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Posted by on Oct 1, 2012 in Nature | 1 comment

Those Crafty Corvids: What will they think of next?

American Crow on the roadside (National Zoo)

“Crows know which side of the white line to stand on.”  That’s what my dad told me decades ago. He was talking about crows on the highway, and sure enough, in all my years of driving and watching crows, I found he was right.

Pay attention some time. If a crow is on the highway chowing down on tasty road kill, and a car comes, what does it do?  It doesn’t overreact and fly off in a frenzy. It doesn’t stand there and stare down your car until you hit it. No, it just walks off to the side of the road, steps across the white line, and waits for the cars to pass.  If the road kill is on the shoulder, outside of the white line, the crow won’t even look up as cars whiz by. When it comes to highways, crows understand the Line of Death.

That may not seem a monumental show of intelligence until you consider that the bounty available to the crows on the roadside exists solely because most animals don’t understand that basic rule.  Crows get it. Raccoons do not.

Crows belong to a family of birds called corvids. Sharing that taxonomy are ravens, jays, jackdaws, magpies and handful of other similar birds. And lately, it seems, corvids are all over the news. Apparently, knowing how to eat road kill without becoming road kill is the least of their talents.  Corvids are considered the most intelligent of all birds and rank high among all animals in terms of smarts.

Corvids are cosmopolitan in every respect, living happily in concert with humans. Rather than be threatened by human encroachment, these synanthropes thrive on the makings (and leavings) of mankind. Crows in particular are spectacularly successful at this. They live on every continent except Antarctica, almost always raising their young within a few miles of human habitation. They eat from our farms, our dumps, and our highways. They hang out in our parks, scold us from the tops of our homes, and laugh at our attempts to outsmart them.  As any farmer will tell you, a scarecrow is great at scaring away everything… except crows.

But it’s their intelligence that continues to floor us.  The canon of corvid intellect is vast, built firmly on a foundation of science, which I will come to. But the science is bolstered by anecdote and observation. In researching this post, I was surprised to see how many people in the comment sections of blog posts and news articles reported interesting corvid encounters.

Many spoke of rescuing and releasing injured or orphaned birds that stayed and became members of the family. In one case a couple bought a new house that came with a resident raven. Each day it stood at their front door and “asked to come inside”. They were terrified of it. Farm owners with habituated corvids related stories about the birds opening gate latches and releasing animals, mimicking the sounds of farm animals, stealing eggs from chickens and items from porches, and riding on farm vehicles, seemingly for the pure joy of it.

Corvids: Quite possibly the sharpest tool in the shed (Behavioral Ecol Research Group, Oxford)

Youtube is filled with clever corvid videos. A quick search for “smart crow” yields ravens and/or crows reeling in fishing line; outsmarting other animals including cats, deer, and eagles; making tools; using bait to fish; solving puzzles; And famously, dropping nuts onto crosswalks, waiting for the cars to crush the nuts, and then waiting for the light to change so they can retrieve their nuts safely.

But while watching corvids in the wild is fascinating, the controlled studies done with birds in the lab or the field are equally, if not more important. A well-designed study serves to isolate specific behaviors in order to narrow down exactly what type of intelligence the animals are displaying.

So, to get a glimpse of how we know what we know about corvids, here’s a rundown of some of the scientific work that has been done with the birds in recent years. To be sure, this is a mere tip of the iceberg  –

Self-Recognition Tests

A crucial step in the emergence of self-recognition is the understanding that one’s own mirror reflection does not represent another individual but oneself. (from Prior, et al)

The magpie mirror test (Prior, et al PLoS)

The Eurasian Magpie (Pica pica), a black and white corvid with a showy blue streak has been shown to recognize itself in the mirror.  Now this is no small feat.  It’s tricky to know it’s you in a mirror. Consider a kitten: No matter how many times you show it a reflection of itself, it still takes one look and thinks, stranger! Pfffttt!

Mirror self-recognition (MSR) is considered an indicator of self-awareness, which itself is considered an indicator of intelligence. In the MSR Test, an animal is exposed to a mirror and is allowed to investigate its properties. Once the animal is familiar with the mirror, the investigators paint a mark on the animal’s face or other area that the animal can’t normally see. Then they watch to see if the animal figures out that the paint smudge in the mirror image is actually on its own body. In other words, does it reach for the mirrored nose or its own nose.

Humans (older than about 1 year), great apes, elephants, dolphins and orcas are the only other animals that realize the creature in the mirror is them.

When provided with a mark, magpies showed spontaneous mark-directed behavior. Our findings provide the first evidence of mirror self-recognition in a non-mammalian species. (from Prior, et al)

Recognizing human faces

Anecdotes abound of corvids recognising people. In many cases it seems to be true. In the story about the crow that became a family member, the bird certainly recognizes the little girl on the playground with her friends. But the question that cannot be answered without doing a controlled experiment is this: What is it about the girl that the bird is recognizing? Is it her face (as we humans would presume) or something else?

Masks and faces used to test facial recognition in crows (Marzluff, et al)

Controlled studies on wild crows, mockingbirds, and magpies show that indeed, it is our faces. In the crow study, humans wearing face masks trapped, banded, and released wild birds. Before being trapped, only a few birds acted aggressively towards mask wearers. After being trapped, more birds scolded the mask wearers, no matter who wore them. However, most birds did not scold people in other masks or people without masks, even if those people had, in fact been the trappers.

In the magpie study, some humans climbed up to the birds’ nests and some did not. Later, the birds were aggressive towards only the humans who had approached the nest.

Both cases show that the birds, once threatened by humans, do not lump all humans into the “dangerous” category, but instead differentiate between the faces of humans who have behaved threateningly and those who have not. Some birds will even warn each other about humans that have posed a danger in the past. Astonishingly, crows have been shown to remember a person for at least 2.7 years.

Flexible Thinking  

Corvids must be quick-thinking and flexible to keep their food safe from competitors (Mark Prins, USGS Alaska)

Many corvids forage food then cache it for winter. That’s a lot of work, so one strategy for saving time and effort is to simply steal food that another bird has already cached.  If that behavior is present in a population, then a counter-strategy of being secretive about where one hides food is necessary. So where is it best to spend one’s time? Stealing from others food or carefully hiding your own?  Leave it to a corvid to be flexible enough to switch strategies depending on the situation.

Eurasian Jays (Garrulus glandarius) are omnivorous but count on storing huge caches of acorns to survive the winter months. During the caching process, the question becomes, should I sneak off and hide these acorns or should I just walk up and take someone else’s food?  After studying the jays, researchers in Cambridge learned that the answer depended on which birds were around.

The jays have a distinct hierarchy. In any pairing, one bird is dominant over the other. It turns out that when a bird is dominant, it is bold, aggressively stealing food. When it’s subordinate, it is more timid and secretive about where it hides food.

But the pairings change frequently, as one birds come and go, so the jays have to be flexible. One minute a bird may be dominant but a minute later the nearby subordinate bird might be replaced with one that is higher ranked.

“When dominant, jays in our study hid many food items, moved these items frequently and boldly approached food that had been hidden by their subordinate competitor, often stealing it in full view..”

“In contrast, when subordinate, the same jays were secretive when hiding food and when attempting to steal from others, preferring to wait until the competitor was at a distance before attempting to approach and steal their food stores.”  —Professor Nicky Clayton, to BBC

So the birds aren’t tied to a single response, but can assess the situation and change their behavior accordingly.

Winning the “war of information”

Wars are won and lost over access to intelligence. Watching what your enemy is doing and protecting your own information are key strategies to winning any battle. In the war of food caching and food pilfering, the same holds true. Knowing where others cache their food and being secretive or misleading about where your own cache is are necessary strategies for winning that war.

One way to do this is to pay close attention to those who are hiding food. A study of western scrub-jays, Aphelocoma californica,a corvid from western North America, found that the jays prefer to spend time watching the activities of birds that are actively caching food as opposed to watching birds carrying out other activities or at a cage with only a food bowl and caching trays.

Of course, you wouldn’t want your victim to know you’re eyeing his store of food. That might provoke an attack. So, until the food storer leaves, it’s probably a good idea to look busy poking around in other sites, especially if the storer is a dominant bird. That’s what ravens, (Corvus corax) were observed doing in a study at the University of Vermont. It’s also interesting to note that the pilferers remembered who had buried the food. If a nearby dominant bird was not the cacher, the pilferer showed little hesitation in stealing the store.

Home cage for shade study (Dally, et al)

When you hide food, it’s smart to hide it where it’s harder for other birds to see you. This might explain why Western scrub jays, when they know they are being watched, prefer to hide their food in shady spots rather than in the bright sun. They also prefer to hide food in quieter substrates when they know competitors can hear them, even if those birds cannot see them.

All of this means that when you are hiding food, you must keep track of who is watching you. This way, if that observer comes back around, you can recover your food and move it. But you don’t want to waste time moving food when it’s not necessary. In another study at UVM, ravens were observed moving food only when observers approached the cache. Western scrub jays have been shown to use the same tactics.

.. our results show that ravens selectively recover caches in the presence of former observers, confirming previous findings that ravens secure hidden food from conspecifics that could pose a threat to their caches (Heinrich 1999; Bugnyar & Kotrschal 2002).

What’s important about all of those studies is that they show that the birds may have a sense of what’s going on in the mind of other birds – in other words, a theory of mind.

Overcoming impulse

Quality vs quantity: Crows get it.

Controlling ones immediate impulses in return for a delayed reward is not something most animals can do. Adult humans, of course, can do it, as can some non-human primates. But in tests of delayed gratification, birds have historically performed poorly. However, since corvids are smart and seem to delay their own gratification by caching food for later eating, it’s conceivable that they might do well on impulse control tests.

After a short delay, individuals were asked to give back an initial reward (cheese) to the human experimenter in order to receive a higher amount of the same reward (two, four, or eight pieces).

In one study of carrion crows (Corvus corone) and the common raven (Corvus corax), the results were intriguing. The carrion crows were able to control their impulse and exchange food for a more desired food. However they performed less well on tests where the “better” option was simply a greater quantity of the same food.

It should be noted that the birds were first tested to see if they understood the concept of quantity. Indeed, in preference tests the birds discriminated between quantities and always favored the higher quantity.

… these results suggest that, although crows may possess the cognitive abilities to judge quantities and to overcome an impulsive choice, they do so only in order to optimize the qualitative but not quantitative output in the exchange paradigm.


As I said, that’s just the tip of the iceberg. There is also research that implies corvids can infer the action of hidden people, that they hold funerals, and much more.

I leave you with Betty, who fashions a hook …






The Behavioral Ecology Research Group at Oxford is an excellent resource for more information. Includes publications, photos, and video.

Dr. John Marzluff, Professor of Wildlife Science at the University of Washington, leads us on an investigation of the inspiration of ravens on people and our effects on these adaptable birds.

Mind of the Raven: Investigations and Adventures with Wolf-Birds, by Bernd Heinrich



ResearchBlogging.orgBugnyar T, & Heinrich B (2005). Ravens, Corvus corax, differentiate between knowledgeable and ignorant competitors. Proceedings. Biological sciences / The Royal Society, 272 (1573), 1641-6 PMID: 16087417

Bugnyar T, & Heinrich B (2006). Pilfering ravens, Corvus corax, adjust their behaviour to social context and identity of competitors. Animal cognition, 9 (4), 369-76 PMID: 16909235


Clayton NS, Dally JM, & Emery NJ (2007). Social cognition by food-caching corvids. The western scrub-jay as a natural psychologist. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 362 (1480), 507-22 PMID: 17309867

Dally JM, Emery NJ, & Clayton NS (2004). Cache protection strategies by western scrub-jays (Aphelocoma californica): hiding food in the shade. Proceedings. Biological sciences / The Royal Society, 271 Suppl 6 PMID: 15801583

Grodzinski, Uri. (2012-5) Peep to pilfer: what scrub-jays like to watch when observing others. Animal Behaviour, 25(5), 271-1260. DOI: 10.1016/j.anbehav.2012.02.018

Lee WY, Lee SI, Choe JC, & Jablonski PG. (2011) Wild birds recognize individual humans: experiments on magpies, Pica pica. Animal cognition, 14(6), 817-25. PMID: 21614521

Marzluff, John M. (2010-3) Lasting recognition of threatening people by wild American crows. Animal Behaviour, 111(3), 159-707. DOI: 10.1016/j.anbehav.2009.12.022

Prior H, Schwarz A, & Güntürkün O. (2008) Mirror-induced behavior in the magpie (Pica pica): evidence of self-recognition. PLoS biology, 6(8). PMID: 18715117

Shaw, Rachael C. (2012-9) Eurasian jays, Garrulus glandarius, flexibly switch caching and pilfering tactics in response to social context. Animal Behaviour, 271. DOI: 10.1016/j.anbehav.2012.08.023

Stulp G, Emery NJ, Verhulst S, & Clayton NS. (2009) Western scrub-jays conceal auditory information when competitors can hear but cannot see. Biology letters, 5(5), 583-5. PMID: 19605383

Wascher, Claudia A. F. (2012) Carrion Crows Cannot Overcome Impulsive Choice in a Quantitative Exchange Task. Frontiers in Psychology. DOI: 10.3389/fpsyg.2012.00118

1 Comment

  1. Fascinating stuff, Kim! Thanks for posting! 🙂


  1. Theropod Thursday 31: Thieving Magpie | dinosaurpalaeo - [...] simply are everywhere, and do the most surprising things. Recently, I linked to a very cool post by Kimberly…

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