Monthly Archives: September 2018

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?

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)

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.

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.

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:

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:

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!”

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.

***

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?”

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.

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.

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 the 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.

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…

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