Category Archives: Science

2018 research round up

As 2018 draws to a close, I want to dedicate a post to five of the most interesting and important publications about our favorite family of birds that came out this year. For the sake of a brevity, the reported studies are largely condensed with some tests/results omitted and little attention to normally key experimental elements like controls, statistical analyses, etc. Please click on the study title to be directed to the full publication.

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1. Townsed AK, Frett B, McGarvey A, and Taff CC. (2018). Where do winter crows go? Characterizing partial migration of American Crows with satellite telemetry, stable isotopes, and molecular markers.  The Auk 135: 964-974

Background: Depending on where you live, the answer to, “Do crows migrate?,” can be quite different.  For example, most Seattle residents would probably say no, since large numbers of crows can be seen here year round, while someone in say, a southern Canadian province, may notice a sharp decline in the number of crows during the winter.  That’s because crows are what’s know as “partial migrant species” meaning that within a population, some individuals may be migratory and others resident with more migratory strategies biasing in areas with harsh winters.  Despite the role of partial migration in how scientists currently explain the evolution of complete migration, little is known about the phenomenon.  Even elemental questions such as: is this behavior fixed or flexible within individuals, is it environmentally influenced, and how might species use it to adapt to changing conditions remain under-explored.

Methods: The study looked at two populations of overwintering crows: one in Ithaca, New York and a second in Davis, California.  They used a combination of intrinsic (meaning originating in the body) and extrinsic (meaning originating outside the body) markers to track the movement and origin of their 18 tagged subjects over 2-4 years.  The intrinsic makers included molecular and stable isotope data, and the extrinsic marker was a satellite tracking device that was attached to the bird via a light backpack.  I won’t go into the details of the molecular and stable isotope data, but suffice it to say that stable isotopes were used to identify the place of origin via the unique properties of the local food and water that embed into an individual’s tissue and the molecular data was used to sex individuals and establish relatedness.

Key findings: Of the 18 tagged crows across both east and west coast populations, they found that almost 78% were migratory.  This was a shock to me, TBH.  I had no idea just how many crow were making these annual trips.  The distance these birds traveled varied widely, with some going as “little” as 280 km (173 miles) and others as much as 1095 km (680 miles). Among resident birds, they found that individuals never ventured further than 25 km (15.5 miles) from the center of their breeding site.  For both resident and migratory individuals they found that birds were very loyal to their breeding sites; returning to the same territory year after year.  Given this finding, it should not be surprising to learn that individuals did not vary from year to year in whether they were migratory or not.  Together these results offer clues to how crows may respond to climate and urbanization induced changes in temperature to their local environments.

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2.  von Bayern AMP, Danel S, Auersperg AMI, Mioduszewska B, and Kacelnik A. (2018). Compound tool construction by New Caledonian crows. Nature Scientific Reports 8

Background: For decades people considered the use of tools to be a uniquely human feature.  Now we know that all sorts of animals, ranging from fish to monkeys, use tools and a handful of animals even create tools.  Among the small number of animals that create tools, we have only seen wild individuals modifying a single object.  For example, stripping a twig of small leaves or branches in order to probe small holes for insects.  Whether any wild animal is capable of making compound tools, those made by combining seperate non-functional parts, is unknown.  Even in captivity, this behavior only has limited observation in the great apes.  Understanding what animals are capable of this complex task and how they achieve it, might give us insight into the evolution of our own exective functions.

Methods: This study used eight wild caught captive New Caledonian crows.  Like many experiments involving novel objects, this one occurred over multiple different phases.  In phase I the birds were provided a long stick and a baited test box where food was within reach when using the stick, but not without it.  In phase II the birds were presented with the same baited test box, except that instead of a single long stick, they were given a hollow cylinder and a second, thinner cylinder that needed to be combined in order to generate a tool long enough to reach the food.  In phase III, the birds were given the same problem, only now with novel combinable items.  In phase IV, the researchers tested whether the birds were combining elements because they understood that they needed to, or if because they derived some other benefit from the process.  To do this, they presented birds with a bait box that had two tracks: one where the food was within reach of a single element and one where it required a compound element. In the final phase, birds were presented a bait box that required the combination of more than two elements.

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Image from von Bayern et al. 2018

Key findings: All birds passed the initial tool use phase handily.  Given that New Caledonian crows frequently use single element tools in the wild, this was not at all surprising. In the second phase, half of the subjects (four) were able to combine the two elements after no more than two failed attempts. These subjects were then able to transfer this knowledge when presented novel combinable objects. When given a bait box with food presented on the close and far tracks, birds most often only made compound tools when it was necessary, suggesting that they don’t do it just for fun.  In the final phase, only one bird succeeded in making a tool that required more than two elements.  These findings demonstrate that New Caledonian crows are not only on par with what’s know about compound tool use in the great apes, but actually exceed them.

Unfortunately what this study does not explicitly answer is whether the birds were able to create the needed tools as a result of mental mapping (i.e imagining the correct tool and how it might be assembled) or by happy accident.  Without this knowledge, what their ability to make compound tools suggests about the evolution of things like insight remains mysterious.  Given all the other remarkable ways New Caledonian crows show innovation when it comes to tool use, however, both myself and the authors of this study are hedging that it’s indeed cognition behind these behaviors rather than more simple mechanisms.

3. Boeckle M, Szipl G, and Bugnyar T. (2018). Raven food calls indicate sender’s age and sex. Frontiers in Zoology 15

Background:  One of the most frequent inquiries that come my way are requests to decipher various crow calls.  Given all we know about crows, this doesn’t seem like such an impossible request, but the reality is that crow communications remains one of the most impenetrable black boxes of crow behavior.  I’ll save more on this for a future post dedicated to an upcoming publication by my colleague Loma Pendergraft, who spent his MS learning this fact the hard way.  But suffice it to say that any progress on this front in the various Corvus species is groundbreaking news.  We do, however, know more about raven calls. For example long “haa” calls are thought to recruit other individuals to sources of food.  What was unknown at the start of this study was whether these calls encoded any class-specific information about the caller, such as their age or sex. Calls that impart class-level information about the caller have been previously demonstrated in some marmots and monkeys.

Methods: The researchers recorded hundreds of “haa” calls from wild ravens which had previously been color banded and whose age and sex were known.  Using acoustic software they analyzed the vocalizations for patterns in call elements like frequency and inflection rate.

Key findings: As the study’s title suggests, ravens appear to encode information about their age and sex in “haa” food calls.  For animals like ravens that live in “fission-fussion” social systems, meaning flexible social groups where individuals regularly reencounter familiar individuals, but also encounter unfamiliar ones, class-level information helps individuals quickly assess important aspects of a caller’s identity.  Such information may be key to helping individuals decide if they want to join a feeding event or not.  This decision is particularly important because aggression at feeding events can cause mortal injury, so grouping with a bad crowd can come at a high price.

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4. Kroner A, and Ha R. (2018). An update of the breeding population status of the critically endangered Mariana Crow (Corvus kubaryi) on Rota, Northern Mariana Islands 2013–2014. Bird Conservation International 28: 416-422 

Background: The Mariana crow or Aga is a native species to the islands of Guam and Rota.  After the introduction of the brown tree snake to Guam in the 1940’s, Guam’s entire population of Aga were wiped out leaving only those found on Rota to continue the species.  In 1982, the population hovered around 1,300 individuals but things were clearly in decline. In 1984 the Aga was officially listed as endangered and today is considered critically endangered by the IUCN.  Unlike on Guam, there is no clear reason why the Aga continues to decline on Rota, though habitat loss, persecution by humans, natural disasters and introduced predators like cats likely all work together.

Methods: During 2013-2014 researchers counted breeding pairs by surveying all known island territories.  During these counts (which took 845 hours of labor and traversed 1,485 hectares!) the researchers also documented any unpaired or subadult birds. Since the entire island could not be surveyed, to ultimately estimate the population size the researchers used models that accounted for missed detections.

Key findings: Spoiler alert: They are A BUMMER.  In all that searching only 46 breeding pairs were detected.  Accounting for unpaired birds and detection failures, the researchers estimate that the current population of Aga hovers around 178 individuals.  Obviously that number alone is a gut punch but it’s especially true when you consider that that’s a 10-23% decline since 2007 and a 46-53% decline since 1998.  Researchers estimate that at least 75 pairs are needed to maintain a viable population of Aga.  Without intensive predator management and community level advocacy for these birds, their future is sadly looking grimmer and grimmer.

5. Walker LE, Marzluff JM, Metz MC, Wirsing AJ, Moskal ML, Stahler DR, and Smith DW. (2018). Population responses of common ravens to reintroduced wolves. Ecology and Evolution 8: 11158-11168

Background: One of the most persistent myths about common ravens is that they have a symbiotic relationship with grey wolves; intentionally showing them carcasses they find and then sharing in the bounty together.  But while the case is actually that ravens are unwelcome dinner guests at the wolves’ table, there’s no question that the two species have profound effects on one another. The reintroduction of wolves to Yellowstone in 1995 therefore offers a valuable way to study how the presence of wolves affects the spatial distribution and feeding behaviors of park ravens.

Methods: This study was a collaborative effort between avian and spatial ecologists at the University of Washington and Yellowstone wolf biologists.  Using data from 2009-2017 on wolf abundance and prey kills, and raven surveys taken both within the interior of the park and at anthropogenic food sources in surrounding areas (ex: the Gardner town dump), the researchers were able to model raven abundance during both the study period and before the reintroduction of wolves.  I won’t go into the details of how these models are created, but suffice it to say that their purpose is to take the data you give them and find what predictors best explain your observed outcomes.  For example if, say, you have a bunch of data about where ravens were located at different times, and have data on different possible predictors, say, wolf abundance, weather, carcass abundance, carcass biomass, and distance to anthropogenic food, etc., the right model could help you identify that carcass biomass is the best predictor of raven abundance.

Key findings: Previous studies have demonstrated that wolves make more kills during severe winters with higher snowpack, because prey have a more difficult time evading them.  As a result, the researchers hypothesized that ravens would depend more heavily on wolf kills during severe winters,  but this is not what they found.  Instead, Yellowstone ravens seem to lean more on consistent, anthropogenic food sources during tough winters, but lean more on wolf provided carrion during more mild winters.  Still, the presence of wolves has increased and stabilized the number of ravens in the park, because they provide a second year-round source of food, in contrast to human hunter provided kills which are seasonally limited.  These findings are yet another demonstration of the value of top carnivores in stabilizing food webs and providing food for a cascade of creatures.

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6. And as a bonus let’s not forget the most important 2018 study of them all, “Occurrence and variability of tactile interactions between wild American crows and dead conspecifics,” which you can read all about here. 😉

leslie

 

 

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Filed under Conservation, Crow behavior, crow intelligence, New Research, Raven behavior, Ravens, Science

You need to know more about jay spit

Look, I’m a reasonable person.  I know what you’re thinking.

“Literally never has it occurred to me I might know too little about jay spit.”

But here’s the thing: it’s actually super interesting and you really can’t understand Canada jays without knowing about their saliva.  It would be like trying to understand the internet without cat videos-you just can’t do it.  So trust me when I tell you this is the information you didn’t know you needed.

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In the early 1960’s Walter Brock was examining Canada jay corpses when he discovered that they have massive salivary glands on par with the ones found in woodpeckers.1 Such generously sized glands are found in no other songbird.  Furthermore, like the woodpeckers, it’s not just that Canada jays make a lot of saliva, but they make a lot of sticky saliva.  At the time this discovery was made, it was already known that the enlarged glands of woodpeckers served to allow for a foraging tactic called “tongue probing” where, like anteaters, the birds use their long sticky tongues to extract food from narrow crevices.  Although Canada jays don’t have especially long tongues, the ability to tongue probe seemed the most parsimonious explanation for this strange adaptation, and Brock suggested that this strategy may actually be the key to the jays’ winter survival.  A study a few years later examining their foraging behavior revealed that they don’t feed in this manner, however.  They feed more or less the same way the other corvids do.2  It seems instead, that it’s what they do with the food after that’s different.

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Rather than using their copious amounts of weird, sticky spit for acquiring food, it’s used for depositing it.  If you watch a jay closely after it’s got a bit of food you’ll notice it seems to have missed Emily Post’s memo about chewing with your mouth closed. Over the course of a few seconds you’ll see the food peek out from the bill as the bird moves it around inside its mouth.

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This jay picked up this bit of food about 60sec before this photo was taken.  Now it’s working it around with its tongue, coating it in sticky saliva.

Once sufficiently spit coated, the bird will deposit the food blob (called a bolus) onto the foliage or trunk of a tree.  No matter the material or angle, once the spit dries the food is safely secured come hell or high-water.  Because these caches are pretty small there’s little fear that many will be found.  More importantly, by stashing food high in the trees instead of burying them into the ground like many other cache-dependent corvids do, Canada jays can thrive in areas that receive much heavier snowfall, allowing them the title of the most northern residing jay in North America.

Here’s where it all really comes together though.  If you’ve seen me write about Canada jays before you’ll have noticed that it’s almost inevitable that I’ll use the phrase “Cute little faces” at some point to describe them.  But have you ever wondered why? Why do they have such cute little faces?  While jays do feed more or less in the same way as other corvids the one exception is that they don’t hammer at objects.  If you’re ever given a crow or a Steller’s an unshelled peanut you’ll know exactly the motion I mean. Without the need the hammer objects, or dig holes for burying food, Canada jays don’t need the heavy bills their cousins do.2  Instead they have the blunt little bill that helps give them their characteristic baby-faced look.  So not only is their spit responsible for their ability to tough it out in some of the harshest winter environments this continent offers, but it also means they get to look super cute while doing it.

So like I said, you don’t really know Canada jays until you know a thing or two about their spit.

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Literature cited

  1.  Brock WJ. (1961). Salivary glands in the gray jay (Perisoreus). The Auk 78: 355-365
  2. Dow DD. (1965). The role of saliva i food storage by the gray jay.  The Auk 82: 139-154

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Filed under Birding, Canada jays, Corvid trivia, Diet, Field work, Jay behavior, Science

Putting the “crow” in necrophilia

It’s early April 2015, and John Marzluff and I are standing with a film crew attempting to capture some footage of a crow funeral to compliment a story they are working on about Gabi Mann.  I’ve already set the dead crow on the ground, it’s placed just out from a cherry tree resplendent in springtime blossoms.  After only a few moments of waiting, the first crow arrives and alights on the tree, its head cocking around to get a better look at the lifeless black feathers beneath it.  I hold my breath for the first alarm call, ready for the explosion of sound and the swarm of birds that will follow it.  But it doesn’t come.  Instead, the bird descends to the ground and approaches the dead body.  My brow knits together in surprise but, ah well, I think, the shots of it getting so close and then alarm calling will make good footage.   The audience will have no questions about what it is responding to.  To my continued surprise, however, the silence persists; only now the crow has drooped its wings, erected its tail, and is approaching in full strut. No, no, this can’t be, I think.  But then it happens.  A quick hop, and the live crow mounts our dead one, thrashing in that unmistakable manner.  “Is it giving it CPR?” someone asks earnestly.  Still in disbelief, John and I exchange glances before shaking our heads and leaving the word “copulation” to hang awkwardly in the air.  After a few seconds another bird arrives to the cherry tree and explodes in alarm calls, sending our first bird into its own fit of alarm, followed by a more typical mobbing scene.  The details of what I’ve just witnessed as still washing over me when I hear John lean over to me…”You need to start your field season tomorrow.”

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What crows do around dead crows is something I’ve dedicated much of my academic life to understanding.  In the course of my first study, my findings made for a nice clear narrative: crows alarm call and gather around dead crows as a way of learning about dangerous places and new predators.  Although there are other hypotheses we can’t rule out, certainly danger avoidance is at least partially driving this behavior.  An important detail of that original study though, is that because of the way it was designed, with a dangerous entity always near the dead crow, our live crows were never in a position to ever get very close to our dead stimulus. So the possibility that they do other things around dead crows, like touching them, couldn’t be explored.

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It’s been 3 years since that day in April and during that time it has taken every ounce of my power to remain tight lipped when journalists would ask “what’s the most interesting thing you’ve learned from your studies?” Because until we were able to scientifically vet the prevalence of this behavior, I wasn’t willing to say much about it for fear of making necrophilia mountains out of mole hills. But with our findings now officially available in the journal Philosophical Transactions B, I am delighted to finally share what has been the most curious secret of my PhD: crows sometimes touch, attack, and even copulate with dead crows.

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Although this statement is jarring in its own right, what really gives it power is that we know this not just from that first fateful day with the film crew, but through an experimental study testing the response of hundreds of birds over several years.  That’s important because it allows us to say not just what they’re doing but possibly why they’re doing it (and at least why they’re not doing it).  So how did we conduct this experiment?

First, I dove into the literature to try and see if there was any precedent for this kind of behavior in other animals.  Although there have been no systematic studies, repeated observations of animals touching, harming, even copulating with their dead occur in dolphins, elephants, whales, and many kinds of primates, among some other animals.  Based on this, we hypothesized that this behavior may arise from: attempts to eat it, attempts to learn from it, or a misuse of an adaptive response (like territoriality, care taking, mate guarding, etc.). To test these ideas I searched the neighborhoods of Seattle until I found a breeding adult pair and (while they weren’t looking) presented one of four stimulus options: An unfamiliar dead adult crow, an unfamiliar dead juvenile crow, a dead pigeon or a dead squirrel.  The latter two stimuli being key in helping us determine if the behavior was food motivated, whereas the nature and prevalence of the interactions themselves (common, uncommon, exploratory, aggressive, sexual) helped us address the other hypotheses.  In all, I tested 309 individual pairs of crows; or in other words, once again I freaked out a lot of Seattle residents wondering why there was a woman with a camera, binoculars, and some dead animals loitering in front of their house for long periods of time.

Our main findings are that crows touched the animals we would expect them to eat (pigeons and squirrels) more than the dead crows, and although crows sometimes make contact with dead crows, it’s not a characteristic way they respond.  Because this behavior is risky, this seems to back up previous studies in crows that suggest that they are primarily interested in dead crows as a way of self preservation and avoiding danger.

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A crow tentatively pokes at one of our dead crows

That said, in nearly a quarter of cases, crows did make some kind of contact with dead crows.  Like with mammals, we saw that these behavior could be exploratory, aggressive and in rare cases even sexual (about 4% of crow presentations resulted in attempted copulations), with the latter two behaviors being biased towards the beginning of the breeding season.  Importantly, the latter two categories of interactions were rarely expressed independently, and it was often a mixture of the first two; in rare cases, all three.  In the most dramatic examples, a crow would approach the dead crow while alarm calling, copulate with it, be joined in the sexual frenzy by its presumed mate, and then rip it into absolute shreds.  I must have gone through a dozen dead crows over the course of the study, with some specimens only lasting through a single trial. It was an issue that may have been insurmountable if not for the donations of dead crows by local rehab facilities and the hard work of my long time crow tech turned taxidermist, Joel Williams.

It’s hard to witness this behavior without wondering if maybe the crows somehow don’t recognize that it’s dead and are instead responding like they might to a living intruder or to a potential mate.  So we tested that idea too, by conducting a second experiment where we presented either a dead crow or a life-like crow mount.  The differences in their response was clear.  They dive bombed the “live” crows and less often formed mobs, just like we would expect them to do for an intruder.  They also attempted to mate with the “live” birds but in these cases it was never paired with alarm calling or aggression.  So the issue doesn’t seem to be that they think it’s alive.

The fact that this behavior was rare, and often a mix of contradictory behaviors like aggression and sex, seems to suggest that none of those hypotheses I outlined earlier are a good fit for this behavior.  Instead, what we think happens is that during the breeding season, some birds simply can’t mediate a stimulus (the dead crow) that triggers different behaviors, so instead they respond with all of them. This may be because the crow is less experienced, or more aggressive, or has some neurological issue with suppressing inappropriate responses.  Only more experiments will help us determine what makes this minority of birds unique, and whether expressing these seemingly dangerous behaviors are the mark of the bird that is more, or less reproductively successful in the long haul.

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So while there’s still much more left to be explore here, I can finally say that this is without a doubt some of the most interesting behavior in crows I’ve ever witnessed.  I hope you will check out the publication here, and seek out all the other amazing work being reported in this special thanatology (death science) themed issue.

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Filed under Being a scientist, Breeding, Crow life history, Field work, Graduate Research, New Research, Science