Nature
Corvid intelligence has passed another test with flying colors, and thrown Aesop's Fables into a whole new light. New Caledonian crows have been shown to understand liquid displacement and the difference between solid and hollow objects.
The intelligence of crows is a matter of legend. Aesop's fables include a thirsty crow who finds a pitcher of water. The water is too far down for its beak to reach. The crow takes pebbles and drops them in the pitcher until, “At last, at last, he saw the water mount up near him, and after casting in a few more pebbles he was able to quench his thirst and save his life.”
Aesop's moral was “Little by Little does the trick.” However, when Sarah Jelbert from the University of Auckland, confronted crows with variations on this problem the conclusion might be, “Corvus moneduloides are really fucking smart.” The crows were even able to distinguish which objects would raise the water level best. In PloS One Jelbert concludes, “New Caledonian crows possess a sophisticated, but incomplete, understanding of the causal properties of displacement, rivaling that of 5–7 year old children.”
Previous experiments have shown that rooks are not only able to grasp the concept of dropping stones into water to raise its level, but realize that large stones work better than small ones, and that sawdust cannot be raised in the same way.
New Caledonian crows are the master toolmakers of the bird world, unlike rooks, which don't make tools in the wild. Jelbert put six crows through six variations of the fable. Two “lacked motivation” and were returned to the wild without attempting all the tests, but the others lived up to their reputation as avian geniuses.
The birds in the study were wild crows, although New Caledonian crows have been the subject to so much intelligence testing its tempting to imagine the population might be starting to pick up extra tricks from researchers. For pre-season training they learned how to to drop stones into a tube to collapse a platform which would allow them to access a treat.
In each of the experiments, rather than get the crows thirsty enough to want to drink the water, the experiment was designed with a floating food reward.
The first test required the crows to work out that dropping stones in water was effective while putting them in a tube of sand was not. All caught on, although some were quicker than others. The second tested their recognition that objects that float would not help them reach their goal, and again the crows all worked this out – usually discarding floating objects when they picked them up, while making full use of those that would sink.
Similarly those birds still trying were able to work out that hollow objects were no use to them, choosing solid ones almost 90% of the time. Two crows may have worked this out without even trying – they never even dropped a hollow object into the tube to see if it worked.
If you're feeling intimidated by the crows' brilliance it may come as a relief to discover they were not able to work out that it was more efficient to drop stones into a narrow tube than a wide one.
In the fifth experiment each crow concluded that a tube with an already high water level was a better bet than one where the water was lower, even though the latter was narrower.
Jelbert discusses the intriguing question of why the crows were able to work out the value of using solid objects, but not the benefits of a narrower tube.
“This result could have stemmed from differences in the size, proximity or quantity of the objects and tubes, or it could reflect the difficulty of recognising that the volume of a tube can be a relevant causal property, compared to the potentially more salient properties of solidity or weight. It could also reveal a more general principle: it may be easier for this tool-using species to recognise the functional properties of various tools than to recognise the functional properties of the substrates their tools interact with.”
A final test, where the tube containing the reward was connected through a hidden passage with one wide enough for stones to fit in, proved a step too far for even these crows. Jelbert says,
"These results are striking as they highlight both the strengths and limits of the crows' understanding. In particular, the crows all failed a task which violated normal causal rules, but they could pass the other tasks, which suggests they were using some level of causal understanding when they were successful."
It might be less amazing than the New Caledonian crow that used a series of tools to access a food in an eight step puzzle set by Jelbert's co-author Alex Taylor , but four crows worked these things out, not one. Still not quite as useful as a different species of crow putting litter in a rubbish bin, though.
The finding casts some doubt on a more famous Aesop fable, where a fox outsmarts a crow through flattery. And we're still a bit suss of whether a tortoise really beat a hare.
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