Category Archives: Uncategorized

Crows removing ticks: helpfulness, opportunism, or something else?

Guest post by Thom van Dooren

If you’ve spent much time at all watching YouTube videos of corvids, you’ve likely come across some of the numerous examples of them engaging in the seemingly helpful act of removing ticks and other ectoparasites from all kinds of other animals. The lucky ‘client’ might be a rhino, a sambar deer, or a cow.  Admittedly, it isn’t always entirely clear that these crows are being exclusively helpful, engaging in what biologists call symbiotic cleaning. In at least some of these cases, there seems to be a reasonable chance that they’re (also) playing, pestering, or even getting ready to take a bite out of an unsuspecting animal. 

In a few cases though, the results of their cleaning work speak for themselves. One recent set of camera trap videos circulating at the moment on social media shows a group of corvids, probably Torresian Crows (Corvus orru), removing ticks from several, somewhat reluctant, wallabies. The ticks have been around for a while, as evidence by their size, and cover large parts of many of the animals’ ears and necks. As the wallabies arrive at a watering station that has been set up for them in the dry summer of southern Queensland, the crows move in. They carefully sidle up to the wallabies while they’re drinking, and then, in one swift movement, a beak flashes out and returns with a tasty, protein-rich, tick.

While these videos fascinate, delight, and frequently disgust, YouTube viewers, these behaviors are not uncommon in the animal kingdom. Symbiotic cleaning, across species lines, is a particularly common practice for fish, crustaceans, and birds. The animals who receive these cleaning treatments are mostly larger fish and herbivores. In the case of corvids, studies have documented the symbiotic cleaning of deerwild boars, camels1, and a handful of other species.

Perhaps my favorite example—sadly, one without footage—is another Australian case involving the same species, the Torresian Crow. At the top of the Northern Territory, on the Cobourg Peninsula, these crows have struck up an unlikely relationship with banteng (Bos javanicus), a species of feral cattle that was introduced to Australia in 1849. Over the last couple of decades, scientists have observed crows landing on the backs of resting banteng. The banteng will then roll onto its side and lift its upper legs—which is not a comfortable or easy posture for a banteng—so that the crow can access the area under the legs and belly. Moving into this space, crows have then been observed removing ectoparasites, likely ticks, from these exposed areas. 

While this kind of cleaning is not in itself exceptional, the degree of attunement between the cleaner and the client in this case is fascinating. In many cases, as with the wallabies, the clients seem not to agree to the treatment at all. In other cases, as in a beautiful set of photos of a house crow cleaning a cow in India, we can see some degree of ‘posing’ and positioning to facilitate access. But in the case of the banteng and the crow, a whole procedure seems to have been worked out and agreed to. Torresian crows aren’t known to have similar relationships with any other mammals, and banteng around the world aren’t known to deliberately expose themselves for grooming by any other bird. Yet this is happening here. We will never know which crafty individuals struck up this mutualism, how those first awkward interactions took place, how a proposal was made, and how an agreement was reached that such vulnerability was worth the risk. But we do know that this behavior is spreading as more and more banteng and crows around the region get in on the action.

There are lots of lessons that might be taken from all these inter-species interactions. One that particularly interests me, visible in the comments below many of these YouTube videos, is the desire to cash this behavior out in simple, black and white terms. Either this cleaning is an example of a helpful altruist, or it is a nasty, opportunistic, crow taking advantage of another. Biologists, too, are not immune to these kinds of explanations—although which ones are preferred does shift over time, as things like ‘selfish genes’ go in and out of fashion. As Robert Poulin and Alexandra S. Grutter noted, writing in the mid 1990s: “Over the past few decades … the opinion of scientists regarding cleaning symbioses has changed, from selfless cooperation, to a mutually beneficial interaction, and finally to a one-sided exploitation.”

Of course, how these interactions are characterized depends a lot on whether we’re after the adaptive, evolutionary, explanation, or the psychological motivations of the individual involved. When it comes to clever corvids, there seems no reason to assume that all individuals engage in this behavior for the same proximate reasons. Some Torresian Crows might clean wallabies or banteng purely for the tasty snack. But can we hold open room for the possibility that others might, at least in part, be motivated also by the desire to help out, to remove a difficult parasite? As I’ve argued elsewhere, I want explanations that, at the very least, refuse to rule out in advance the possibility that corvids might be engaged in something like “targeted helping” across species lines. More than this, I think we need explanations that refuse one size fits all, black and white, options.

When it comes to the ethical behaviors of humans, many people have likewise often looked for either/or explanations, altruism or selfishness. But for hundreds of years at least some philosophers have been arguing for the need to destabilize these categories. When we look closely, our own actions are very rarely purely selfless or selfish, they are a complex, shifting, sea of grey. Perhaps it is time to recognize that similar, even if thoroughly distinctive, dynamics might be at work in the black and white worlds of corvids and other nonhuman animals?

Thom van Dooren is a philosopher and a corvid enthusiast. His most recent book is The Wake of Crows: Living and Dying in Shared Worlds (Columbia University Press, 2019). He is an associate professor and Australian Research Council Future Fellow at the University of Sydney and a Professor II at the University of Oslo. www.thomvandooren.org

You can read an excerpt of Kaeli’s review of The Wake of Crows for the journal Oryx here.


Literature cited
1Lewis, A. D. (1989). Notes on two ravens Corvus spp. in Kenya. Scopus13.2, 129–131. 

4 Comments

Filed under Uncategorized

Join me virtually tonight!

Tonight at 7:00pm PST, Timberland Regional Libraries is hosting an evening of crows and literature featuring myself and Hollow Kingdom author Kira Jane Buxton.  The night will start with a reading from Kira’s book, which features an irreverent crow navigating a zombie apocalypse in Seattle.  Afterwards Kira and I will host a game of crow and literary themed trivia, followed by a Q&A with myself.  It promises to be a family friendly event for crow lovers and book worms alike.  Registration is required but the event is free.  I hope to meet some of you there!

Register here 

Crow trivia

7 Comments

Filed under Uncategorized

The adorable guide to distinguishing American crows and common ravens

Recently, I published what I hope is one of the most comprehensive crow vs. ravens guides readily available on the web. But sometimes, you don’t want to pour through a bunch of text and details, you want just a quick reference, or a shorthand way of explaining to an inquiring newbie that crows and ravens are actually different.  To that aim, I am so excited to share that I teamed up with artist Rosemary Mosco of Bird and Moon comics to create a guide that is equal parts charming and informative.  Share it widely and spread the corvid love!

raven vs crow

UPDATE
Since initially sharing our comic Rosemary and I have been absolutely delighted at its reception, including the number of education programs asking to use it.  Among those is the Kituwah Preservation and Education Program, whose mission is to preserve and promote the Cherokee langue through engagement and service.  Cherokee is a beautiful, complex language that like all other indigenous American languages is endangered of dying out due to the cultural genocide that the US government inflicted on indigenous Americans during the late 19th and early 20th century.  I am incredibly honored to contribute to KPEP’s mission even in this small way.  Please feel free to share this image widely, just make sure to attribute the design to Rosemary Mosco, and the translation to the Kituwah Preservation and Education Program.

raven vs crow Cherokee

17 Comments

Filed under Uncategorized

Tickets for 3/3/20 talk in Portland

Hi all! From time to time I get messages on here from people lamenting that they missed the ticket sales for upcoming talks, so this time I am doing my due diligence and letting you be the first to know. For the third time (!) Portland’s Science on Tap is having me back to talk crows and this time it will be in my biggest venue yet, the Aladdin Theater.  Tickets are going on sale 12/13/19 at 10:00am (the link below will not be active until then).  I hope to see you, or someone who deserves a super sweet holiday gift, there!

Buy Tickets

IMG_0953

 

9 Comments

Filed under Uncategorized

A letter to my readers

Hello blog followers.  It’s been a long time.  Looking over my stats today, I was pretty horrified to see I’ve only published a whopping 4 times this year.  I don’t have a very good excuse.

2019 was certainly professionally busy, but not more so than in years past.  Probably the biggest reason for my absence is that I am sinking considerably more scicomm time on places like twitter and Instagram, and it doesn’t leave a lot of room for blog articles. Which is a shame, because that’s where some of my best science communication has happened.

I also think that because I’ve historically treated the blog as a place where I only publish researched, 800 word articles, I’ve been hesitant to just send out quick updates a la a more traditional blog.  But since at least some of you don’t follow me on the other platforms, you end up missing a lot of stuff.

Like, for example, did you know I published a short fiction story about crows (a very specific crow to be exact) on Audible this year?  I did! And I’m damn proud and excited.  And as followers of my writing, you all are probably among the most interested audience for that kind of material, and I didn’t even tell you.

So take this as a vow that 2020 will be better.  I already have a ton of ideas for new articles, at least some of which will come out before the end of the year.  Teaching an ornithology class has vastly improved my arsenal for communicating some pretty cool bird biology through the vehicle of corvids.  Oh right, did you know I am no longer a postdoc but a lectuter at the Unviersity of Washington?  Yeah, we need to catch up more.

Thanks for your patience and continued readership.

Kaeli

EA0FDD5C-8AD8-433D-BC71-E8CF6A6A7F07

14 Comments

Filed under Uncategorized

Denali field notes: A hare of another color

Up until the last few weeks, spotting snowshoe hares before they darted out from the adjacent vegetation was something I considered myself fairly lucky to do.  After all, it’s literally a matter of life and death for them to remain as undetected as possible.  As September came to a close, however, I found myself spotting them more often and from further away than I had previously, and not just from weeks of practice.  Their concealment was being betrayed by the very mechanisms designed to keep them safe.

DSC_1168.jpg

There aren’t a lot of animals that call Denali home all year long, but those that do need effective strategies for staying alive during the essentially 8 months long winter.  For Denali’s snowshoe hares, one of these strategies is to adapt in an entirely new winter outfit, something only 20 other animals in the northern hemisphere do.1 While in the summer they are a mottled reddish brown, starting around late September the hares grow in their nearly all white coats. The ears are typically the first to change, with the rest of the body following suit shortly thereafter.

 

This transformation is mediated by changes in the photoperiod that affect melanin production.  Although the full explanation is quite complex, the core mechanism is that the shorter day length increases the hormone melatonin, which suppress the melanin producing hormone prolactin.1

 

While this strategy is good one for long winters blanketed in snow, changes in snow regimes are making this transition more precarious.  Camouflage mismatch–which is generally considered when more than 60% of the coat is different from the surrounding environment–can result either from winter coats that have come in too early, before the snow arrives, or because the snow pack lingers inconsistently.  This year, the lower elevations of the park have yet to see so much as a flake of snow, though you wouldn’t know that by looking at the hares.  As I have already experienced, such mismatch makes hares considerably easier to detect, a big problem for basically everyone’s favorite winter meal.2

DSCN1302

Although hares can adjust the timing of their molts to a small extent, it won’t be enough to keep them in sync with the more dramatic shifts climate change has in store for the future.  This is especially problematic because hares don’t seem to be very aware of their mismatch and attempt to compensate behaviorally by say, hiding behind vegetation or choosing resting spots that more closely match their color.3 Other animals, particularly birds, seem better at this.  Rock ptarmigan for example will actually dirty themselves to more closely match patchy snow.4

DSC_1160

Given the immense selection pressure on these animals to match their environment and the high variation in the traits responsible for such color changes, it’s possible that hares will be able to keep pace with an already changing arctic landscape, but we don’t know for sure.  The alternative will be to add hares to the growing list of once common animals that now require invasive management strategies to stay afloat in the anthroproscene.

 

Literature cited

1. Zimova M, Hackländer K, Good JM, Melo-Ferreira J, Alves PC, Mills SL. (2018). Function and underlying mechanisms of seasonal colour moulting in mammals and birds: what keeps them changing in a warming world? Biol. Rev. 93: 1478 – 1498.1478 doi: 10.1111/brv.12405

2. Pedersen S, Odden M, Pedersen HC. (2017). Climate change induced molting mismatch? Mountain hare abundance reduced by duration of snow cover and predator abundance. Ecosphere 8: 01722

3. Zimova M, Mills SL, Lukacs PM, Mitchell MS. (2014). Snowshoe hares display limited phenotypic plasticity to mismatch in seasonal camouflage. Proc. R. Soc. B DOI:

4. Montgomerie R, Lyon B, Holder K. (2001) Dirty ptarmigan: behavioral modification of conspicuous male plumage, Behavioral Ecology 12: 429–438.

5 Comments

Filed under Uncategorized

No, that crow is not sharing with a mouse

I know, I know, it looks so much like that’s what’s happening.  The mouse is clearly interested; the crow walks the direction the mouse exited to, drops food, and leaves.  But rather than sharing, this is simply a coincidental intersection of two animals trying to claim what they can.

First, a little bit of background information: In animal behavior, food sharing is defined as the transfer of a defendable food item from one individual to another, or the joint use of a monopolized item.1 Obviously these definitions are both broad and somewhat subjective. Because of this, you’ll see a range in how different papers describe how common this behavior is. Some might say it occurs frequently, while others say it is rare depending on if they’re including tolerated theft, food transfers during courtship, etc. Among corvids, evidence of food sharing has been suggested in ravens, rooks, scrub-jays and northwestern crows, but it’s been most formally studied in jackdaws.

Studies on jackdaws show that donor initiated sharing is a more common behavior for that species than it is in primates (though more rare than recipient initiated sharing), sharing decreases as the birds get older, and that patterns of food sharing support both the altruism hypothesis (tit for tat) and the harassment hypothesis (take this and leave me alone!).  A key thing among all these studies is that food sharing is taking place within social groups, not with strangers and not with other species.

Given the rarity and circumstances with which this behavior has been observed, even within corvid social groups, it is exceedingly unlikely that this crow is so generously sharing with a mouse.  So how else could we explain it?

theif

The crucial moment of truth comes at the 0:58s mark.  Crows, like many other corvids, hide food for later consumption. If you’ve ever shared enough food with a crow that it constituted multiple morsels, you might have noticed this behavior.  The crow will walk or fly to a location not generally too far away and tuck the food into a tree branch, under the soil, into your gutters, etc.  Then, like clockwork, they will use some nearby debris to cover it.  In the most adorable, “Damn why don’t I have my camera!?,” moments, I’ve seen them pick pansies to cover and hide their treats.  At the 0:58sec mark you can see the crow pick up some sidewalk detritus and do exactly this.  That’s not a signal of sharing, that’s caching loud and clear.

Some of you might be wondering why, given how smart crows are, it did such a poor job, considering the mouse was so clearly watching and ready to lift it as soon as the crow walked away.  Crows are smart, and very good at some tasks, but as I’ve said before, the ability of an animal to excel at a cognitive task requires that that task in some way relates to their natural history.  American crows are not good at, or perhaps even capable of, making hook tools because, unlike New Caledonian crows, there was no available food niche which selected this behavior.  Likewise, American crows aren’t very cache protective because they don’t really need to be. There is so much food in the urban ecosystem that most crow caches will go uncollected anyway.  On the other hand, the corvids that rely more on their caches are quite sensitive to onlookers.  As for why it picked that particular spot to begin with, that was probably the closest cache location, and since the crow probably couldn’t see the mouse, there wasn’t any reason not to cache there.

One of the more elaborate explanations I’ve read falls decidedly in the opposite direction, however.  The idea is that instead of sharing, the crow is actually baiting the mouse and the sprinkling of detritus on top was meant to slow the mouse down and facilitate the kill.  While there are some birds, such as green herons, that do use bait to catch prey, this is not a consistent feature of crows’ hunting repertoire.  That said, you can find the occasional video of crows using bait much like the herons.  We can also rule out this explanation because 1) a leaf on top of a piece of bread will slow the mouse down for approximately 1/100 of a second and 2) the crow very clearly wandered away from, and lacked the focus on the food that would be required to actually catch the mouse.

So consider the food sharing crow video officially debunked.  And don’t worry, there are still plenty of videos out there that are as exactly pure and wonderful as they seem.  I’m not coming for your golden retriever.

Literature cited

  1. De Kort, S.R., Emery, N.J, and Clayton, N.S. (2005).  Food sharing in jackdaws Corvus monedula: what, why and with whom? Animal Behaviour 72: 297-304. (scroll down to see PDF of paper)

 

 

19 Comments

Filed under Uncategorized

Denali Diaries Part I: The Place

My first trip into the park, the Ranger warned us that we would fall in love with Denali.  I could tell her sincerity wasn’t manufactured, it was that kind of genuine love for something that can make cynical people feel a bit embarrassed, but I was careful to temper my expectations nonetheless. By the end of the trip (who am I kidding, more like after about 20min), however, I was ready to write my own effusive love note.  Denali is unlike any place I’ve ever been, and I hope I can share a small fraction of what seeing it in real life is really like, and offer a few tips for anyone planning their own trip.

DSC_0666

View from Eielson visitor center

The first thing to understand about the park is that it is managed quite differently than most other national parks in the country.  Unlike say Yellowstone where private vehicles, service vehicles, and tour buses clog the roads, in Denali private vehicles are only allowed for the first 15 miles of the park.  To see the other 77 miles you’ll need to take a bus. Buses come in three forms: Private tour buses offered by the hotels and resorts at the road’s end in Kantishna, NPS tan buses that offer interpretive, structured tours, or the NPS green buses that function like city transit buses.

DSC_0171

There are four possible destinations you can bus to in the park: Toklat at mile 53, Eielson at mile 66, Wonder Lake at mile 85 and Kantishna at mile 92. Out of my four separate trips into the park, I traveled as far as Eielson (8 hour round trip) twice and Wonder Lake (11 hour round trip) the other two times.

DSC_0452

No matter which destination you select, you’ll stop at one of Denali’s prettiest look-outs, Polychrome Pass.

DSC_0011

Things can change a lot at the end of the season. This is Polychrome Pass only a week after the above photo was taken.

Choosing which destination you want to go to is a matter of price, travel time, and priorities. My experience was that although Wonder Lake is beautiful, unless you can camp overnight, or have additional trips into the park planned, it wasn’t worth the full day trip. Instead, I would suggest spending more time off the bus at an earlier stop like Eielson.  The only exception would be if the weather is completely clear and your goal is to get phenomenal views of Mt. Denali, in which case continuing on to Wonder Lake is absolutely worth it.

DSC_0229

Mountain views on the way from Eielson to Wonder Lake.

DSC_0489

Less than 30% of visitors get to see even a section of Mt. Denali. We were exceptionally lucky to get an end of season full view.

If you’re balking at the idea of being stuck on a bus, I hear you, and based on the feedback I’ve already gotten I know that a “terrible bus ride” is a lot of people’s impression of what visiting Denali is like.  But there are three reasons to embrace the bus. The first is the whole reason for their existence: keeping people on buses rather than personal vehicles keeps wildlife safer.  Look no further than they annual stories out of places like Yellowstone to appreciate how necessary this is.  Second is that with more eyes you’re much more likely to spot wildlife, especially small or cryptic wildlife.  And lastly, and this is the real beauty of Denali, when I said that the green buses function like city transit buses I should have included “but better” because you can request to get off anywhere outside of Sable Pass which is a wildlife protection area.

DSC_0270.jpg

In Denali what do you do when you see a spectacular view and want to go experience it yourself?  You get off the bus and go to there.

Coming from Washington, it was a bit hard to understand why my Denali-based colleagues couldn’t offer very specific hiking suggestions.  My entire outdoor life has been ruled the the trail, so I was baffled when inquires about trail names were met with blank stares.  It wasn’t until I actually got onto the tundra that I appreciated the possibility of a trail-less adventure.  Of being able to shout “Stop!” at the first beautiful area that struck me and having the driver pull over to let me off.  If the brush is high you can walk the road, taking in the scenery at your own pace.  If it’s low, as it is between Toklat and Eielson, you can easily range off road and as deep into the park as you like.  With 6 millions acres at your feet, it would take nearly a lifetime to explore everything.

DSC_0311

dsc_0363.jpg

 

Once you’re done, you head back to the road and flag down any green bus.  If you’re heading out of the park, there’s no need to even show them your ticket. Alternatively, you can get on a bus heading to your ticketed destination if you want to maximize your wildlife mileage as I did on a number of occasions.  As long as you’re attentive to the bus schedule and prepared to wait for an empty bus, you ping-pong back and forth like this as many time as you can fit into the day.  Although it adds up to a lot of driving, it can be one of the best ways to see wildlife because you’ll hit the park at different times of day and you can get a lot closer than you can on foot.

DSC_0796

With so much flexibility to explore such a beautiful area, it’s no mystery why people fall  in love with this place so easily.  And we haven’t even gotten to the wildlife yet.  For that you’ll need to stay tuned for Part II

***

7 Comments

Filed under Field work, Just for fun, Uncategorized

Gray is the new black

For the past six years, my life has been dedicated to the “funerals” of birds perpetually dressed for the occasion.  This pursuit has provided some of the highlights of my life and lodged crows so deep in my heart I suspect they might show up on an ultrasound.  The still vastly untapped world of comparative thanatology (the study of animal death) is one I have every hope of returning to, but I’d be fooling myself if I didn’t admit I was ready for a change.  There is much more wildlife and natural history to be had in the urban jungle than people realize, but as a fieldsite it can start to grate on your mind and spirit.  The suspicious onlookers, inattentive texters, cranky homeowners, fastidious leaf blowers, relentless piles of dog shit, and generous helpings of stuff you couldn’t even imagine, like the truck that blew bubbles as it drove around the block and always seemed to drive by just as I started the experiment.  After so many years, this work left me yearning for a new set of challenges and maybe the opportunity to scrub earth from my feet, rather than the film of carbon, copper, and zink that peels invisibly off the passing cars.

You can imagine my great delight then, when I was offered the opportunity to conduct a year long PostDoc position in Denali National Park, studying the foraging behavior of Canada jays; adorable corvid cousins to my beloved crows.  The project is a collaboration between the University of Washington’s John Marzluff and the National Parks Service.  Apart from its brief duration, the project is everything I wanted.  A complete change of scenery, a new and challenging study question, loyalty to my favorite family of birds, and limited field seasons that would not require me to uproot my family.  Needless to say I jumped at the opportunity.

DSC_0666

At mile 66 of the park road, the Eielson visitor center offers spectacular views.

In a nutshell, the big picture question we are after is: are rising and fluctuating temperatures reducing the shelf life of the food jays store to survive the winter? The worry is that without a deep freeze to keep the perishable things that jays cache like mushrooms, meat scraps, and berries well preserved, their food is going bad earlier in the winter than in years past.  Since they start breeding well before the spring food flush, this would be a major problem.

DSCN0425-2

Although toxic to humans, the jays are happy to dine on the iconic amanita mushroom.

Answering this question will take years of study, so don’t expect any headlines from me.  Instead, my research will help fill the existing knowledge gaps and create a foundation on which future studies can build.  Namely our goal is to document what and where Denali’s jays are caching.  To accomplish this I, along with a tech, are spending 6-8 weeks in the park this fall to document the kinds of foods they cache and where they stash them. Then returning in “spring” (March, but there’s still 6ft of snow on the ground) to watch them retrieve caches. To the great relief of my parents, this fieldwork will not take place in the more predator rich back country of the park, but rather the front country near park headquarters, where over the past two years the park’s avian ecologists have been working to identify and color-band the existing pairs.

Documenting these activities is a textbook example of “Easier said than done”. The birds move quickly and can disappear out of sight with magician-like talent. There is often dense foliage blocking my view, and what they are eating is typically small.  Seeing what/if/where in terms of foraging and caching can sometimes be done in the field with binoculars, but the main way we are accruing data is by using a video camera with a powerful zoom to film the behaviors we see, and then rewatching it in slow motion to try and tease out details we missed in the field.  Attempting to get this stuff on film comes with its own set of challenges though, and the majority of our film clips are well, see for yourself…(NSFW language).

Out of the dozens of bad clips we get each day though, there are enough containing some actual data to start seeing a meaningful picture of what’s happening (like the one below), and to keep us pressing forward. We’ve already documented food items that have surprised the more senior jay scientists we are collaborating with, and I’m starting to envision what kinds of questions future researchers could ask based on my work here.

Needless to say, I am tremendously excited for this new, albeit short-lived chapter of my life. It’s a delight to be challenged by a new set of questions and contribute to something with more direct conservation implications.  And the new office isn’t half bad either.

It even comes with the usual cast of colorful office characters.  There’s the chatterbox:

DSC_0410

Pine squirrel

And the prankster that thinks it’s funny to hide around corners and jump out at you, damn near scaring you out of your skin:

DSC_0783

Spruce grouse are slow to flush, preferring to wait until the last possible moment before exploding out of the shrubbery.

And of course, there’s that one grumpy gus that is constantly paranoid someone is going to steal her lunch despite labeling her food like a dozen times.  “No one wants your crappy squirrel, Brenda!”

DSC_1005

Northern hawk owls are among the few daytime hunting owls.

Plus everyone else that’s just trying to mind their p’s and q’s and make it through the day.

So, while I miss my crows a great deal (did I mention there aren’t crows in Denali at all?) I think this new gray look is suiting me quite well.

***

17 Comments

Filed under Uncategorized

Can crows and ravens hybridize?

This feels like a simple enough question but answering it requires a check on how scientists define ‘species,’ a look at the phylogeny of crows and ravens (that’s the study of the evolutionary development and diversification of species i.e. the study of the tree of life), as well as an understanding of how their biology determines whether “Can they hybridize?” turns into “Do they hybridize?”

dsc_5014-2

A rather curious question indeed.

For decades, the textbook standard for defining the term ‘species’ in introductory biology classes has been the biological species concept.  First coined in 1942 by ornithologist Ernst Mayr, this is the idea that a species is a population of animals that share a gene pool, and can successfully mate with each other, but not with animals outside their gene pool. Almost always, the pudding offered as proof is a mule; the sterile offspring of a breeding attempt between a horse and an ass.  “See,” your teacher would say, “The offspring are sterile because it’s the result of two different species,” and then they would slap their hands together like a satisfied chef.  But of course, nature is rarely so inclined to follow such rules.

In fact, lots of animals we consider distinct species can and do interbreed.  Possibly even more problematic is that there are plenty of living things that don’t reproduce sexually at all. So how do you define them under this concept?  These issues have left scientists jaded and unsatisfied with the biological species concept and given rise to new classification schemes that don’t consider sex.  One of the most robust rivals to the biological concept is the phylogenetic species concept, where species are defined by their evolutionary histories.  The problem with this concept though is that sometimes the amount of hair splitting that goes on divides formally unified species beyond what is perhaps biologically appropriate.  Meaning, if two species have a distinct evolutionary history, but fulfill the same ecological role, look the same, and freely interbreed, are they really distinct species? Without going too far further down this rabbit hole suffice it to say that there still remains no universally accepted scientific definition of a species, and that more than likely the most comprehensive approach is one that takes into consideration an each organism’s unique situation and leans on multiple concepts as appropriate.

This is relevant to American crows and common ravens because knowing that they are two different species might incline some people to believe that the answer to the question “Can they interbreed?” is, by definition, “No.”  However, hopefully now it’s clear that such dogmatism will only hamper our understanding of the situation.  To advance further on this question, we instead need turn our attention to the tree of life.

For animals to freely interbreed, they generally need to share a very recent relative.  Given their geographical overlap and how similar American crows and common ravens look, it might be tempting to assume they must be quite closely related, but of course by now you know what assuming will get you. As you can see from the phylogentic tree below, you need to go back four ancestors (or about 7 million years) to find the relative shared by both birds.1  American crows are actually more closely related to the collared crows of China than they are to common ravens.  So, while we haven’t ruled on our original question one way or another just yet, this suggests it shouldn’t happen extremely freely.

Crow phylogeny

Jønsson et al. 2012

For the last piece we need to consider the real life, biological relationship between these birds.  Most crucially, we need to ask if, when in close proximity, they generally tolerate each other and socialize.

A paper out earlier this year by Freeman and Miller answers this question with a resounding, “No.”2 Thanks to thousands of observations provided by citizen scientists, the authors were able to show that most interactions between the two are aggressive, and that crows are almost always the aggressor, particularly during the reproductive period. This is thanks in no small part to the fact that ravens will depredate crow nests. Therefore, come breeding time, crows will be most anxious to evict ravens, not bed them.

DSC_0675 (2)

A crow attempts to chase a raven out of its territory

With all this in mind, it seems we can finally conclude that the most informed answer would be, “Ravens and crows do not hybridize…

…most of the time.”

Ok time for me to come clean.  The truth is that I’ve buried the lead a little bit here, because in fact we know the answer to the question “can they hybridize.” In 1990’s Beth Jefferson documented a successful breeding attempt between a wild American crow and common raven in Toronto, Canada.3  In 1990, the single raven started showing up in the area; a surprise to the local birders since they didn’t usually encounter ravens for another 145km north.  Over the course of the next three years, the raven appeared to be buddying up to a crow in the area.  It wasn’t until 1993, however, that Beth and other locals were able to document the pair nest building, attending to nestlings, and successfully fledgling two young.

hybrid

One of the hybrid young documented by Beth Jefferson

So why on earth did I drag this out so much? Because your take home message should not be that crows and ravens are going buck wild making little cravens.  Just as one swallow does not a summer make, one or two craven babies does not a habit make.  By and large, American crows and common ravens are reproductively isolated and do not hybridize.  But under the strangest of circumstances there’s no questioning that…

life

Literature cited

  1. Jønsson K.A., Fabre P.H., and Irestedt, M. 2012.  Brains, tools innovations and biogeography in crows and ravens.  BCM Evolutionary Biology 12
    https://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-12-72
  2. Freeman B.G. and Miller, E.T. 2018.  Why do crows attack ravens? The roles of predation threat, resource competition, and social behavior.  The Auk 135: 857-867
    http://www.bioone.org/doi/full/10.1642/AUK-18-36.1
  3. Jefferson E.A. 1994.  Successful hybridization of common raven and American crow.  Ontario Birds.  https://sora.unm.edu/sites/default/files/27-35%20notes%20OB%20Vol12%231%20Apr1994.pdf

18 Comments

Filed under Uncategorized