated to making scientific knowledge accessible to all. Sign up for Knowable Magazine's newsletter.
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An innovative new approach to building safer, more efficient small unmanned aerial vehicles is taking a cue from nature.
Anticipating worsening turbulence due to environmental concerns in the coming years, engineers studying the unmatched hovering ability of kestrels hope to reverse engineer the secrets of their exceptional flight capabilities to help assemble better future drones.
Smaller drones, known as sUAVs, have a range of different applications that include delivery services, photography, and more. However, wind conditions can often cause turbulence that either grounds them temporarily or, when caught off guard, can lead to their destruction.
Now, a team of researchers has published their findings in the Journal of the Royal Society Interface, which details an ongoing collaboration between RMIT University and the University of Bristol that seeks to overcome such issues by building drones that operate more like some of nature’s best flyers.
Specifically, the nankeen kestrel is an avian species whose amazing prowess while flying during adverse conditions still confounds researchers. To unravel the secrets of their flight, the researchers used motion capture technologies made possible in RMIT’s special Industrial Wind Tunnel facility, one of the largest such facilities anywhere on the Australian continent.
“Birds don’t rely on a single response to wind gusts,” according to RMIT researcher Matt Penn. “They constantly adjust their wings and tails to stay balanced, while the natural flexibility of their feathers and joints helps absorb sudden changes in airflow.”
“They can also sense disruptions very quickly, which allows them to respond almost instantly and maintain control,” added Penn, one of the team members who co-led the studies into how birds can cope with turbulence.
According to Penn and his colleagues, these birds possess a range of unique strategies that help them overcome the dangers of turbulence, which help them navigate under conditions that would cause most sUAVs to come tumbling from the sky.
To attempt to understand what makes kestrels such adept fliers, the team created a robotic replica to mimic some of their key flight movements and characteristics, which allowed the team to unravel some of the secrets to how these incredible birds perform.
“By creating a robot replica, we were able to measure how specific movements were contributing to steadiness in flight,” said Dr Mario Martinez Groves-Raines, a researcher now with the Royal Veterinary College in London, adding that precision measurements of kestrel flight that normally could not be achieved in nature were possible using the team’s robotic version.
“We uncovered several unique techniques behind the kestrel’s impressive stability,” Groves-Raines said. “Many of these techniques have the potential to improve [the] manoeuvrability of small aircraft, which encounter similar challenges to kestrels.”
“This research shows what’s possible when engineers look to nature for solutions,” added RMIT senior researcher Associate Professor Abdulghani Mohamed.
“Our findings open new pathways for designing aircraft that can better handle turbulence.”
Going forward, the researchers say they hope to apply their findings toward the construction of novel sUAVs, which can help future drones operate safely and more efficiently, much like their agile avian forebears.
Ultimately, similar engineering in the coming years may have applications that extend well beyond just drone flight, and could also help to ensure safe operations for much larger aircraft.
The team reported their findings in a pair of new studies, ‘Bridging the Gap: A Review of Gust Mitigation in Birds and Small UAVs’, and ‘Stability and control benefits of coupled wing and tail morphing in kestrel wind-hovering flight explored using a robot replica’, both of which were published in the Journal of The Royal Society Interface.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.
Inbreeding depression, a genetic phenomenon where offspring result from the mating of related individuals, has long been understood to compromise individual fitness and elevate the risks of population decline or extinction. However, the depth and nuances of these effects remain largely unexplored in many non-model species, especially those facing dire conservation challenges. A groundbreaking new study investigates this complex relationship in a threatened songbird species, the aquatic warbler (Acrocephalus paludicola), unveiling how inbreeding depression manifests in a phenotype-dependent manner, particularly emphasizing body size as a critical factor in male fitness outcomes.
The aquatic warbler, a habitat specialist bird known for its recent precipitous population decline, poses a unique opportunity to study the genetic intricacies and their ecological consequences. Unlike widely studied model species, this bird’s depleted genetic diversity coupled with habitat specificity creates an ideal system to understand how inbreeding depression impacts survival and reproduction under real-world environmental pressures. Employing cutting-edge genomic tools such as single nucleotide polymorphisms (SNPs), researchers could quantify inbreeding at an unprecedented resolution, offering new insights into the genetic health and viability of this endangered population.
One of the pivotal breakthroughs of this research lies in demonstrating that inbreeding depression is not a uniform or straightforward process but is intricately moderated by the physical and ecological context of the individual. For male aquatic warblers, the genomic markers revealed a pronounced negative effect of inbreeding on breeding success—but only among males characterized by smaller body size, denoted by tarsus length, an established morphological proxy. This finding highlights how phenotypic traits can mediate the severity of inbreeding effects, suggesting that smaller-bodied males experience disproportionately harsh fitness reductions as a consequence of their genetic relatedness.
Equally compelling is the study’s exploration of wing length as another morphological trait potentially influencing inbreeding depression. While the results here were more subtle, a notable weak interaction was found between the degree of inbreeding and shorter wing length in males, correlating with decreased survival probabilities. This interaction suggests that even relatively minor morphological differences may buffer or exacerbate the impacts of genetic relatedness on survival outcomes, adding another layer of complexity to how we understand inbreeding depression in natural populations.
Importantly, the research also reveals a stark contrast between sexes. Female aquatic warblers showed scant evidence of inbreeding depression affecting key fitness components such as clutch size, hatching success, nestling survival, or the number of fledged fledglings. Moreover, no interactions emerged between female fitness measures and phenotypic or environmental variables examined. This asymmetry in inbreeding effects underscores sex-specific pathways in evolutionary and ecological responses to genetic challenges, warranting further inquiry into the mechanistic underpinnings driving such differences.
Prior studies investigating inbreeding depression often relied on relatively simplistic models that did not incorporate potential interactions with individual phenotype or environmental heterogeneity. This omission might explain why many previous estimates of inbreeding load—reflecting the cumulative fitness consequences of deleterious recessive alleles—have been inconsistent or highly uncertain. By integrating detailed phenotypic data with high-resolution genomic analyses, this study advances the field by showing that accounting for individual variation unveils clearer, biologically meaningful patterns of inbreeding depression.
The implications for conservation biology are profound. Small-bodied males, disproportionately burdened by inbreeding loads, might represent a critical demographic subgroup whose reproductive success most strongly influences population trajectories. This insight challenges conventional approaches that treat all individuals as homogenous units within populations, advocating instead for conservation strategies that are finely tuned to the nuanced genetic and phenotypic architectures of endangered species.
The environmental factors considered—timing of breeding and brood size—proved less influential in mediating inbreeding depression compared to phenotypic variables, at least in this particular system. While breeding timing and brood size have been previously hypothesized to influence fitness by affecting resource allocation or offspring survival probabilities, their interaction with genomic inbreeding proved minimal. This result refines our understanding of the complex ecological determinants of fitness and suggests that underlying genetic and phenotypic factors can overshadow the modifying effects of certain environmental variables.
Methodologically, the use of SNP-based genomic inbreeding coefficients represents a significant step forward from traditional pedigree-based measures. Genetic markers enable more precise estimates of genome-wide relatedness and inbreeding, which is especially crucial for populations like the aquatic warbler, where pedigree data are incomplete or unavailable due to logistical challenges in tracking individuals across generations. This technological advance paves the way for more accurate assessments of inbreeding consequences in conservation genetics.
Another noteworthy contribution is the nuanced interpretation of inbreeding load estimates. The study found that models omitting phenotypic interactions yielded highly uncertain inbreeding load figures for both survival and breeding success in males, whereas incorporating body size interactions produced more robust and meaningful estimates. This methodological insight calls for revisiting traditional models that treat inbreeding load as a fixed entity, promoting dynamic frameworks that integrate individual heterogeneity.
The study also contributes to evolutionary theory by demonstrating how inbreeding depression may exert selective pressure that maintains or increases body size in male aquatic warblers. Since smaller males suffer more pronounced fitness detriments due to inbreeding, natural selection may favor larger males, which effectively buffer against the genetic disadvantages. This dynamic adds a rich dimension to sexual selection and life-history evolution, where morphology and genetic fitness interplay to shape population structure.
In broader ecological contexts, understanding conditional inbreeding depression helps elucidate population resilience amid environmental change. As habitats degrade or fragment, and populations become increasingly isolated, the compounding effects of genetic bottlenecks and reduced phenotypic diversity could spell catastrophic declines if certain phenotypic groups disproportionately suffer. This research underscores the urgent need to consider phenotype-genotype-environment interactions in any assessment of vulnerable species.
Moreover, the findings raise compelling questions about the management of breeding programs aiming to mitigate inbreeding effects. Conservation initiatives often emphasize genetic diversity restoration in general terms, yet this study advocates for precision strategies that recognize differential vulnerabilities among phenotypic classes. For the aquatic warbler and similar threatened species, such targeted approaches could optimize reproductive success and survival rates, substantially enhancing recovery prospects.
Overall, this study reshapes scientific paradigms surrounding inbreeding depression by foregrounding the conditionality of its effects. It insists that blanket statements about the genetic risks of inbreeding are oversimplifications. Instead, fitness consequences emerge as a complex tapestry woven from genomic architecture, individual morphology, and ecological context, challenging researchers and conservationists alike to embrace multidimensional models.
Future research inspired by these findings will likely delve deeper into the mechanistic bases for the observed interactions, exploring, for instance, how developmental pathways link body size and genetic load expressions. Additionally, cross-species comparisons incorporating habitat specialists versus generalists may reveal whether this pattern of phenotype-dependent inbreeding depression is widely applicable or highly species-specific.
In conclusion, this landmark study presents a compelling case that it is indeed “hard to be small” for male aquatic warblers facing the genetic drawbacks of inbreeding. Such a phenotype-dependent dimension to inbreeding depression not only enhances scientific understanding but also provides invaluable practical guidance for conserving one of Europe’s most threatened songbirds. As the world grapples with biodiversity loss and climate challenges, integrating genomic, phenotypic, and ecological data remains paramount in crafting effective conservation blueprints.
Subject of Research: Inbreeding depression and its phenotypic and environmental conditionality in a threatened songbird species, the aquatic warbler (Acrocephalus paludicola).
Article Title: It is hard to be small: Inbreeding depression on male breeding success depends on body size in a threatened songbird.
Article References:
Kubacka, J., Arantes, L.S., Herdegen-Radwan, M. et al. It is hard to be small: Inbreeding depression on male breeding success depends on body size in a threatened songbird. Heredity (2026). https://doi.org/10.1038/s41437-026-00859-0
Image Credits: AI Generated
DOI: 21 June 2026
Remote Scottish islands turned tiny wrens into giant birds that may be evolving into new species. Tiny wrens living on remote Scottish islands are providing scientists with a rare look at how evolution can reshape animals in isolated environments. A new study led by researchers at the University of Birmingham found that several island populations [...]Corvids, including ravens and crows, have earned a reputation for being some of the smartest birds on Earth. They can recognize human faces, use tools, solve multistep puzzles and even hold funerals.
In recent years, scientists have added another surprising skill to the list: a grasp of numbers. Research suggests that corvids can distinguish between different quantities and, in some cases, even perform statistical analysis.
These discoveries have raised a provocative question: How deep do the roots of mathematical thinking go? While modern humans are the only known species to develop formal mathematics, studies of crows, monkeys and other animals suggest that the cognitive building blocks of math may have evolved hundreds of millions of years before people began doing calculations.
Few researchers have done more to uncover these abilities than Andreas Nieder, a professor of animal physiology and the director of the Institute of Neurobiology at the University of Tübingen in Germany. By examining the neural activity of both monkeys and crows, Nieder has found evidence that very different animal brains can process numerical information in surprisingly similar ways. Live Science spoke with Nieder about how these findings may reveal that the origins of mathematics are far older than our species.
Kenna Hughes-Castleberry: How did you get into researching crows and primates?
Andreas Nieder: After studying the auditory and visual systems for several years as a student, I realized that I wanted to focus on the neuronal foundations of intelligence and cognitive control functions. How does the brain give rise to abstract concepts, working memory, or the ability to follow complex rules deliberately? Of course, to study intelligence, you need intelligence specialists. I therefore began working with primates — more specifically, rhesus monkeys, arguably among the most intelligent mammals. They are closely related to humans, possess relatively similar brains, and share many cognitive abilities with us.
However, as a trained biologist, I was always interested in a broader evolutionary question: Are sophisticated cognitive abilities unique to mammals, or can they emerge in very different brains? Crows provided an ideal comparative species. Birds diverged from mammals more than 360 million years ago — roughly six times longer ago than the extinction of the dinosaurs. Yet despite this immense evolutionary distance, corvids display remarkably flexible behavior; in many ways, they are the feathered primates of the air.
Some are reliable, highly motivated workers that perform consistently day after day. Others are sensitive divas who seem to require special treatment before they are willing to cooperate.
Andreas Nieder
KHC: Are crows difficult to study? Are they fun? Do they have personalities?
AN: Both primates and crows are demanding to study. Because they are highly intelligent, they require special housing and care. Since we investigate cognitive abilities under tightly controlled experimental conditions, they also require extensive behavioral training by skilled personnel. And, of course, the regulations governing research with these species are particularly strict.
At the same time, they are fascinating research subjects. Individual monkeys and crows differ noticeably in temperament and behavior. Some are curious and adventurous, others are cautious and reserved, and some learn new tasks much faster than others.
Their qualities can be quite striking. Some are reliable, highly motivated workers that perform consistently day after day. Others are sensitive divas who seem to require special treatment before they are willing to cooperate. Researchers generally avoid anthropomorphizing, but there is little doubt that individual animals exhibit stable behavioral traits that we would reasonably describe as distinct characters.
KHC: You've found that crows understand the concept of zero. How do we know this?
AN: We trained crows to recognize different numbers — sets containing one, two, three, four objects, and so forth. We then included trials in which no items appeared at all. The remarkable finding was that the birds treated an empty set as a quantity and as part of the numerical continuum. They behaved as though the empty set belonged before "one." Their error patterns were particularly revealing: They confused zero more often with one than with larger numbers, exactly as one would expect if zero were mentally represented as a numerical quantity next to one. In addition, recordings from neurons in the crow brain showed cells that responded selectively to empty sets. This suggests that zero is not merely a behavioral trick but is represented neurally as a meaningful numerical category.
KHC: To play devil's advocate, is the ability to recognize the difference between something and the lack of something really that surprising? Is it akin to the human concept of zero?
AN: That's an important question. Simply recognizing that nothing is present or absent is not surprising. Many animals can distinguish between the presence and absence of objects. The key issue is whether they treat "nothing" as a quantity that can be compared to other quantities.
Our experiments suggest that crows do more than detect absence. They position empty sets within an ordered numerical sequence, at the low end of the number line, and show distance effects similar to those observed with other numbers. For example, they find it easier to distinguish zero from two than zero from one.
That said, we should not claim that crows possess the full human mathematical concept of zero. Humans use symbolic zero in arithmetic, algebra, calculus and formal mathematics. What we have demonstrated is a foundational precursor: a nonsymbolic numerical representation of zero. This may represent an evolutionary building block from which more sophisticated human concepts eventually developed.
KHC: You've also found that crows can perform statistical analysis. Tell me more about these findings and what you showed.
AN: We trained carrion crows [Corvus corone] to associate a series of arbitrary visual signs, colored geometric images displayed on a touch screen, with different reward probabilities. One sign might yield a food reward 90% of the time, another 70%, another 50%, and so on down to 10%. Importantly, the signs themselves carried no inherent meaning; the birds had to learn these probabilities through experience.
Once the birds had learned these sign-probability associations, we presented them with pairs of signs and asked them to choose one. To maximize their rewards, they should select the sign associated with the higher reward probability. The crows did exactly that. Even when confronted with many novel pairings, they reliably chose the statistically more favorable option.
This does not mean that crows perform formal statistics in the human sense. They are not calculating percentages or solving probability equations. However, the results show that they can extract probabilistic regularities from experience, store this information in memory, and use it flexibly to make reward-maximizing decisions under uncertainty. These are fundamental ingredients of statistical reasoning and are highly valuable in natural environments, where animals constantly have to make decisions based on incomplete information.
KHC: Have researchers tried similar tests on babies or young humans, and if so, how do we stack up?
AN: Yes. Developmental psychologists have conducted many related studies with infants and young children. Even before learning language, infants show sensitivity to quantities, numerical differences and simple probabilistic information. By around 4 years of age, children also display an intuitive understanding of empty sets.
What is striking is that the behavioral signatures observed in crows resemble those seen in preverbal human infants and nonhuman primates. Of course, human children eventually develop symbolic mathematics through language, education and culture, which takes them far beyond the abilities observed in animals.
KHC: Can these findings tell us anything about how the human understanding of math evolved?
AN: They suggest that some foundations of mathematics may predate humans by a very long evolutionary timescale. Humans did not invent numerical intuition from scratch. Rather, we appear to have inherited ancient cognitive systems that allow brains to estimate quantities, compare numbers, and represent concepts such as zero approximately.
The fact that crows and primates — two distantly related groups — show similar numerical abilities suggests that these capacities may emerge whenever evolution favors flexible intelligence. Human mathematics likely built upon these ancient cognitive foundations and then expanded dramatically through language, symbols and culture.
What is striking is that the behavioral signatures observed in crows resemble those seen in preverbal human infants and nonhuman primates.
Andreas Nieder
KHC: What's something a general audience might be surprised to learn about crows?
Many people are surprised to learn that crows are songbirds. Most people associate songbirds with melodic singers such as nightingales or blackbirds, not with the harsh cawing of a crow. Yet crows do learn a significant part of their vocal repertoire and are even known as apt imitators, even of human speech sounds — the internet is full of funny videos in which crows imitate human speech.
What makes this particularly interesting is that their vocalizations are not merely reflexive calls. In our experiments, we found that crows can exert volitional control over their vocal behavior. They can learn to produce vocalizations in response to arbitrary cues and withhold them when required, demonstrating a level of cognitive control over vocal output that is considered an important prerequisite for complex communication and, ultimately, language.
More recently, we showed that crows can even use their vocalizations to count. In a recent study published in [the journal] Science, we trained crows to produce a specific number of calls — between one and four — in response to visual or auditory cues. Remarkably, they not only generated the correct number of vocalizations but also appeared to prepare the intended number before they started calling. This suggests that they can represent numerical information and use it to guide a self-generated sequence of vocal actions.
Taken together, these findings show that crow vocalizations are far more sophisticated than most people assume. Their characteristic "caw" is not just a simple call — it can be brought under deliberate cognitive control and even used to express numerical information. For a bird, that is a remarkably advanced ability.
This interview has been edited for length and clarity.
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In a groundbreaking study recently published in eLife, researchers have unveiled compelling evidence that female North American mountain chickadees, a species traditionally characterized by socially monogamous mating systems, selectively seek out males exhibiting superior spatial cognitive abilities for extra-pair copulations. This revelation adds a fascinating dimension to our understanding of mating behavior and cognitive evolution in wild avian species, highlighting the intricate interplay between cognitive traits and reproductive strategies.
The study centers on mountain chickadees, non-migratory birds inhabiting temperate environments, renowned for their reliance on sophisticated spatial navigation to store and retrieve food caches. These birds’ survival depends heavily on their capacity for spatial learning and memory, making them an ideal model to investigate the role of cognitive function in mating preferences. By focusing on this species, the research addresses a critical question: do female birds prioritize cognitive skills of potential mates, beyond social pair bonds, to maximize reproductive success?
To quantify spatial cognition, the researchers engineered an innovative experimental setup involving ‘smart’ feeder arrays configured to dispense sunflower seeds at unique locations, individualized for each tagged chickadee. By systematically tracking how many errors each male made in locating their designated food source, the team accurately gauged spatial learning efficiency and memory retention. This mechanistic approach enabled an objective assessment of cognitive performance under naturalistic conditions, bypassing subjective behavioral observations.
Over the course of three breeding seasons, the research team meticulously documented parentage using genetic analyses to detect the prevalence and paternal identity of extra-pair offspring. Their findings revealed that nearly 70% of nests contained offspring sired by males other than the social partner, with approximately one-third of sampled chicks resulting from such extra-pair copulations. Notably, males demonstrating superior spatial cognition sired significantly more extra-pair young, suggesting these cognitive traits confer a conspicuous reproductive advantage.
Importantly, the data showed that males excelling in spatial tasks could sire between six and seven extra-pair offspring annually, dwarfed not by age but by cognitive prowess. This finding disrupts conventional assumptions that reproductive success correlates linearly with age or physical dominance, positioning advanced cognitive abilities as a pivotal determinant in extra-pair mating dynamics. Females appear to leverage these abilities when selecting mates beyond their social partner, potentially securing superior genetic quality for their progeny.
Further analyses indicated that males with higher cognitive scores did not suffer reproductive deficits within their social nests, maintaining comparable clutch sizes and chick mass relative to cuckolded males. Intriguingly, cognitive superiority was associated with heavier offspring overall, which likely translates to increased survival probabilities and long-term fitness benefits. Thus, spatial cognition appears to enhance male reproductive success both directly and indirectly.
Exploring female factors, the study unveiled a compelling pattern: females exhibiting lower spatial cognitive capabilities were more prone to have extra-pair young. This suggests a strategic response where cognitively limited females seek extra-pair copulations as an adaptive mechanism to compensate for their own deficiencies, thereby securing mates with enhanced spatial abilities. This bidirectional cognitive dynamic underscores complex sexual selection pressures governing mate choice.
Crucially, the likelihood of social males being cuckolded did not directly relate to their cognitive performance. Instead, female choice driven by assessing alternative males’ spatial skills determined extra-pair mating outcomes. This decouples social pair bond stability from cognitive prowess, signaling selective pressures favoring cognitive traits beyond social fidelity. It also emphasizes the nuanced cognitive evaluation females employ during mate selection processes.
These findings have profound implications for evolutionary biology, particularly in understanding how cognitive characteristics influence sexual selection. By demonstrating that spatial cognitive abilities contribute to reproductive success and offspring viability, the study offers robust empirical evidence supporting natural selection acting on cognitive traits in natural populations, reinforcing the theory that cognitive evolution is intertwined with reproductive strategies.
Moreover, the research methodology blending fieldwork, engineered cognitive assessments, and genetic parentage analysis exemplifies a cutting-edge interdisciplinary approach. This allows researchers to unravel the subtle yet consequential behavioral and genetic factors shaping population dynamics in wild animals, paving the way for future studies investigating cognition’s role across diverse taxa.
Senior author Vladimir Pravosudov summarizes the significance: “Our results demonstrate that sexual selection extends into cognitive domains, with females preferring males whose spatial skills enhance offspring survival. This contributes to shaping cognitive evolution in species where spatial memory directly influences fitness.” The study thereby enriches our comprehension of how natural and sexual selection jointly sculpt complex traits like cognition.
In conclusion, the evidence presented calls for a paradigm shift in considering cognitive traits as central to mating strategies and reproductive fitness. This research not only deepens insights into mountain chickadee behavior but also broadens the evolutionary narrative, spotlighting cognition’s integral role in survival and reproductive success. Future explorations may unravel similar patterns in other species, expanding our grasp of the cognitive dimensions of sexual selection.
Subject of Research: Animals
Article Title: Male chickadees with better spatial cognition sire more extra-pair young
News Publication Date: 16-Jun-2026
Web References: http://dx.doi.org/10.7554/eLife.110905.1
Image Credits: Vladimir Pravosudov (CC BY 4.0)
Keywords: Ecology, Animals, Birds, Cognition, Spatial memory, Evolutionary biology, Natural selection
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