Monthly Archives: September 2015

Why crows gather around their dead

As long as I’ve been interested in crows, I’ve wondered the same thing that many other people who study or simply enjoy crows have wondered; why do they gather so predictably around their dead?  It’s with great pleasure that this week not only do I get to address this question, but I get to do so with my own research, which was recently published in Animal Behaviour.  Banished are the days of cagey descriptions of my work and results, the peer review process has finally green-lighted full disclosure.

Photo: Michael Werner, Michael Werner Inc

Photo: Michael Werner, Michael Werner Inc

Anecdotes of crows’ attraction to their dead have long been documented and accounts usually go something like this: a dead crow is observed laying in the grass and other crows, sometimes an individual, sometimes a large group, are perched nearby silently or very raucously.  People have reported that sometimes these onlookers stay for minutes, sometimes days.  So what are we to make of this?  Well it could be explained a number of ways:

    • Maybe it’s purely coincidental, the crows have no idea there’s a dead crow on the ground.  Anyone who has observed this event in action, of course, will know that this explanation seems unlikely, but from a scientific perspective, we’ve also been able to invalidate it through previous experiments on corvids1,2,3.  So we can pretty safely ex-nay that explanation.
    • Maybe they gather because it’s a foraging opportunity.  Well, as far as we know crows rarely cannibalize each other, and even if they did, they probably wouldn’t be simultaneously scolding.  So this also seems like an unworthy hypothesis.
    • Maybe they’re experiencing a deep sense of mourning and have come together to grieve and pay their respects.  Having spent as much time around crows as I have, I hold little doubt that they have emotional intelligence.  But testing this scientifically remains problematic because there’s still no way we can truly know what’s happening on an emotional level in an animal’s head.  We’ve tried4, using noninvasive brain imagining (invasive meaning lethal or surgical), but this method is imperfect and while it can tell us the parts of the brain used, it still doesn’t tell us what they’re thinking or feeling.  So for now, the question of crows and grief remains open (scientifically anyway).
    • Which brings us to danger learning.  If I were to find a dead person in the woods I might be feeling sad but I’d also be alarmed and likely looking for the cause of death to make sure I’m not next.   Perhaps the crows are doing the same thing; looking for the source of danger and remembering key elements of the experience that will help keep them safe in the future.

Previous experiments in jays1, crows2 and ravens3, suggested that danger learning is likely a key motivator behind these gatherings since corvid effigies can be an effective deterrent, a fact that is old hat to many farmers.   So I wanted to look at this question with a little more nuance and ask, 1) whether crows would avoid food in areas previously associated with crow death, 2) could they learn new predators (i.e specific people) they see near dead crows, 3) for how long will they remember these people and, 4) how do these same responses compare if we substitute a “dead” crow for a “live” familiar predator.

Untitled

A volunteer demonstrates the one of the experimental set-ups. A masked person holds a “dead” crow. In the following days we looked for changes in latency to approach the food. A week later this same masked person would reappear and we would see how the birds responded to them.

To do this I would locate a territorial pair and feed them for 3 days.  This would give me a baseline of their feeding behavior and allow me to say, “on average, crows take x minutes to arrive at a fresh food pile.”  Then I would introduce one of my three dangerous scenarios: a masked person holding a dead crow, a masked person standing near perched hawk, and a masked person standing near a perched hawk with a dead crow.  In all these cases the birds were taxidermy-prepared mounts.  In a handful of cases (4 to be exact) the birds simply observed the scene in silence and left.  In most other cases however, the response was pretty stereotyped; the discovering bird (usually the territory holder) would scold and typically attract 5-11 additional birds.  The mob would stick around for 10-20 minutes, scolding loudly and gradually growing more silent and dispersing before all but the territory holders were left.  Exposure to the dangerous stimuli would only last 30min, after which they were removed.  I found that crows responded most strongly when they saw a person and a hawk with a dead crow as opposed to a person holding a dead crow or a person near a hawk.  This tells us that context matters, and crows are most sensitive to dead crows when they’re with familiar predators.

Following this event, for the next three days I would continue feeding the birds.  By doing this I was able to test if exposure to the dead crow, hawk, or hawk with dead crow would make them act differently in the area, despite the presence of their favorite food.  Spoiler alert, it did!  While the crows usually approached the food it would take about 15-30min longer than it used to.  This suggests that dead crows are used, in part, to assess that an area is dangerous, and that this information is retained and used for future decisions about spatial use.

The last question was if and for how long they would remember that masked person associated with the dangerous event.  Provided the birds had scolded them in the first place, a week after the main experiment wrapped up, I would reintroduce the dangerous person.  I found that, of birds that were administered this test (N=84) the majority of them remembered and scolded the person.   Even after 6 week, 38% of the 65 pairs eligible for all 6 tests continued to respond to the ‘dangerous’ person.  It’s incredibly cool to me that crows cannot only learn new predators based on their proximity to dead crows but to other predators and remember them for so long.  It’s really amazing.  Crows are likely learning and remembering an incredible number of humans faces over their lifetime.

So why do crows gather around their dead, according to the best available science?  At least in part, it’s to learn about dangerous places and new predators.  Could there be other, more emotionally intelligent reasons?  Sure.  Scientists simply haven’t devised a way to address that yet, but we’re trying to think of ways to do so that are minimally invasive.  Until then, I know there are many folks out there who need no scientific evidence to believe that that’s precisely what’s going on and I see no reason studies like this should disabuse them of that belief.  Studies of animal emotions are the next frontier and I couldn’t be more excited to watch crows continue to blow people away.

To read the scientific article in full, which covers the many additional elements of this experiment, click here.

Thank you to GO who was one of my original test subjects and who continues to be a regular source of friendship and delight.

Thank you to GO who was one of my original test subjects and who continues to be a regular source of friendship and delight.

Literature cited

  1. Iglesias, T.L., McElreath, R., & Patricelli. G.L. (2012) Western scrub-jay funerals: cacophonous aggregations in response to dead conspecifics. Animal Behaviour 84: 1103-1111.
  2. Avery M.L., Tillman E.A., & Humphrey J.S. (2008) Effigies for dispersing urban crow roosts. Proceedings of the 23rd Vertebrate Pest Conference. Davis, CA: University of California, Davis: 84–87
  3. Peterson, S. & Colwell, M. (2014) Experimental evidence that effigies reduce corvid occurrence. Northwest Naturalist 95: 103-112.
  4. Cross, D.J., Marzluff, J.M., Palmquist, I., Minoshima, S., Shimizu, T., & Miyaoka, R. (2013) Distinct neural circuits underlie assessment of a diversity of natural dangers by American crows. Proceedings of the Royal Society B. 280: 20131046

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The facts about crows and West Nile virus

Ah fall, the changing colors, the arrival of mushrooming season, and the gratuitous consumption of hot drinks makes this season a favorite of mine.  But if you live in a temperate zone like the PNW, there’s one drawback to fall; West Nile virus (WNV).  Make no mistake, since its first appearance in the US in 1999, cases of WNV have been documented year round.  Outbreaks, however, typically peak in late summer and early fall which is why you often start to see increasing media attention directed towards the discovery of crow corpses.  Such is the case with a recent die-off in Spokane which inspired a fleet of recent articles.  My favorite was a post from an otherwise predominately car-focused site called The News Wheel which included the following passage:

“In either case, authorities are telling residents that, should they find a dead or dying crow in the street, that they should under no circumstances handle them…In the mean time, it may be a good idea to replace your car’s windshield cleaning fluid with holy water (just in case).”

I suspect some sarcasm was at work there but, in truth, people can get very worked up about WNV.  So here are some FAQs about crows and WNV intended to keep you safe and informed.

What is WNV?
WNV is an arthropod-borne virus which can cause febrile illness, encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord).  It was first documented in the US in 1999, and exists across most of the world (sigh; the costs of globalization)1.

Why are crows associated with WNV?
No, this isn’t unfair prejudice at work, corvids really do have it rough when it comes to this disease.  In all the documented cases of WNV 80% of them have affected corvids, despite its presence in 300 other species of bird2.  The connection between WNV and climate change means that corvids could be in big trouble which you can read about in a previous post here.

What does a bird sick with WNV look like?
Most birds who contract WNV will survive, but for the many crows and other corvids what won’t, symptoms include trouble with balance both at rest and while mobile, and lethargy.  There won’t be any way to tell if a dead crow is infected simply by looking at it2.

Can I get WNV from touching a crow?
According to the CDC, there is no evidence that a person can be infected by direct contact with infected birds, dead or alive3. That being said, always use gloves when handling wildlife.

Can people get WNV?
Yes, though keep in mind that the virus really doesn’t want to be in you.  Humans and other mammals are considered ‘dead-end’ hosts meaning we generally don’t develop a big viral load and the virus cannot be transmitted from you to other humans via mosquitoes1.

How is it spread?
Mosquitoes, mosquitoes, mosquitoes.  They bite the infected bird and then bite you.   And very, very rarely through blood or organ transfusions, and from mother to baby during pregnancy or by breast feeding1.

transmission cycle

How worried should I be?
I’m not a doctor so let’s let the numbers speak for themselves on this one.  Here are the key stats you should know c/o the CDC1:

  • 80% of infected humans will not develop any symptoms.
  • Of the 20% that show symptoms, most will look like the flu.  It will suck, but you’ll recover just fine.
  • Only 1% of infected individuals develop life threatening symptoms.
  • As of September 22, 877 people have tested positive for WNV in 2015.  Of those 43 have died. That’s about half the number that will probably be killed by bee stings and twice as many as will be killed by cows4.
  • For a look at the average annual WNV incidence by state from 1999-2014, check out this map from the CDC.  Suffice it to say, in most states the incidence per every 100,000 people is less than 1.  As a reference point, about 5-20% of the population gets the flu every year and about 36,000 die as a result5.

How can I protect myself?
The CDC recommends an integrated management plan that includes:

  • Mosquito surveillance (are there lots around, how often are you getting bitten, are illnesses being reported etc.).
  • Reduction of breeding sits (i.e eliminate standing water around your home).
  • The use of chemical and biological mosquito control.
  • And finally, education (which by reading this you’re already doing!).

So, does a dead crow in your yard mean it’s time to break out the hazmat suit? Definitely no, but do Fish and Wildlife a favor and report your crow carcasses.  And if any of your neighbors start to panic, calmly and gently give them the facts.  Or just send them this meme I made you:

IMG_2711(1)

Literature cited

  1. http://www.cdc.gov/westnile/faq/genquestions.html
  2. http://www.seattleaudubon.org/sas/LearnAboutBirds/SeasonalFacts/WestNileVirus.aspx
  3. http://www.cdc.gov/westnile/faq/deadbirds.html
  4. http://wnyyradio.com/news/25-shocking-things-more-likely-to-kill-you-than-a-shark/
  5. http://www.cdc.gov/flu/about/qa/disease.htm

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Filed under Crow behavior, crow conflicts, Crow disease, Crows and humans

Baby crow detective work

By now, most of us have come across these images of  “baby crows” so often it induces more of a yawn than a fit of aggravation.
Image-1If, somehow, these images are new to you feel free to check out my post fully debunking them, as I will not dedicate any further time to them here.   But there’s a new photo circulating social media, and it makes for a much more compelling crow doppelganger;

baby crow
You’ve got three, black, altricial baby birds in a nest and really, they’re not terribly un-crow like.   It doesn’t make you a complete crow rookie to make this mistake, but there are some key things wrong here.  And this is the moment where, as a scientist, these photos elevate from being simply another source of annoying misinformation (which, they are) to the kind detective work that childhood doctor visits fostered a deep love for.  Because, not unlike my favorite activity in the Highlights magazines I anxiously parsed through in those waiting rooms, there are 4 things that are different between these two photos and it’s up to you to find them.  So take a minute and see what jumps out at you….

Crows_v_no crows
Figured it out (or given up)?  The first thing to know is that all bird species are very specific in terms of both nest materials and nest construction.  Sure, some birds can happily use some ribbon in place of straw (like orioles) or build nests in old shoes just as easily as in gutters (like bewick’s wrens) but the basic style is always the same.  Robins will always use mud as a binder and bushtit nests will always look like cozy sleeping bags made of moss.  Knowing that, the material used in the nest on the right should jump out as a red flag.  Of course you’ll find crow nests with a bit of string, fabric or grass (especially for lining) but the bulk of the nest is always made of pinky-wide sticks.   Really, you need look no further at this point to know immediately that the photo on the right is an impostor but let’s keep going because it’s fun.

Next let’s look at the babies themselves, which is where the three remaining differences are.  Two of them are color-coded, did you catch them?  Ah yes, gape and eye color.  See that brightly colored area on the corner of the bird’s developing beak?  That’s called the gape, and the bright color that flashes when they open their mouths is a powerful signal that tells parents to “insert food here”.  Crows have bright pink gapes, whereas these other birds have yellow gapes.  Our other color coded giveaway is the eyes.  Granted the lighting is not great, but it’s clear that the crows on the left have light blue eyes whereas these other birds have dark eyes.  In some species of crow the babies are born with brown eyes that turn blue as they age, but such is not the case with our American crows and you can expect that nestlings will always have blue eyes.  The last clue, which takes more expert level knowledge to notice, is the bill shape.  The birds on the right have a slightly more embellished curve to the bill than a typical crow chick.

The final mystery, of course, is what the birds on the right actually are.  Unfortunately, I failed to track down the original poster, but as best I can tell they’re black drongo chicks.  Black drongos are members of the drongo family (Dicruridae) and are native to Southern and Eastern Asia.  Here’s another photo I found that looks consistent with the previous one.  If any drongo experts read this blog though and want to correct me, I’d love to hear from you!

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