Eureka! Canny crows figure out water displacement to get noms

March 31, 2014 • 6:19 am

In the Aesop fable “The Crow and the Pitcher,” a thirsty crow manages to get water out of a near-empty jar by dropping pebbles into it, raising the water level so he could reach it with his beak. The moral was “Little by little does the trick.”

That fable is a title reference in a new a new paper in PLoS ONE by Sarah Jelbert et al. (reference and free download below), showing that crows can not only displace water this way—in this case to get a treat, not a drink—but also understand some principles of water displacement: use heavy rather than floating “stones,” avoid hollow objects, use vessels where the water level is higher rather than lower, and—it doesn’t work with sand.

The authors used six wild New Caledonian Crows (Corvus moneduloides), a species already known for its smarts and its ability to use tools (see my earlier post here, which has some nice videos). The experiment was in fact conducted in New Caledonia, and I’m pleased to read that the birds were released back into the wild after the study.

The birds were first trained how to drop stones to achieve ends, but not to raise water levels. The training involved getting them to drop stones to collapse a Plexiglas platform, giving the birds a reward.  After they learned the concept of dropping stones, they were then given six tasks involving water displacement. They passed four of them, and it’s a remarkable achievement. I

‘ll briefly summarize the four tasks. But be sure to watch the video below, which summarizes the behaviors nicely.  Here is the experimental setup, showing the six experiments designated with letters, as below:


A. Water versus sand. The birds were presented with a treat (a piece of meat affixed to a cork) in a clear tube filled with either sand or water. The level of both sand and water were too low for the crows to reach. Result: the crows preferred to drop stones into the water rather than the sand. This was done from the outset rather than by simple learning, and the crows were given a chance to inspect the tubes before starting their task.

B. Sinking versus floating “stones”.  The birds were given a choice of using light versus heavy objects (the light ones, made of polystyrene and identical in appearance to the heavy ones, floated, ergo didn’t displace water) to raise the water level to get their noms.  Again, they succeeded, rejecting the light stones from the outset. The birds were able to perceive the weight differences, rejecting the light ones without even dropping them into the water. This shows that they have a concept of water displacement, a concept they probably don’t deal with in nature.

C. Solid versus hollow “stones”.  Again the birds were given choices between objects of similar size and weight, but half of them were hollow, displacing only 2 mm of water in the tube versus 7 mm for the non-hollow “stones”.  And again, the crows preferentially used the non-hollow ones.

D. Narrow versus wide tubes with similar water level.  In this case it is better to put stones in the narrow tubes, as the water rises more quickly. But here the crows failed! In fact, they dropped significantly more objects into the wide than the narrow tubes: it took 7 stones to get the treat from the wide tubes as opposed to only 2 in the narrow tubes.

E. Different water levels in narrow versus wide tubes. I’m not sure why the researchers used tubes of different diameter if they simply wanted to test water level, as there were two factors conflated here. Nevertheless, the water level was set higher in the wide tube (only 6 mm below the crow-reachable distance) than in the narrow one (50 mm). In the narrow tube, though, it was impossible to raise the water level high enough to get the treat. Here the crows succeeded: although all started by dropping stones into the narrow tube, they learned quickly that it didn’t work.

F. Tricky connected tubes! Here, as you see in the diagram above, there were three tubes: a central one containing the floating treat, and two ancillary tubes. One of these was connected to the central tube in a hidden way, below the base, and the other wasn’t. Since the central treat-containing tube was too narrow to drop stones in, the water level in the treat tube could be raised only by dropping stones into one of the side tubes (marked with different colors). Again the birds failed, dropping about half the stones into each of the two side tubes. They didn’t learn, even though they could have, that only one of the tubes worked.

What do the results show? First, that Caledonian crows are smart, but we knew that already. More important, as the authors note, the results suggest that the birds have a “causal understanding of displacement, but this understanding has limits.” They also note that studies of displacement haven’t been done in other species, most notably primates, where it would be really interesting to replicate this study. They also note that this level of reasoning ability makes the crows equivalent to a seven-year-old child, though those eight and older usually figure out things like the tricky three-tube experiment. Still, that’s pretty impressive for a bird!

What about the two failures? In the three-tube experiment, the authors suggest that the crows simply had limited visual ability to assess the situation, i.e., were “less able to use perceptual-motor feedback when they need to focus their attention on more than one location.” Perhaps, they note, the experiment could be made more compact so that all tubes are in the birds’ line of sight.

It’s harder to explain why the birds failed in the wide- versus narrow-tube study. Note that this was a general failure: none of the birds preferred the narrow tubes, as opposed to the other studies in which successes were general among all birds.  The authors float (pardon the pun) several explanations, but none are immediately convincing. Perhaps we need to know what it is like to be a crow!

I’ve probably just wasted a lot of time writing the above, for here’s a video clearly showing the behaviors. Though it’s short, notice that the crows inspect the tubes before deciding what to do. You can almost hear their little crow brains working!


h/t: Diana MacPherson (who’s wondering whether crows can figure out how to reverse rolls of toilet paper)


Jelbert SA, Taylor AH, Cheke LG, Clayton NS, and Gray RD. 2014. Using the Aesop’s Fable Paradigm to Investigate Causal Understanding of Water Displacement by New Caledonian Crows. PLoS ONE 9(3): e92895. doi:10.1371/journal.pone.0092895

Jelbert SA, Taylor AH, Cheke LG, Clayton NS, Gray RD (2014) Using the Aesop’s Fable Paradigm to Investigate Causal Understanding of Water Displacement by New Caledonian Crows. PLoS ONE 9(3): e92895. DOI: 10.1371/journal.pone.0092895Read more at:


Jelbert SA, Taylor AH, Cheke LG, Clayton NS, Gray RD (2014) Using the Aesop’s Fable Paradigm to Investigate Causal Understanding of Water Displacement by New Caledonian Crows. PLoS ONE 9(3): e92895. DOI: 10.1371/journal.pone.0092895Read more at:
Jelbert SA, Taylor AH, Cheke LG, Clayton NS, Gray RD (2014) Using the Aesop’s Fable Paradigm to Investigate Causal Understanding of Water Displacement by New Caledonian Crows. PLoS ONE 9(3): e92895. DOI: 10.1371/journal.pone.0092895Read more at:

41 thoughts on “Eureka! Canny crows figure out water displacement to get noms

      1. I don’t know but I have plan to enlist a whole murder of crows to carry out my toilet paper orientation domination plans.

    1. My hometown has a crow infestation (not sure if that is the right term), they can sometimes blacken the sky (only a little hyperbolic). It can be quite frustrating because they end up ripping up garbage bags and spreading garbage everywhere. The city has spent quite a large sum of money to get rid of them but I think the crows have out smarted them every time.

      1. Surely a falconer could help disperse them? They work every year at Wimbledon to scare away pigeons during the tennis…

        Can I reccommend this book – Crow Country by Mark Cocker.

  1. I once observed a dove building a nest in a pine tree. She (or he?) would fly down to the ground, pick up a pine needle, drop it, then pick up another one of a different length. She would do this several times before selecting one and flying back up to the nest. How could she have done this without the ability to remember what length she needed for the particular spot where she planned to put it? My attitude toward animals changed that day. I started viewing them as more intelligent than I had previously thought. I no longer think of them as “dumb animals”.

  2. If crows can figure out how to reverse tp rolls, then undoubtedly other crows can be trained to put them back the right way.

  3. I feel compelled to include an Ancient Greek lesson – heureka (εὕρηκα) is Greek for “I have found it” the 1st person singular perfect indicative of heurisko, “I find”. Greek doesn’t have “h” they have “rough breathings” in front of vowels which give an “h”. I guess in English transliteration we ignored the rough breathing & didn’t add an “h”.

      1. Perhaps though the correct transcription would include the “h”. The word “halo” comes to mind.

        1. Greek words that start with the rough breathing are generally transliterated with initial ‘h’. E.g., helios, Hera, Hermes, hoi polloi. I did a little poking around but can’t come up with a definitive reason why “eureka” is an exception. Wikipedia says that the initial ‘h’ is retained in (e.g.) Finnish, Danish and German.

            1. I don’t think so, but that wouldn’t explain it in any event. Greek loans into Latin were also spelled with initial ‘h’ for the rough breathing. For example, we get “hematite” from Latin “haematites”, which in turn was from Greek “haimatitês lithos”.

          1. ….& my favourite one, hybris or hubris depending on your y or u transliteration preference.

      1. So do Indian mynah birds (an introduced species here in NZ, though they may have introduced themselves). I always try to run them over, in the knowledge that I’ve never succeeded yet and probably never will. What is especially insolent about their behaviour is, they don’t fly away, just estimate the speed and course of your car and walk casually to the side of the road just before you reach them.

  4. For E you say “all started by dropping stones into the narrow tube,” but the crow in the video starts with the wide tube.

  5. I think E would have been more interesting if the crows had passed D.

    Both seem to me to be tied up with conservation (of volume) which is a developmental landmark that’s been extensively studied in the psychology of young children.

    D asks “can you see that the wide tube has a lot more volume to go even when the water levels are the same” they apparently couldn’t.

    E asks, can you take into account both height and diameter when comparing volumes? It rules out a simple preference for the narrower tube always. Since the crows didn’t solve D and didn’t seem to prefer narrow tubes anyway, it wasn’t very interesting by itself, IMHO.

    In both cases. the volumes being compared are the volumes remaining in the container, not the volumes present.

    1. I’m not sure I’d call D a failure. Maybe the crows were just optimizing for something other than number of stones dropped.

      As someone who has used long-nosed pliers to fish small parts out of the bottoms of computer cases, I can testify that it’s much easier to do so in a wider space than in a narrow space. Maybe the crows prefer the wider tube for similar reasons, and simply don’t care that it takes more stones to bring the treat within reach, so long as they have adequate room to grab it when it gets there.

      1. They weren’t optimizing for anything, as they seemed to choose a tube at random and stick with it until completing the task.

        From the paper:
        “Importantly, on their first object drop per trial, across 20 trials, birds showed no preference for either the narrow or wide tube (narrow tube chosen first on 56% of trials, binomial test, p = 0.27), and individually no bird dropped significantly more objects into either tube. Birds showed no sign of developing a preference for the narrow tube over 20 trials”

  6. Great study. Just one tensy quibble – my Aesop’s has the moral being something like “sometimes a little cleverness is all you need to solve a problem.” Don’t have the exact wording with me, but it was supposed to be about the value of using your brain, not about the value of step-by-step solutions.

    Incidentally,I would put Aesop’s fables in the top 10 literary sources which have lead to common English turns of phrase. There’s the Bible, some of Shakespeare’s plays, and then Aesop. Its pretty amazing going through them and realizing just how many references to them we still make, probably in most cases without realizing where the reference comes from.

  7. Reblogged this on Evidence & Reason and commented:
    Rather than rewrite everything myself I will just reblog this post on a new study of crow intelligence that I found on WEIT. For what it’s worth, I believe there was a paper a year or two ago that found human children performed about the same in these tasks as crows did. I will have to go reread it before I can say for sure though.

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