Category Archives: Crow behavior

15,000 crows

I had imagined it like a beckoning flood.  A small sputter of water followed with increasing force until a great river finally makes its way.  Rather than water though, the flood I was trying to envision was the ascent of 12-15,000 crows to their nightly roost in Bothell, Washington.

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Witnessing it in person, I found that my water analogy was not entirely accurate.  Rather than being a steady stream with a predictable course, their arrival ebbed and flowed, sometimes leaving the sky lonely with only its fading grey light while other times exploding into seemingly endless black clouds.  They arrived from all cardinal directions, colliding into a mass that could be deafening at close range.  Although the movement of the flock as a whole was more restrained, individually they showed off with spontaneous dived and barrel-rolls.  Soon the light receded completely, and all I could sense was the cacophony of so many crows settling into the willow trees they would call their beds for the evening.

Time lapse of Bothell crow roost I took with my GoPro in December of 2016.  Music by Andy McKeen.

Since that first experience, I have visited the Bothell roost many times, each as awe inspiring as the time before.  This behavior isn’t unique to my region, however.  Cities and rural areas all over the world call themselves home to the upwards of thousands, even hundreds of thousands of crows that may seek their refuge when darkness falls.  Even in the greater Seattle area, Bothell is only one of two roughly equally sized roosts.  This kind of mass sleepover, known as communal roosting, isn’t unique to crows, but it certainly captures our attention in ways most other birds don’t.  So what exactly are the characteristics and functions of roosts?

For all species of corvid, roosts are places where anywhere from a small handful to hundreds of thousands of individuals may converge to spend the night together.  Though roosting occurs year round, it peaks in winter, when territorial pairs are free from the eggs or nestlings that demand all-night attention.  They may occur in wildlands, but more typically occur in cities, where sequestration of heat is higher than in surrounding areas.  Here in Bothell, the roost converges in a wetland outside of the University of Washington’s Bothell campus, but in other areas they may take over the rafters of abandoned buildings or trees dotted within a business district.

Historically Danville, IL hosted North America’s largest roost, a whopping 325,000 birds but I do not know if they remain the contemporary record holder.  The midwest is particularly primed to host such large numbers because many thousands of crows head there during winter from their too cold territories in Canada and because appropriate roosting locations are few and far between.

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Prior, or just after roosting crows attend “staging” or “pre/post-roost” areas where they gather in the trees or on the ground by the hundreds or thousands.  Since these staging areas often occur on asphalt or turf where there’s little food or water, their function continues to elude scientists though social or anti-predator implications seem likely.1 A new UW research study is attempting to parse why crows are so vocal during the staging period and what they might be trying to communicate.  Perhaps their findings will shed some much needed light on these events. 

Corvids get different things out of roost itself depending on the species or possibly even the region they live.  For example, for ravens roosts act, in part, as mobile information centers.2  A raven knowledgeable of a food bonanza such as a moose carcass will display to other ravens at first light, and recruit others to the food.  Rather than being a sign of food altruism, this kind of recruitment is often the only way a lone raven can gain access to a large carcass.  Finding and gaining access to an animal carcass is challenging both because its arrival is unpredictable but also because it’s intensely guarded by the pair whose territory happened to claim the animal’s life.  Overpowering a pair takes a small army, so by recruiting other birds, rather than giving up food in the name of helping others, the lone raven actually gains access to a resource it would have otherwise been boxed out of.

American crows on the other hand do not have this need because urban waste and invertebrate filled yards are so easy to come by.  For crows, roosts act in large part as predator protection.  The odds of successfully fleeing an incoming owl are much better when there are thousands of you, rather than just you and your mate.  They may serve other purposes as well though including socialization, mate finding, and thermoregulation.  Lastly, while there isn’t strong evidence of information sharing among crows it would be arrogant to claim we know it doesn’t occur.

How roosts are organized remains largely mysterious.  For example some evidence shows that ravens that come from the same food bonanzas also sleep near each other in a roost,2 whereas other work done on crows suggested that group cohesion is low at roosts.3  Still, other research suggests that while group cohesion from the territory is low,  it’s high leaving the staging area.  So perhaps there is deep rhyme and reason for who they sleep with, it just hasn’t been captured by the questions we’ve so far asked.  One thing is for certain though; the one place you don’t want to be is low in the trees with others above you.  There would be no escaping the white shower raining down throughout the night.

Even the people who share the UW’s campus are sensitive to this reality.  In perfect synchrony with the incoming cloud of birds, the umbrellas bloom like moonflowers.  Here in Seattle, people seem willing to take such measures to coexist with the birds (though I’m sure there are many who only do so only by rule of law).  In other areas though the cultural attitude or resulting damage makes such cohabitation difficult, even deadly.  In the most extreme case, 328,000 crows were killed in 1940 when the city of Rockford, IL elected to dispose of a local roost with dynamite.4  Today, crows are protected under the migratory bird treaty act and cities are usually required to take more creative, non-lethal approaches including noise and light deterrents.

City living doesn’t always lend itself to witnessing the kind of mass animal movements we fawn over when they appear in Planet Earth footage, but that doesn’t mean they are devoid of such spectacles.  The mass micro-migration of thousands of crows is an awe inspiring event,  grand in both scale and the mysteries it contains.  Any corvid or birdwatcher would be remiss to ignore such an opportunity and I encourage everyone to get outside, head to your roost, and watch the magic unfold.

Literature cited

  1. Moore JE, and Switzer PV. (1998).  Preroost aggregations in the American crow, corvus brachyrhyncos.  Canadian Journal or Zoology.  76: 508-512.
  2. Wright J, Stone RE, and Brown N. (2003).  Communal roosts as structured information centers in the raven, Corvus corax. Animal Ecology 72: 1003-1014.  DOI: 10.1046/j.1365-2656.2003.00771.x
  3. Donald F. Caccamise, Lisa M. Reed, Jerzy Romanowski and Philip C. Stouffer
    (1997). Roosting Behavior and Group Territoriality in American Crows. The Auk 114: 628-637
  4. Marzluff, J.M. and Angel, T. 2005. In the company of crows and ravens.  Yale University Press

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Filed under Birding, Crow behavior, crow conflicts, Crow curiosities, Crow life history, Crows and humans

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

Crow curiosities: Why their feet don’t freeze

With a peanut visible in my gloved hand, we square off.  The crow eyes me from its snow covered perch, weary of such gifts offered by strangers.  Above us a raven castes a disinterested look, croaks, and flies away.  I toss the nut into an empty parking space and the crow descends to quickly collect its prize.  The space between us must be widened before it will comfortably eat however, so I decide to leave the crow to its snack and return to my car.

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Scrunching my feet in my shoes, I’m relieved for the excuse to retreat.  It’s about 10˚F outside and despite the wool socks and insulated boots between my skin and the snow I can tell that my feet are numb from the cold.  Even stashed in my pocket, my gloved hands are having trouble articulating to their full range.  How is it that my extremities lose function even with so much coddling and yet the crows can continue using their bare toes to steady and manipulate food in such cold weather?

To be clear, birds are endothermic, or warm blooded, just like mammals.  In fact, on average, birds run a little hotter than mammals.  And their feet, like ours, requires warm blood both to function and to prevent the tissues from outright freezing and causing cell death.  Yet despite these needs birds can comfortably walk, stand, or even sleep on ice.

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Glassy ice makes cold footing!

To do so, they can take advantage of two important adaptations.   The first is that the size of the arteries carrying blood into the legs and feet is exceedingly small.  Given this high surface to volume ratio, the blood has already lost most of its heat by the time it reaches the feat, and can’t lose much more to the outside world.  The second is that they employ what’s called a counter current heat exchange system.  Essentially, warm blood traveling away from the core and towards the feet via the arteries comes into close contact with colder blood traveling away from the feet and towards the core via the veins. At this point of contact, heat from arterial blood is transferred to blood traveling in the veins. This heat exchange system allows for the tissues in the feet receive just enough heat to prevent cell death, and can reduce heat loss by up to 90%1.

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A crow wades out across a frozen Drumheller Fountain in search of scraps

As an additional strategy, you’ll see them protecting their exposed legs under their body feathers, as if they’re incubating them.  This is the same reason you often see winter birds standing on one leg.  By switching back and forth, birds can minimize overall heat loss by reducing the exposure to only a single leg.

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A crow covers its feet while it waits (hopes) for a handout

So while I need special clothing to keep my extremities at a similar temperature range to my core, the physiology of most birds is adapted to simply allowing extremities to exist at near ambient temperatures with no tissue damage.  In other words, rather than crows’ feet not getting cold, their feet simply are cold.  That said, frostbite is still a possibility even in birds, particularly for: nonnative species, birds in wire cages, birds with metal legbands, and birds in unseasonably cold conditions.  If frost bite occurs, early treatment at a rehab facility can prevent long term damage2.

Still, the idea that cold-adapted birds can keep their hearts beating away at around 105˚F even while their feet are exposed to freezing temperatures is marvel of adaptation and thermoregulation!

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Literature cited:

  1. Elphick C, Dunning JB Jr., Sibley DA (eds).  (2001) The Sibley guide to bird life and behavior.  New York: Alfred A. Knopf
  2. Wellehan JFX. (2003). Frostbite in birds: Pathophysiology and treatment.  Compendium 25: 776-781

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Filed under Corvid health, Crow behavior, Crow curiosities

Why crows sunbathe

With its bill agape, I watch as the crow fans out awkwardly across the cedar shingles. Pressing the camera to my face I snap a couple photos, pleased to finally capture on film a moment I so often encounter in the field.  Unlike the crow, who’s keeping a watchful eye on the sky, I’m completely taken with my admittedly creepy behavior.  Until, of course, I hear the stiff “Excuse me, can I ask what you’re doing?” from the driver’s window as the homeowner’s minivan pulls up behind me.

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Fortunately for me, crow curiosity isn’t hard to come by and quickly the homeowner is as taken with watching this bird as I am.  “So, what is it doing up there?  I see them like this on my roof all the time” he asks after I give him my credentials.  It is a rather odd sight.  It’s nearly 90˚ and the crows is sitting in direct sunlight, mouth open, head cocked and wings outstretched like it’s injured.  Rather than escaping to shade, it’s joined by its fledgling and together they bake their bodies in the hot sun for a few minutes before gathering themselves and carrying on down to the grass to forage.

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Although the specifics can vary slightly, this general kind of posture can be observed across hundreds of bird species, even those you might not expect to have much opportunity for it like owls.  Often it’s used to dry wet feathers or warm up on a crisp winter morning but, given that they do it even when it modestly heat stresses them, it must have some other physiological benefits beside thermoregulation.

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There are a handful of other reasons that birds may sunbathe, but the big picture is that applying intense heat to feathers is critical to maintaining them in good condition.  For example, sunlight exposure has been shown to suppress feather degradation caused by the bacteria Bacillus licheniformis 1.  Heat also helps control ectopatasites, possibly by making them more mobile and easier for birds to remove2.  Lastly, sunning may relieve discomfort caused by molting and promote vitamin synthesis3.

So, far from being a signal of distress or heat exhaustion, observing this posture in crows is like watching them ruffle around in a puddle. It’s a routine, and important part of their self care regimen. Plus, everyone knows a few minutes in the sun just plain feels nice.

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Photo c/o Kathy Brown.  Find more of her great photos on Instagram @kat2brown

Literature cited

1. Saranathan, V., and Burtt, E.H. Jr. (2007).  Sunlight on feathers inhibits feather-degrading bacteria.  The Wilson Journal of Ornithology 119: 239-245

2. Blem, C.R., and Blem, L.B. (1993).  Do Swallows sunbathe to control ectoparasites? An experimental test.  The Condor 95: 728-730

3. Potter, E.F., and Hauser, D.C. (1974) Relationship of anting and sunbathing to molting in wild birds.  The Auk 91: 537-563

 

 

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Filed under Crow behavior, Crow curiosities, Crow disease

5 reasons to leave baby crows alone 

Those blue eyes, that awkward gate, their seemingly constant precariousness, they’re all calling to you to intervene. Here are 5 reasons second guessing that instinct might be in the bird’s best interest.

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1) The vast majority don’t need your help. It’s totally normal for baby crows to be on the ground and flightless as long as they’re covered with feathers and appear otherwise alert and mobile. Even nestling crows are usually on the ground on purpose. Not because they are ready, but because their parents have intentionally rejected them for one reason or another. They will die and that’s ok. Part of coexisting with wildlife is giving them the agency to be wild. The story is different of course for species where the survival of individuals may mean the difference between population survival and extinction, especially because these situation are almost always driven by human activity.

2) It’s hard to tell when they’re stressed.  Recently, I saw a video on Facebook of a Steller’s jay fledgling in the care of a very well intentioned good samaritan.  She was giving it gentle strokes with her fingertips, each touch resulting in the young bird turning its head towards its back and opening its mouth.  The comment thread filled with ooo’s and awww’s and general comments of encouragement or gratitude for her actions.  For me it was like watching an alien attempt to care for a human child, the child recoiling and screaming while its caretakers congratulated themselves on how kind they were being.  Having handled baby corvids before, I know what that kind of posturing means, it means “I’m scared and stressed.”  To an untrained eye though, it may not look much different than the kind of gaping that means ‘feed me.’  Being stressed to death is a reality for young, or even adult animals, so any handling best be done by experts whenever possible.

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3) It’s illegal to rehab crows without a license. You can provide temporary care until you can get them to a licensed facility, but do not attempt to rehab them on your own.  Mistakes like the one I just described are a prime example of why the law seeks to protect animals by ensuring they are only raised or rehabilitated by experts.  For more information on how to handle them until you can get them to a facility visit my previous post.

4) Imprinted crows do not survive well in the wild.  Even if baby crows are receptive to being treated like a pet, doing so is both a legal violation and I would argue a violation of their right to be a wild animal with a healthy fear of people.  Of all my daydreams, at the top of the list is having a wild but imprinted crow that follows me around.  I even have a name picked out.  This fantasy of mine will forever remain just that, however, because it’s too dangerous to allow a crow to become that comfortable with people.  All it would take is one cranky neighbor with a pellet gun and it would be over.  Not to mention being imprinted on people, instead of crows, denies them access to skills and relationships with other crows that will help them survive into adulthood.

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Killing them with kindness is a real risk.

5) It may do more harm than good.  The conventional wisdom suggests “well, worse case scenario is I try and rehab this baby crow and it dies, which it would have done anyway so really, nothing’s been lost.”  The more we study death in social animals the more we are beginning to realize there may be a cost to prematurely removing ailing or dead animals from their groupmates, however.  Being able to interact with their dead may serve an important role for social animals, and denying them this opportunity may have serious implications in their ability to process that death.  So be thoughtful about how slim the chance of survival is.  It might be that the kindest, most responsible action is no action at all.

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Filed under Corvid health, Crow behavior, Crow curiosities, Crow life history, Crows and humans, Uncategorized

Everything you want to know about crow nests

Spring marks one of my favorite times of year.  Cherry blossoms abound, the rain smell sweet and the birds get busy putting their carpentry skills to good use. Starting early March, the silhouettes of crows with bill loads of timber or wads of soft material dot the skies as they shuttle back and forth to their nest tree. Like a townhouse development, these construction projects are over in the blink of an eye and soon, their bill loads of twigs will be replaced by food for their mate and, eventually, their insatiable young. Spotting these nests is both a great way to observe and engage with your local crow family and avoid unpleasant conflicts with protective crow parents.  With a little knowledge and a bit of practice, tracking down your resident crow nest will become one of your favorite spring traditions in no time.

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Nest construction and site selection

Nest construction begins in early March and will continue (as nests fail) through about June. It takes 1-2 weeks to finish a nest after which the female will lay a clutch of 2-6 eggs. Unlike similarly sized squirrel nests (aka: dreys) which are made of leaves, crow nests are made mostly of pencil-width twigs. A new nest is usually about 1.5 ft across and 8-10 in deep.  After the bulk of construction is complete, they’ll line the cup of the nest with soft materials like grass, tree bark, moss, flowers, paper or fur. Once we saw a crow ripping out the hair of an outdoor manakin, no doubt to use as lining material.

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A crow gathers moss off the branches of a big leaf maple to use as lining material.  


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This bird toyed with this branch for a few minutes before rejecting it and letting it fall to the ground.  


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A sidewalk littered with twigs is good evidence that the nearby deciduous tree is a favorite among the local crows to pull branches from.  I’ve only once seen a crow try and retrieve a branch it dropped, so these are all rejects.  

 

Crows will nest in an astounding array of places, from the eaves of skyscrapers to the crooks of well concealed tree limbs. They can tower in the sky or be almost within reach. Most commonly, I see them built close to the trunk in the top third of Doug fir trees, but this is, of course, specific to the PNW.  Both partners participate in nest construction. Helpers will aid to some degree but most of the work is left to the parents.

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Differences among corvids

Crow, jay and raven nests are similar in shape and material but differ in overall size in accordance with the size of the bird. The main standout are magpies,  which build incredible domed-shaped nests the size of a large beach ball.  The nests require so much material, they can take as much as 40 days to build.  Japanese jungle crows are another species of note, as they have a (relatively) new and problematic habit of building nests out of wire hangers and causing massive blackouts.

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A Jungle crow nest in urban Japan. Photo: Götz


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The magpie’s nest is the big clump in the middle of the tree.

Reuse

The life of a typical nest is only about 9 weeks (1-2 weeks of building, 6 days of laying, 20 days of incubating and 4 weeks of nestlings) though they are hardy structures and can remain intact in a tree for years.  After the young fledge, the crows will not return to the nest.  Crows will only use a nest once, and generally only fledge one brood a year. They will, however, build on top of an old nest particularly in areas where nest trees are especially sparse like downtown Seattle. This also appears to be more common in the Midwest.

Avoiding conflicts

Most breeding related dive bombs occur as the result of a person being too close to a fledgling, but some crows get feisty around their nest too. Crows in areas where they are less persecuted (like cities) tend to be more aggressive than their rural counterparts. If you know where a nest is and can avoid it, do so and save everyone the aggravation. Otherwise carry an umbrella or paint eyes on the back of a hat. Crows rarely attack from the front so having eyes on the back of your head can be an effective deterrent!

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Putting all this together to actually find nests, is one of the most rewarding moments an urban naturalist or crow enthusiast can have.  Be warned though: crows are wary of potential predators (including people) spying on them and they have a few tricks for throwing you off, so don’t be surprised if a nest location you were certain of turns out to have been a ruse!

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Can you spot the nest? 

Have more questions? Let me know in the comments!

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Filed under Birding, Breeding, Crow behavior, Crow life history, Crows and humans

Why the crow smiles

There’s hardly a corvid species that doesn’t strike me as beautiful but there’s only one that’s always struck me as particularly gleeful.  Looking at the New Caledonian crow it’s evident there’s something different about the shape and proportions of its bill. It’s a bit shorter and more blunt, and it lacks the obvious downward curve of a typical crow bill, with lower mandible actually curving slightly up. Put together, these features appear to give it the perpetual grin that trademarks this species.  I’ve joked that this must be because they’re always feeling very pleased with themselves for being so smart, and thanks to new research, I’ve come to learn my joke had it backwards.

By using tomography scans, Hiroshi Matsui and his team were able to compare the shape and structure of the NC crow’s bill with that of its close relatives. Their conclusion, which they report in the March issue of Scientific Reports, is that this shape makes the handling and manufacturing of tools easier. Looking at photos of the birds in action, it feels intuitive that the more exaggerated curve of a raven or American crow bill would have a hard time achieving the dexterity that NC crows need to use their stick and hook tools.

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Given this new research it’s time to amend my joke. It’s not that NC crows grin because they’re smart, they’re smart because they grin.

Literature cited

  1.  Matsui, H., Hunt, G., Oberhofer, K., Ogihara, N., McGowen, K., Mithraratne, K., Yamasaki, T., Grey, R., and Izawa, E. 2016.  Adaptive bill morphology for enhanced tool manipulation in New Caledonian crows.  Scientific Reports 6. doi:10.1038/srep22776

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Filed under Crow behavior, crow intelligence, Crow life history, New Research