Category Archives: Raven behavior

2018 research round up

As 2018 draws to a close, I want to dedicate a post to five of the most interesting and important publications about our favorite family of birds that came out this year. For the sake of a brevity, the reported studies are largely condensed with some tests/results omitted and little attention to normally key experimental elements like controls, statistical analyses, etc. Please click on the study title to be directed to the full publication.

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1. Townsed AK, Frett B, McGarvey A, and Taff CC. (2018). Where do winter crows go? Characterizing partial migration of American Crows with satellite telemetry, stable isotopes, and molecular markers.  The Auk 135: 964-974

Background: Depending on where you live, the answer to, “Do crows migrate?,” can be quite different.  For example, most Seattle residents would probably say no, since large numbers of crows can be seen here year round, while someone in say, a southern Canadian province, may notice a sharp decline in the number of crows during the winter.  That’s because crows are what’s know as “partial migrant species” meaning that within a population, some individuals may be migratory and others resident with more migratory strategies biasing in areas with harsh winters.  Despite the role of partial migration in how scientists currently explain the evolution of complete migration, little is known about the phenomenon.  Even elemental questions such as: is this behavior fixed or flexible within individuals, is it environmentally influenced, and how might species use it to adapt to changing conditions remain under-explored.

Methods: The study looked at two populations of overwintering crows: one in Ithaca, New York and a second in Davis, California.  They used a combination of intrinsic (meaning originating in the body) and extrinsic (meaning originating outside the body) markers to track the movement and origin of their 18 tagged subjects over 2-4 years.  The intrinsic makers included molecular and stable isotope data, and the extrinsic marker was a satellite tracking device that was attached to the bird via a light backpack.  I won’t go into the details of the molecular and stable isotope data, but suffice it to say that stable isotopes were used to identify the place of origin via the unique properties of the local food and water that embed into an individual’s tissue and the molecular data was used to sex individuals and establish relatedness.

Key findings: Of the 18 tagged crows across both east and west coast populations, they found that almost 78% were migratory.  This was a shock to me, TBH.  I had no idea just how many crow were making these annual trips.  The distance these birds traveled varied widely, with some going as “little” as 280 km (173 miles) and others as much as 1095 km (680 miles). Among resident birds, they found that individuals never ventured further than 25 km (15.5 miles) from the center of their breeding site.  For both resident and migratory individuals they found that birds were very loyal to their breeding sites; returning to the same territory year after year.  Given this finding, it should not be surprising to learn that individuals did not vary from year to year in whether they were migratory or not.  Together these results offer clues to how crows may respond to climate and urbanization induced changes in temperature to their local environments.

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2.  von Bayern AMP, Danel S, Auersperg AMI, Mioduszewska B, and Kacelnik A. (2018). Compound tool construction by New Caledonian crows. Nature Scientific Reports 8

Background: For decades people considered the use of tools to be a uniquely human feature.  Now we know that all sorts of animals, ranging from fish to monkeys, use tools and a handful of animals even create tools.  Among the small number of animals that create tools, we have only seen wild individuals modifying a single object.  For example, stripping a twig of small leaves or branches in order to probe small holes for insects.  Whether any wild animal is capable of making compound tools, those made by combining seperate non-functional parts, is unknown.  Even in captivity, this behavior only has limited observation in the great apes.  Understanding what animals are capable of this complex task and how they achieve it, might give us insight into the evolution of our own exective functions.

Methods: This study used eight wild caught captive New Caledonian crows.  Like many experiments involving novel objects, this one occurred over multiple different phases.  In phase I the birds were provided a long stick and a baited test box where food was within reach when using the stick, but not without it.  In phase II the birds were presented with the same baited test box, except that instead of a single long stick, they were given a hollow cylinder and a second, thinner cylinder that needed to be combined in order to generate a tool long enough to reach the food.  In phase III, the birds were given the same problem, only now with novel combinable items.  In phase IV, the researchers tested whether the birds were combining elements because they understood that they needed to, or if because they derived some other benefit from the process.  To do this, they presented birds with a bait box that had two tracks: one where the food was within reach of a single element and one where it required a compound element. In the final phase, birds were presented a bait box that required the combination of more than two elements.

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Image from von Bayern et al. 2018

Key findings: All birds passed the initial tool use phase handily.  Given that New Caledonian crows frequently use single element tools in the wild, this was not at all surprising. In the second phase, half of the subjects (four) were able to combine the two elements after no more than two failed attempts. These subjects were then able to transfer this knowledge when presented novel combinable objects. When given a bait box with food presented on the close and far tracks, birds most often only made compound tools when it was necessary, suggesting that they don’t do it just for fun.  In the final phase, only one bird succeeded in making a tool that required more than two elements.  These findings demonstrate that New Caledonian crows are not only on par with what’s know about compound tool use in the great apes, but actually exceed them.

Unfortunately what this study does not explicitly answer is whether the birds were able to create the needed tools as a result of mental mapping (i.e imagining the correct tool and how it might be assembled) or by happy accident.  Without this knowledge, what their ability to make compound tools suggests about the evolution of things like insight remains mysterious.  Given all the other remarkable ways New Caledonian crows show innovation when it comes to tool use, however, both myself and the authors of this study are hedging that it’s indeed cognition behind these behaviors rather than more simple mechanisms.

3. Boeckle M, Szipl G, and Bugnyar T. (2018). Raven food calls indicate sender’s age and sex. Frontiers in Zoology 15

Background:  One of the most frequent inquiries that come my way are requests to decipher various crow calls.  Given all we know about crows, this doesn’t seem like such an impossible request, but the reality is that crow communications remains one of the most impenetrable black boxes of crow behavior.  I’ll save more on this for a future post dedicated to an upcoming publication by my colleague Loma Pendergraft, who spent his MS learning this fact the hard way.  But suffice it to say that any progress on this front in the various Corvus species is groundbreaking news.  We do, however, know more about raven calls. For example long “haa” calls are thought to recruit other individuals to sources of food.  What was unknown at the start of this study was whether these calls encoded any class-specific information about the caller, such as their age or sex. Calls that impart class-level information about the caller have been previously demonstrated in some marmots and monkeys.

Methods: The researchers recorded hundreds of “haa” calls from wild ravens which had previously been color banded and whose age and sex were known.  Using acoustic software they analyzed the vocalizations for patterns in call elements like frequency and inflection rate.

Key findings: As the study’s title suggests, ravens appear to encode information about their age and sex in “haa” food calls.  For animals like ravens that live in “fission-fussion” social systems, meaning flexible social groups where individuals regularly reencounter familiar individuals, but also encounter unfamiliar ones, class-level information helps individuals quickly assess important aspects of a caller’s identity.  Such information may be key to helping individuals decide if they want to join a feeding event or not.  This decision is particularly important because aggression at feeding events can cause mortal injury, so grouping with a bad crowd can come at a high price.

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4. Kroner A, and Ha R. (2018). An update of the breeding population status of the critically endangered Mariana Crow (Corvus kubaryi) on Rota, Northern Mariana Islands 2013–2014. Bird Conservation International 28: 416-422 

Background: The Mariana crow or Aga is a native species to the islands of Guam and Rota.  After the introduction of the brown tree snake to Guam in the 1940’s, Guam’s entire population of Aga were wiped out leaving only those found on Rota to continue the species.  In 1982, the population hovered around 1,300 individuals but things were clearly in decline. In 1984 the Aga was officially listed as endangered and today is considered critically endangered by the IUCN.  Unlike on Guam, there is no clear reason why the Aga continues to decline on Rota, though habitat loss, persecution by humans, natural disasters and introduced predators like cats likely all work together.

Methods: During 2013-2014 researchers counted breeding pairs by surveying all known island territories.  During these counts (which took 845 hours of labor and traversed 1,485 hectares!) the researchers also documented any unpaired or subadult birds. Since the entire island could not be surveyed, to ultimately estimate the population size the researchers used models that accounted for missed detections.

Key findings: Spoiler alert: They are A BUMMER.  In all that searching only 46 breeding pairs were detected.  Accounting for unpaired birds and detection failures, the researchers estimate that the current population of Aga hovers around 178 individuals.  Obviously that number alone is a gut punch but it’s especially true when you consider that that’s a 10-23% decline since 2007 and a 46-53% decline since 1998.  Researchers estimate that at least 75 pairs are needed to maintain a viable population of Aga.  Without intensive predator management and community level advocacy for these birds, their future is sadly looking grimmer and grimmer.

5. Walker LE, Marzluff JM, Metz MC, Wirsing AJ, Moskal ML, Stahler DR, and Smith DW. (2018). Population responses of common ravens to reintroduced wolves. Ecology and Evolution 8: 11158-11168

Background: One of the most persistent myths about common ravens is that they have a symbiotic relationship with grey wolves; intentionally showing them carcasses they find and then sharing in the bounty together.  But while the case is actually that ravens are unwelcome dinner guests at the wolves’ table, there’s no question that the two species have profound effects on one another. The reintroduction of wolves to Yellowstone in 1995 therefore offers a valuable way to study how the presence of wolves affects the spatial distribution and feeding behaviors of park ravens.

Methods: This study was a collaborative effort between avian and spatial ecologists at the University of Washington and Yellowstone wolf biologists.  Using data from 2009-2017 on wolf abundance and prey kills, and raven surveys taken both within the interior of the park and at anthropogenic food sources in surrounding areas (ex: the Gardner town dump), the researchers were able to model raven abundance during both the study period and before the reintroduction of wolves.  I won’t go into the details of how these models are created, but suffice it to say that their purpose is to take the data you give them and find what predictors best explain your observed outcomes.  For example if, say, you have a bunch of data about where ravens were located at different times, and have data on different possible predictors, say, wolf abundance, weather, carcass abundance, carcass biomass, and distance to anthropogenic food, etc., the right model could help you identify that carcass biomass is the best predictor of raven abundance.

Key findings: Previous studies have demonstrated that wolves make more kills during severe winters with higher snowpack, because prey have a more difficult time evading them.  As a result, the researchers hypothesized that ravens would depend more heavily on wolf kills during severe winters,  but this is not what they found.  Instead, Yellowstone ravens seem to lean more on consistent, anthropogenic food sources during tough winters, but lean more on wolf provided carrion during more mild winters.  Still, the presence of wolves has increased and stabilized the number of ravens in the park, because they provide a second year-round source of food, in contrast to human hunter provided kills which are seasonally limited.  These findings are yet another demonstration of the value of top carnivores in stabilizing food webs and providing food for a cascade of creatures.

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6. And as a bonus let’s not forget the most important 2018 study of them all, “Occurrence and variability of tactile interactions between wild American crows and dead conspecifics,” which you can read all about here. 😉

leslie

 

 

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Filed under Conservation, Crow behavior, crow intelligence, New Research, Raven behavior, Ravens, Science

RAVENous for crow eggs 

Given their similarities, it might surprise folks to see crows occasionally harassing and chasing ravens. After all, birds of a feather right? Not in this case.  Rather than being in cahoots, the relationship between crows and ravens is most often competitive, though it can also be predatory.

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A raven barrel rolls to scold an attacking crow.

Such is the case in a recent video shared with me by a reader, Ty Lieberman.  To the dismay of him and his colleagues, a crow nest they had been observing outside their Los Angeles office window was partially dismantled, and at least one egg taken by what they believed was a pair of crows.   Concerned for the survival of the nest, Ty reached out for my interpretation.  Based on his initial description, I wondered if maybe he had witnessed egg transport, something I knew had been observed in black-billed magpies and pinon jays.1  Previous accounts of these species included descriptions of eggs being taken, and then returned to the nest, as well as eggs being deposited into the nests of neighbors, both of which are utterly fascinating behaviors and probably warrant their own post.

To date, however, there are no accounts of crows engaging in this behavior, though there is one documented observation of a nestling being deposited into a nest from which it did not originate.2  Again, utterly fascinating, but not helpful here.

Later, a more detailed account from Ty made mention of the size of the intruding birds, which quickly led me to the story’s true explanation.  Shortly after my ‘ah ha’ moment, to the dismay of he and his colleagues the nest raiders returned, and this time were caught on video by one of Ty’s colleagues (who you can follow on twitter, @namnam).  Rather than being crows, these literal homewreckers were common ravens.

Instead of being something out of the ordinary, Ty had witnessed a typical breeding season interaction between crows and ravens.  It’s no wonder then, that crows can be so hostilie when ravens enter their territory. 

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Crows (top) mobbing a raven (bottom) in Kent, WA

Eggs of all kinds are one of the most power-packed meals in the animal kingdom, so it’s no surprise ravens would take advantage of crow nests when they find them.  Around this same time back in 2015, a black bear made a similarly memorable meal out of a raven nest, reminding us that for corvids of all kinds, it’s a constant fight between being predator or prey.

Literature cited

  1.  Trost CH and CL Webb. 1986. Egg moving by two species of corvid. Animal Behaviour 34: 294-295.
  2. Schaefer JM and Dinsmore JJ.  1992.  Movement of a nestling between American crow nests.  The Wilson Bulletin 104: 185-187

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Filed under Birding, Breeding, Crow behavior, Raven behavior

I spy with my raven eye…

…someone trying to steal my lunch.  Turns out, humans are not the only ones wary of peeping Toms; new research shows raven can imagine being spied on by a competitor.

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***

The other day my friend and I were having a very merry time at the thrift store when, without cause or provocation, this women decides to up and ruin our trip.  Well really, she simply spotted the same gorgeous caste iron dutch oven that my friend wanted and reached it first, but the consequence was the same (it was a tragically beautiful dutch oven). This dynamic-my friend having her own intentions (to obtain and own that dutch oven for herself) and recognizing that this other women had her own intentions (to obtain and own that dutch oven for herself) is something so second nature to being human we rarely give it any thought.  But the ability to attribute mental states to those around us is an incredibly profound and complex cognitive task.  Understanding if this ability, called Theory of Mind, exists in other animals has been among our top interest as ethologists.

Like other corvids, ravens cache food and, as a consequence, run the risk of their caches being stolen by others.  It has long been known that if ravens can see that they are being watched, they behave differently when it comes to caching than if they are alone.  This is interesting, but doesn’t necessarily speak to whether they posses theory of mind because of the confounding effect of “gaze cues”.   Basically, the correlation between head cues and competitor behavior make skeptics doubtful about non-human animals having the ability to know what others might be seeing.  So raven master Thomas Bugnyar and his colleagues Reber & Bruckner recently published an elegant study to address just this issue.

By training captive ravens to look through a peephole, and then allowing them to cache food with the peephole opened or closed, the researchers were able to show that ravens behaved as if they were being watched when they could hear ravens and the hole was open, but not when they could hear ravens but the peephole was closed.  What this suggests is that ravens are capable of remembering their own experience of looking through a peephole to see into another room, and can imagine that another bird might be doing the same thing even if they cannot see this bird.

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Experimental set up.  Bugnyar et al. 2016.  Nature Communications

Theory of mind and imagination (which are not mutually exclusive) are the cornerstones of what makes for a powerful cognitive toolkit and have long been thought to be uniquely human.  As we continue to build on the body of work showing non-human primates, corvids and some other animals posses some of the same skills we do, many will be challenged to redefine what it means to be human.  Personally, framing the question that way doesn’t interest me.  To me the more interesting question is not how are humans different from ravens, but how are we the same and why? What is it about being human and being raven that make possessing imagination important?  Fortunately there is still loads more research to be done, and when it comes to teasing out this question I can only imagine the possibilities.

Literature cited:

Bugnyar, T., Reber, S.A., and Buckner, C.  (2016) Ravens attribute visual access to unseen competitors.  Nature Communications 7.  doi:10.1038/ncomms10506

 

 

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Filed under Crow behavior, crow intelligence, New Research, Raven behavior, Raven intelligence

Raven nest provides tasty meal for Alberta bear

I’ve talked before about how the claim that crows (and ravens) are “destroying the ecosystem and songbird populations” is mostly unsupported by science.  Breeding plovers and desert tortoises are among the handful of exceptions1,2.  Nevertheless I still see, even in the comment threads of this very blog, people claiming that corvids are out of control and have no predators.  If it wasn’t such a misguided and ultimately dangerous sentiment I might just ¯\_(ツ)_/¯ whenever folks claimed that crows and ravens have no natural predators because those of us who spend even a small amount of time observing them in the summer will know this is anything but true.

Eggs and baby birds are a key summer food source for lots of animals and, while seeing a downy little gosling in the mouth of an arctic fox makes me cringe a little, knowing a healthy population of breeding birds is helping to sustain a community of predators is the kind of ecological balance that, in the long run, makes my heart sing.

Stillframe from Planet Earth

Stillframe from Planet Earth

Corvids are part of this system too, which means their babies are also getting eaten.  Usually it’s by things like hawks, eagles, owl and racoons but a recent video taken in Alberta shows yet another predator we can bear in mind.  It’s never gonna be fun to see the birds I care about taking a hit like this, but knowing that corvid babies are helping to sustain top predators only deepens my love and appreciation for them.  Predators and prey make the world go round and corvids have the badass role of being both.

Photo: Linda Powell

Photo: Linda Powell

Literature cited:

1. Johnson, M. and Oring, L.W. 2002.  Are nest enclosures and effective tool in plover conservation? Waterbirds 25: 184-190

2. Kristin, W.B. and Boarman, W.I. 2003.  Spatial pattern of risk of common raven predation on desert tortoises.  Ecology 84:2432–2443

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Filed under Ecosystem, Raven behavior

Best books for corvid lovers

This post was prompted by someone on my twitter feed who asked that I put together a reading list for people who want to learn more about corvids; a totally kick-ass idea if I say so myself.  The following are all the books I have read and can speak personally to, however, I’m sure there are others and I encourage folks to add them in the comments section.  As a preface, I’ll remind readers that John Marzluff is my graduate adviser, nevertheless, I assure you that I genuinely believe he is a fantastic writer and my review of his books are not inflated in the hopes of getting approval on my dissertation. 🙂  So without much further adieu, here’s a list of all the corvid books I’ve read with a brief synopsis of the material and my recommendation.

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In the Company of Crows and Ravens by John Marzluff and Tony Angell
If watching, feeding or rehabilitating corvids is something you do in your free time, consider this your crow bible.  Curious how long crows live?  What they do as juveniles?  The sounds they make?  The ways the interact with people?  It’s all in there.  This book remains my go-to guide for general crow knowledge.  Yet, despite the fact its backbone is rigorous science, it’s written in a way that feels very easy to digest.  John and Tony wrote it with the intent that it would be for a wide audience and I think they achieved that beautifully.  After reading this book, I have no doubt you’ll have a deeper understanding for these birds, not to mention a new admiration for Tony’s artwork.  I even used one of his drawings for the book on the invitations for my wedding (with permission, of course).

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Dog Days Raven Nights by John and Colleen Marzluff
This is the book I most often recommend to my own friends and family.  Not because it offers superior or more easily read  information on corvids, but because this book gives you the best insight into what it really means to do fieldwork.  Nearly the entirety of the book focuses on the period of time after John and Colleen had finished their graduate work in Arizona, and were conducting a post-doctoral study on ravens with Bernd Heinrich in remote Maine.  It’s organized as a back and forth between John and Colleen, which means you get two perspectives on the raven work and Colleen’s development as a dog sledder and trainer.  As a reader, you experience what it means to completely dedicate every moment, piece of sanity, and dime you have on conducting a field experiment and you walk away with a much deeper appreciation for how difficult it is to answer questions of animal behavior.  If the human dimension of science isn’t your interest, however, fear not.  The book is still loaded with fascinating information on ravens including, in my opinion, one of John’s most important contributions which is information sharing among ravens.  An excellent read for sure.

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Gifts of the Crow by John Marzluff and Tony Angell
For those looking for a more scientifically dense reading on crow behavior and neurology this is the book for you.  It doesn’t make for the lighthearted Sunday reading that ITCOCR does, but it still satisfies the trademark Marzluff style of mixing rigorous science with the anecdotal stories of crow behavior that makes us love them.  If you’ve been fascinated by the story of Gabi Mann, the little girl who feeds and gets gifts from crows, then this is the scientific background you need to see the whole picture.

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Mind of the Raven by Bernd Heinrich
Long before John Marzluff started writing books, his post-doc adviser, Bernd Heinrich, was already an expert at the game.  Heinrich has a reputation for being one of the most eloquent and engrossing natural history writers and it’s a reputation that’s been well earned.  Mind of the Raven is actually what initially peaked my interest in corvids, so in many ways I have this book to thank for the work I am doing now.  For anyone who lives with ravens, or simply has a fascination for them, I can’t recommend it enough.  Bernd’s writing will nurture your passion and give you the science to back up what you already know: ravens are badass, awesome animals.

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Crow Planet by Lyanda Lynn Haupt
Crow Planet it a book best characterized by Haupt’s journey to find curiosity and loveliness in an increasingly urban landscape where the natural world can feel further and further away.  Crows therefore, offered the perfect vehicle for looking at and appreciating what remains when the forests retreat and box stores and neighborhoods take their place.   By the author’s own admission her journey through writing Crow Planet made Haupt appreciate, “but not quite love”, crows.   Despite this, she manages to write about them with grace, and her stories will make even the biggest skeptic take another look at these animals.  Although Haupt’s background is not in science, she doesn’t omit the scientific facts, though she does take more artistic liberty when describing their antics than John or Heinrich do.  All that being said, this is an excellent book for the urban naturalist or crow watcher.

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Crow by Boria Sax
If you’re interested in crow mythology this is the book for you.  Sax takes you through time and space to explore the role of corvids in human myth, religion and art.    His thoroughness is without compare, but if anthropology is not your interest this book will prove taxing.  It’s one I happily keep on hand, but not one that I’ve ever had the patience to read all the way through.  Nevertheless, I probably should, since it’s chalk full of information and historical context that I would be better off knowing.

Bird brain

Bird Brain: An exploration of avian intelligence by Nathan Emery
Although not exclusively dedicated to corvids, Bird Brain, written by corvid cognition expert Dr. Nathan Emery, offers an incredible look at the minds of your favorite birds.  Although his book tackles some of the more difficult concepts of avian cognition, it feels and reads more like a coffee table book, complete with beautiful artwork, some of which was done by Emery himself.  Each chapter is themed around a particular aspect of cognition (communication, spatial memory, etc.) and walks the reader through the fundamental biological principals and samples the most interesting studies that have been done on the topic.  The book is rich with the kinds of analogies and descriptions that break through the barriers of dry scientific writing.  Perfect for the budding young scientist or the long time corvid fan.

I’m sure there are many others I haven’t read which subsequently didn’t make this list.  Feel free to make recommendations in the comments section!

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Filed under Crow behavior, Crow curiosities, Crows and humans, Raven behavior, Raven intelligence

The politics of ravens

With the (insert relevant election cycle) coming up, a 2014 publication from Massen et al. reminds us that we are not alone in our affection/affliction for political scheming. No, ravens don’t hold elections or engage in thinly veiled attempts at voter suppression, but they do build relationships and attempt to thwart the alliances of others.   Massen’s team observed the interactions of hundreds of individually marked birds and categorized their relationships as either (1) breeding pair with territory, (2) strongly bonded w/o territory, (3) loosely bonded w/o territory or (4) nonbonded.   Ravens build bonds by participating in affiliative behavior such as allopreening (mutual grooming), and interrupting these interactions can be risky if the pair fight back. Over six months they recorded affiliative interactions and found that 18.8% were

Photo credit: Jorg Massen

Photo credit: Jorg Massen

interrupted by a third bird, giving rise to their curiosity over the function of these interventions.  Given that ravens have been previously shown to track the dominance relationships of others, they wanted to know if there was a relationship between bonding status and the likelihood of being an intervener or being intervened.  In both cases they found that there was a significant relationship between the two.  Birds who were either a pair or at least strongly bonded were most likely to intervene, and those attempting to forge loose bonds were the most likely to be interrupted.  The researchers interpreted these results to show that strongly allied birds attempt to preclude the threat of competition by squashing the alliances of future coalitions.  Importantly, they were less likely to bother with nonbonded birds since, for now, their social ties are too weak to become threatening.  Conversely, they also stayed out of the interactions of other pair or strongly bonded birds since these interventions pose a greater physical risk to the third wheel.

These results are quite intriguing but there’s still a lot to vet and better understand before they will be widely accepted.  Critically, there is no immediate benefit to the intervener.   Embracing the results of this study requires accepting the idea that an animal, especially a bird, is capable of putting future rewards ahead of current risk or losses.  Loose bonds are too weak to act as a competitive threat, so this effort on behalf of the intervener is only useful if you assume that those loose bonds will become a threat if allowed to flourish and become stronger over the course of days, weeks or months. Risking a fight now to thwart a relationship that only may be problematic next month is a big temporal leap.  As humans, this kind of future planning is an ability we take for granted but it’s quite a cognitive feat.  Although the scales are beginning to shift, I imagine we have many more studies to go before results like this are considered representative of the temporal flexibility of ravens and other corvids.

  1. Jorg J.M. Massen, Georgine Szipl, Michela Spreafico, Thomas Bugnyar. Ravens Intervene in Others’ Bonding Attempts. Current Biology, 2014; DOI: 10.1016/j.cub.2014.09.073

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Filed under New Research, Raven behavior, Raven intelligence