Many scientists think that corvids – the family of birds that includes crows, ravens, rooks and jays – may be among the most intelligent animals on earth, based on their ability to solve problems, make tools and apparently consider both possible future events and other individuals’ states of mind.
“There’s a lot of research that has been done with both ravens and crows because they are such intelligent species,” said Margaret Innes, an assistant curator at the Maryland Zoo in Baltimore.
Even in humans, defining and measuring intelligence is difficult, and it’s more complicated in other species, which have very different body shapes and have evolved for their niche in the environment.
However, scientists who study cognition have defined a few measures of intelligence: recognising oneself in a mirror, solving complex problems, making tools, using analogies and symbols, and reasoning about what others are thinking.
For a long time, biologists expected most of these mental feats to be unique to primates.
The great apes – chimpanzees, orangutans and gorillas – succeed at nearly all of these tasks, from making and using tools to learning large vocabularies of symbols, as well as recognising themselves in mirrors.
A select few other mammals also meet most of the accepted criteria for intelligence.
Dogs and dolphins, for instance, are very good at tasks involving social intelligence, such as communication, conflict resolution and reasoning about what others are thinking.
Dolphins are also capable of basic tool use – for instance, carrying sea sponges in their mouths to shield their noses from scrapes and bumps as they forage on the ocean floor.
However, the greatest intellectual rival to the brainy apes may be a noisy scavenger with a sharp beak, bright eyes and a brain about the size of a walnut: the crow and its corvid relatives.
Crows and ravens are clever problem-solvers, expert toolmakers and adept social movers, but scientists haven’t reached a consensus about how corvid minds handle abstract thinking or how closely their mental processes resemble those of humans.
Researchers from the University of Iowa in the US and Lomonosov Moscow State University in Russia reported early this year that crows can use analogies to match pairs of objects. To reach that conclusion, the scientists trained crows to recognise whether two objects were identical or different, which the birds indicated by pressing one button when shown pictures of objects that matched and a different button when the objects didn’t match.
Once all the birds were good at matching objects, researchers showed the crows images of pairs of objects. Some images depicted matched pairs, while others depicted two mismatched objects with different shapes or colours. In response, crows could press buttons to choose between a matched pair or a mismatched pair.
The researchers wanted to see if crows could figure out the relationship between pairs of objects and then choose a pair with the same relationship: matched or mismatched. For instance, a crow looking at a mismatched pair would then select the mismatched pair from their response choices. Nearly 78 per cent of the time, the birds succeeded. According to the researchers, the birds recognised that the relationship between the two pairs of objects was the same. In other words, they were making analogies.
Other scientists contend that a type of reasoning less sophisticated than analogies could have produced the same results. For instance, the crows in the analogy test could have simply chosen images with similar characteristics, such as objects of the same colour, instead of reasoning about the relationship between the objects, to get the correct answer.
Some behaviours, such as those employed in the analogy test, could have more than one explanation, and until recently, scientists could only see what the birds did, then make inferences about the mental processes behind the behaviour.
Now, researcher John Marzluff and his colleagues at the University of Washington are using positron emission tomography, or PET, scans to study which parts of a crow’s brain are active when it performs such tasks as recognising friendly and unfriendly birds. And he says that another team of researchers, at the University of California at Davis, is preparing to use the same technique to study the brain activity of New Caledonian crows, a species that makes sophisticated tools. The team hopes to see the crows’ brains at work while they’re crafting tools.
Corvids seem to understand that other birds have minds like theirs, and their decisions often take into account what others might know, want or intend, according to several studies of crows, ravens and jays.
Psychologists call this a theory of mind, and it’s a fairly sophisticated cognitive ability. Humans don’t develop it until late in childhood. Crows and their fellow corvids are social animals, much like primates, so theory of mind probably offers significant evolutionary advantages.
For one thing, it may help prevent food theft. Crows and ravens often hide food in caches and retrieve it later.
“You can actually see them watching both the other birds that they are with and the humans, and if they sense that they have been seen, they will take that food and they’ll go and hide it somewhere else,” Innes said of the Maryland Zoo’s ravens.
The birds appear to realise that watchers will know where they’ve hidden the food and might use that knowledge to steal it later.
Studies of several corvid species have documented this re-caching, as it is called. Sceptics of the birds’ advanced intelligence say simpler mental processes might prompt re-caching, such as making an association between being seen and later having a cache stolen.
However, Innes is convinced that the re-caching is a sign that ravens have a theory of mind, based on her observation of re-caching behaviour in ravens at the Maryland Zoo. “Definitely,” she said. “I think it definitely indicates that.”
Other test results are harder to dismiss as simple association. When researchers in Austria hid food behind a partition, ravens found it, apparently by noticing where the humans were looking and following their gazes to the hidden food.
“You’re using the person’s gaze to infer information about something you can’t see,” Marzluff said.
Brain imaging studies could settle the question, Marzluff said, because advanced cognition in all animals uses different areas of the brain than simpler associative learning.