Category Archives: Crow life history

May 15, 2022 · 10:07 pm

Crow curiosities: do crows migrate?

That birds travel seasonally is perhaps one of the most familiar facts about the natural world. Whether it’s the arrival of technicolor spring migrants, or the din of waterfowl above our heads in the fall, it takes no formal training to recognize that something novel and beautiful has suddenly erupted into our lives. Their ephemeral presence offers an opportunity to ground ourselves in time and place and reflect on the shape of our lives since our last meeting. Or, if you don’t want to get that deep with it, there’s always Looney Tunes or any number of other children’s cartoons to remind us that some birds come and go with the seasons.

At the same time, for most of us living in the continental United States, that crows will be nearby to accompany us throughout our year is something we take for granted. Their predictability on our telephone poles and near our garbage cans is one of those quiet details not everyones thinks of often, but whose consistency surely calms us as so many other things feel unsteady. But at the intersection of these truths is an interesting question: if migration seems such an essential part of bird life, why don’t crows do it? Or do they?

Of the world’s ~ten thousand birds species, only 18% actually undertake annual long distance migrations.¹ But there are other types of migration including short distance migration, altitudinal migration (short migrations from shorter to higher altitudes) and partial migration. Partial migration is when only certain individuals within a population migrate, while others are sedentary, and it’s this one that applies to crows. Because while crows in temperate Seattle may be quite comfortable year round, those that call higher latitudes “home”, say central Canada, would have a tougher time making it through the winter unscathed. So much like a wealthy aging relative, they snowbird it to more welcoming climates for the winter.

Until the last decade or so that’s really all we knew about about crow migration. Some did it, some didn’t, and that was that. But given the value of better understanding this behavior, as well as the technological advances that make it possible, western science has finally turned its eye to inspecting these patterns more closely. Because there are so many questions one could ask about this phenomenon. Why do some crows migrate and others do not? Do the same crows migrate each year or can they opt out? Do they return to the same place? How far do they fly? How do they survive the journey? How do they sleep?

In some cases the answers to these questions are surprisingly nuanced. For instance, birds living in the Central and Southern Canadian provinces will nearly always migrate hundreds of miles south into the US, which makes sense because Canadian winters can be especially harsh. But studies have shown that other crows make even shorter migrations, only about 350 miles between contiguous US states with similar climates.² What motivates these shorter-distance trips remains to be seen. Likewise, whether migrating is a discretional activity is still in question. From a climate change perspective, that individual birds might be able to choose whether or not to migrate each year would have important implications in their ability to adjust to changing conditions. But in the studies conducted so far there’s been no evidence that crows pick and choose each year, rather it seems that you’re either a migratory individual or not, albeit this is based on low sample sizes.³ As a result, migrating crows show extremely high site fidelity; returning to the same breeding and winter sites each year.

As far as distances go, there’s huge variability there as well. In a study that looked at crows from both sides of the country, the average distance traveled by east coast crows was 287 miles, while west coast crows traveled an average of 366 miles.³ The longest migration on record is 1740 miles.⁴ One thing that’s for certain is that migration is a daytime affair. While most birds migrate at night, crows are among the minority that stick to business hours for their travels. The most compelling explanation for strategy is the “fly-and-forage” hypothesis. Look, even for the arguably most efficient fliers in the animal kingdom, flying is hard work and crows need a lot of calories to sustain flight speeds up to 37mph covering as much as 186 miles in a single day.² To accomplish this, crows will make up to several pit stops during a marathon flight session, allowing them to refuel and prepare for the next leg. Importantly, although crows migrate both in groups and alone, when stopped they almost always make sure to dine with others. Traveling in the daytime probably helps facilitate finding other crows, and by extension, the local feeding grounds.

When it comes to sleeping, crows leave the fancy mid-flight naps to the swifts and the frigatebirds, opting instead to sleep at night in communal groups. In fact crows will sometimes abruptly change direction just to follow local birds to the nearest roosting grounds. In other cases though, crows have been know to travel routes that allow for stops at the same roosts year after year, underscoring why roost locations should be considered key pieces of habitat.²

So as it stands, crow migration can be summarized the same way as most other parts of their lives: we know some stuff, not everything, we’re probably wrong about a few things, and our best bet is to accept our role as eager pupil and celebrate the gifts of knowledge as they are granted to us.

Literature cites
1. Sekercioglu C.H. 2010. Partial migration in tropical birds: The frontier of movement ecology. Journal of Animal Ecology: https://doi.org/10.1111/j.1365-2656.2010.01739.x

2. Ward M.P. and Raim A. 2011. The fly-and-social foraging hypothesis for diurnal migration: Why American crows migrate during the day. Behavioral Ecology and Sociobiology 65: 1411-1418

3. Townsend A.K., Frett B., McGarvey A. and Taff C.C. 2018. Where do winer crows go? Characterizing partial migration of American crows with satellite telemetry, stable isotopes, and molecular markers. The Auk 135: 964-974

4. Brewer D., Diamond A.W, Woodsworth E.J, Collins B.T.,, and Dunn E.H. 2000. Canadian Atlas of Bird Banding, vol 1: Doves, Cuckoos, and Hummingbirds through Passerines, 1921-1995. Canadian Wildlife Service, Ottawa, ON, Canada

11 Comments

Filed under Climate change, Crow behavior, Crow life history, Movement

Tagged as crow behavior, flight, how far do crows fly, how fast can crows fly, migration, Movement, roost

April 16, 2020 · 6:45 pm

The definitive guide for distinguishing American crows & common ravens

For two birds that are surprisingly far apart on the family tree, American crows (Corvus brachyrhynchos) and common ravens (Corvus corax) can be awfully hard to distinguish, especially if you rarely see both together. But with the right tools and a little practice you can most certainly develop the skill. Fortunately, there are many different types of clues you can use to tell one from the other, so feel free to use the links to skip around to what interests you.

Physical Differences

Although crows and ravens are superficially quite similar, there are variety of features that can be used to tell one from the other. Overall size can be a good place to start. This especially helpful if you live in an area where they overlap, but even if you don’t, I find that people who are used to seeing crows take notice when they see a raven in person because it feels ~aggressively~ large. That’s because ravens, by mass, are about twice the size of an American crow.

A common raven specimen (top) with an American crow specimen (bottom). On average, ravens are about twice as big as crows, but individually there are certainly large crows and diminutive ravens.

This size difference becomes most obvious is when you look at their face. Raven’s are much more adapted for consuming carrion than crows are (crows cannot break through the skin of a squirrel) and their bills give the distinct impression that they could, in fact, pluck your eyes from your face with little effort. So if your sense of things is that you’re looking at a bill with a bird attached, then you’re probably looking at a raven, not a crow.

With practice, judging the proportion of crows’ and ravens’ features, like bill size, becomes easier.

With practice, judging relative size becomes easier and more reliable, but for a beginner it may not be useful because it’s so subjective. Instead, it’s easier to look at the field marks (birder speak for distinctive features) which provide more objective clues.

When looking at perched birds, the most helpful attribute is to look at the throat. Ravens have elongated throat feathers called hackles, which they can articulate for a variety of behavioral displays. Crows meanwhile have smooth, almost hair like throat feathers typical of other songbirds.

Even when the feathers are relaxed, the textural differences between the two species throat feathers are apparent. Note that in this photo, the crown feathers of the crow are erect, while the raven’s is not. The difference in crown shape should not therefor be judged in this comparison.

When vocalizing or displaying the raven’s hackles become especially obvious.

In addition to the hackles, ravens can also articulate some of their other facial feathers in way crows cannot. During threat displays for example, ravens will fluff out both the throat hackles and their “ear” tufts.

For birds in flight, however, it’s often difficult—if not impossible—to clearly see the throat feathers. Fortunately, the tail offers a reliable field mark in this case. Whereas crows have a more squared or rounded tail (depending on how much they’ve fanned the feathers) a raven’s tail will have a distinct wedge shape. Additionally, although they are a bit more subtle, there are also some differences in the primary wing feathers. While both birds have 10 primary feathers, in flight, ravens will look like they have four main “finger” feathers while crows will appear to have five. Ravens also have more slender, pointed primaries relative to crows.

Vocal differences

With a little practice American crows and common ravens can easily be distinguished by their calls. The call of a raven can be best described as a deep, hollow croak. Crows on the other hand, caw. Of course, they can both make at dozens of other sounds including rattles, knocks, coos, clicks, and imitations. With practice even these can be recognized by species, but that level of detail is not necessary for most identification purposes.

Common raven call (Recording by Davyd Betchkal-Denali National Preserve, Alaska)

Juvenile common raven yell (Recording by Antonio Xeira-Chippewa County, Michigan)

Common raven water sound (Recording by Niels Krabbe-Galley Bay, British Columbia)

American crow call (Recording by David Vander Pluym-King County, Wasington)

American crow juvenile begging call (Recording by Jonathon Jongsma Minneapolis, Minnesota)

American crow rattle (Recording by Thomas Magarian-Portland, Oregon)

American crow wow call (Recording by Loma Pendergraft King County, Washington)

American crow scolding (Recording by Kaeli Swift-King County Washington)

Geographic/habitat differences

While both American crows and common ravens have wide distributions across North America, there are some key differences in where you are likely to find them. The most notable difference is that ravens are absent throughout most of the midwest and the southeast. Crows on the other hand, occupy most American states with the exception of the southwestern part of the country. The below maps from Cornell’s All About Birds website offer more specific breakdowns (hover over the images to see the caption).

Common raven range map

American crow range

With respect to habitat, both birds are considered generalists, with ravens erring more towards what one might describe as an “extreme generalist”. Ravens can be found along the coast, grasslands, mountains (even high altitude mountains), forests, deserts, Arctic ice floes, and human settlements including agricultural areas, small rural towns, urban cities (particularly in California) and near campgrounds, roads, highways and transfer stations. Crows meanwhile are more firm in their requirement of a combo of open feeding areas, scattered trees, and forest edges. They generally avoid continuous forest, preferring to remain close to human settlements including rural and agricultural areas, cities, suburbs, transfer stations, and golf courses. In cases where roads or rivers provide access, however, they can be found at high elevation campgrounds.

Behavioral differences

There are books that could be (and have been) written on this subject alone, so we will limit ourselves to what is likely to be most essential for identification purposes.

Migration
While common ravens are residents wherever they are found, American crows are what’s called a “partially migratory species” because some populations migrate while others do not. Most notably, the northern populations of crows that occupy central Canada during the summer breeding season, travel south to the interior United States once the snow-pack precludes typical feeding behaviors

Breeding
Although trios of ravens are not uncommon, and there have been observations of young from previous years remaining at the nest, ravens are not considered cooperative breeders. Crows are considered cooperative breeders across their entire range (though specific rates vary across populations and not much is known about migratory populations). If helpers are present they typically have between 1-3. So if a nest is very busy with more than two birds contributing to nest construction, feeding nestlings, or nest defense, it’s more than likely a crow’s nest, not a raven’s.

Common raven eggs left | American crow eggs right

Diet
Although both species consume a host of invertebrates, crows consume a larger proportion of inverts and garbage relative to ravens. Mammals, especially from carrion, meanwhile make up the largest proportion of a raven’s diet across surveyed populations. Access to refuse and population location, however, can dramatically shift the dietary preferences of both these omnivores.

Flight
Because ravens consume a lot more carrion, which is unpredictable in its availability and location, they spend a great deal more soaring than crows do. So if you see a black bird cruising the sky for more than a few seconds, it’s most likely a raven. Ravens are also unique from crows in that they barrel roll to advertise their territory. So if you see a barrel rolling bird, there’s a better chance it’s a raven.

Interactions
In places where they do overlap, interactions between the two are often antagonistic, with crows acting as the primary aggressors in conflicts. Ravens will depredate crow nests if given the chance.

A raven defends itself from a crow by rolling upside down. Someday I’ll get a better photograph…

Genetic differences

Throughout most of our history, we have used external cues like appearance, voice and behavior, to sort one kind of animal from another. Now that we have access to a plethora of genetic tools, however, we can ask a new level of the question “what’s the difference between an American crow and a common raven.”

To put it simply, American crows (Corvus brachyrhynchos) and common ravens (Corvus corax) are different species in the same genus, just like lions (Panthera leo) and tigers (Panthera tigris). Species and genus refer to different levels of the taxonomic tree, where species represents the smallest whole unit we classify organisms. The issue of species can get complicated quickly, however, so I’ll direct you here if you want to learn what a mess it really is. Most important thing to appreciate now, is that if you want a quick, back of the envelope way to evaluate if two animals are closely related, look at the first part of their latin binomial (scientific) name. If they share that part then they’re in the same genus (ex: crows and ravens belong to the genus Corvus). If they don’t (ex: American crow is Corvus brachyrhynchos and the Steller’s jay is Cyanocitta stelleri) then they are more distantly related.

Within the Corvus genus, however, there is still a ton of evolutionary space available. In fact, to find the closest shared relative of common ravens and American crows you’d need to go back approximately 7 millions years. Although they are more visually distinct and don’t overlap geographically, American crows are more closely related to the collard crows of China, or the carrion crows of Europe, than they are to common ravens.

Image from Jønsson et al. 2012

Laws and protections

US laws
In the United States, both American crows and common ravens are protected under the Migratory Bird Treaty Act. This means that, like with nearly all native birds species, you cannot kill, possess, sell, purchase, barter, transport, or export these birds, or their parts, eggs, and nests, except under the terms of a valid Federal permit. It is this law that prohibits the average person from keeping these birds as pets, and requires that rescued crows be turned over to a licensed professional. The MBTA also prohibits the civilian hunting of ravens under any circumstance. Under 50 CFR 20.133, however states are granted an exception for crows, wherein with some restrictions, states can designate regulated hunting seasons.

In addition, under 50 CFR 21.43 of the Migratory Bird Treaty Act, you can also kill crows without a license and outside of the regulated hunting season if they are in the act of depredating crops, endangered species, or causing a variety of other destructive issues. You can obtain the specifics of the Depredation Order here. Such lethal control must be reported to Fish and Wildlife to remain within the law. No such depredation exceptions exist for ravens.

Canadian laws
In contrast to the US, no corvids receive federal protections in Canada. Crows and ravens may receive provincial protections, however.

Concluding thoughts

Before we pack it up, I want to leave you with one last useful piece of information. This whole article was dedicated to the question of how American crows are different from common ravens. Hopefully, you’re walking a way with a solid understanding that these animals are in fact different morphologically, behaviorally, and genetically. Asking if American crows are different from common ravens is a different question, though, than asking if “crows” are different than “ravens”. Because while that first answer is a hard, “yes,” there is no one thing that initially classifies a bird as either a type of raven or a type of crow. Generally ravens are bigger and have those elongated throat feathers, but there are plenty of crow named birds that could have been named raven and vice versa. So proceed cautiously and consider the specific types of birds the question’s author is referring to before offering specific answers.

If you want to continue to hone your skills I invite you to play #CrowOrNo with me every week on twitter, Instragram and facebook, all at the @corvidresearch handle. While it’s not to quite this level of detail, I promise it will help advance your ID skills and introduce to to more of the world’s fantastic corvids. For a head start, keep this charming and informative guide illustrated by Rosemary Mosco of Bird and Moon comics handy!

Reference literature
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

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

Verbeek, N. A. and C. Caffrey (2020). American Crow (Corvus brachyrhynchos), version 1.0. In Birds of the World (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA.

Boarman, W. I. and B. Heinrich (2020). Common Raven (Corvus corax), version 1.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.

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Filed under Birding, Corvid diversity, Crow behavior, Crow curiosities, Raven behavior, Ravens, Taxonomy, Vocalizations, Wildlife

Tagged as #CrowOrNo, Crow, crow calls, Crow vs raven, guide, raven, raven calls, Vocalizations

April 13, 2020 · 8:47 pm

A tale of two crows: northwestern vs. American

If you search nearly anywhere along the west coast from California to southern Alaska, you will find our most persistent avian neighbors: crows. Cloaked in their Gothic outfits and uttering that all too familiar harsh caw, most people—even many experts—might not register that the neighbors in the north are not exactly like their counterparts in the south. While it’s the American crows (Corvus brachyrhynchos) that have staked their claim to the contiguous states, it’s the northwestern crow (Corvus caurinus) that calls the coast home from British Columbia to southern Alaska. That is, at least as far as the field guides have been telling us since northwestern crows were first described scientifically in the mid 19th century. Despite this early recognition that one of these things was not like the other, however, differentiating American crows from northwestern crows on the basis of phenotypic features like size, voice, and behavior has since proven to an almost impossible challenge; especially in places like Washington where the two ranges meet. This has resulted in questionable hand waving by people like me about what those crows in Seattle really are. I’ve always called them American crows without any qualifiers, but are they really? Might they be northwestern crows? Or something in between? Fortunately, a new study by Slager et al. (2020) lays bare the reticulated evolutionary histories of the two crows of the Pacific Northwest.

By examining differences in both nuclear DNA from 62 specimens and mitochondrial ND2 markers from 259 specimens collected across North America, the team was able to evaluate when these two likely initiated speciation, the process of becoming distinct species from a shared ancestor. What they found is that American and northwestern crows likely split some 440,000 years ago when late Pleistocene glaciers really made mess of things by geographically separating formally intact populations. Isolated in their respective pockets of livable habitat (called glacial refugia) the formally united species did what all organisms do in the face of new selective pressure: they changed, and from one species emerged two. Well, kind of.

Eventually of course, the Pleistocene ice ages came to a close and the glaciers that had divided their ancestors receded away. Although their time apart made a lasting impression on their genome, it did not appear to make a lasting impression on their taste in sexual partners. The team found extensive genomic admixture (the presence of DNA in an individual that originated from a separate population or species), suggesting pretty pervasive hybridization between the two species. In fact, along their shared 900 km range from coastal Washington to British Columbia they found not one “pure” individual. For just how long American and northwestern crows have been hybridizing remains unknown, but the evidence suggests that it’s been happening since well before colonial landscape changes.

These revelations beg two important and contradictory questions. The first is the answer to just what the hell crows in Seattle are. The answer appears to be option C: a hybridized mix of American and northwestern crow, but with slightly more all-American genes. That seems to be true throughout the Washington coast. Once you hit Oregon though, you’re getting almost all ancestral American crow. The opposite pattern is true moving from British Columbia north: what starts as hybrids with a stronger northwestern crow bias, are “pure” northwestern crows once you hit Juneau.

Figure shows the extent of hybridization between American crows (red) and Northwestern crows (blue) along the PNW coast. Image from Slager et al. (2020).

The second question, however, is whether those distinctions are really of any biological value. After all, what appears to have happened is that while these two “species” may have gotten started on a path to different destinies, a changing climate brought them back together before any firm reproductive isolating mechanisms (i.e. physical features, behaviors, or physiology that prevent different species from breeding with one another) could take hold. While that kind of genetic evidence is already pretty damning, the phenotypic evidence that they might be different has likewise eroded. When closely examined, the features that appeared to be diagnostic in the 19th century like the northwestern crow’s smaller size and intertidal habitat use, and a difference in vocalizations, seem to simply be reflections of local adaptions and individual differences present in both species. So, while the guidebooks might still call them different things, the fact that neither the crows themselves nor the ornithologists can really tell them apart warrants serious consideration of whether northwestern crows should be officially absorbed into the American crow.

As it happens, the authority on such things, the American Ornithological Society’s North and Middle America Classification Committee, is currently examining that very proposal as apart of the 2020 Proposal Set C. Expect an official ruling soon. While I don’t know for sure what they will decide, the evidence out of this current study does not bode well for the continued recognition of Corvus caurinus as a species. So if you want to see a northwestern crow, my advice is to do it sooner rather than later.

Literature cited

Slager DL, Epperly KL, Ha RR, Rohwer S, Wood C, Van Hemert C, and Klicka J. 2020. Cryptic and extensive hybridization between ancient lineages of American crows. Molecular Ecology doi: 10.1111/mec.15377

14 Comments

Filed under Corvid diversity, Crow life history, Diversity, New Research, Science, Taxonomy

Tagged as American crow, AOS, hybrid, hybridization, name change, Northwestern crow, Taxonomy

July 16, 2018 · 3:55 pm

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.”

***

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.

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.

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.

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.

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

Tagged as Crow, dead crows, necrophilia, new publication, thanatology

December 8, 2017 · 7:28 pm

15,000 crows

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

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

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

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

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

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

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

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

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

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

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

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

Literature cited

Moore JE, and Switzer PV. (1998). Preroost aggregations in the American crow, corvus brachyrhyncos. Canadian Journal or Zoology. 76: 508-512.
Wright J, Stone RE, and Brown N. (2003). Communal roosts as structured information centers in the raven, Corvus corax. Animal Ecology 72: 1003-1014. DOI: 10.1046/j.1365-2656.2003.00771.x
Donald F. Caccamise, Lisa M. Reed, Jerzy Romanowski and Philip C. Stouffer

(1997). Roosting Behavior and Group Territoriality in American Crows. The Auk 114: 628-637
Marzluff, J.M. and Angel, T. 2005. In the company of crows and ravens. Yale University Press

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

Tagged as bothell, crow river, Crow roost, people and crows, Ravens, roosting, where are the crows going, why are crows so noisy, why do crows gather at night, why do I see crows at night

October 26, 2016 · 7:03 pm

New research on the cause of the AKD outbreak

Since the nineties, Avian Keratin Disorder has been an increasingly common disorder among Alaskan and PNW crows, chickadees (~17% of northwest crows¹, ~6% of black-capped chickadees²) and a handful of other species, that causes gross deformities of the beak such as elongation, curvature or crossing. I’ve written previously about the details of this disease before, but at that time there was little progress in determining the underlying source of the outbreak. While AKD can be caused by a variety of things, at the scale it’s being observed now scientists questioned if there was a more consistent underlying factor. Since AKD can cause discomfort or even death (primarily through the inability to feed or preen) understanding what might be the source of this outbreak has clear management and conservation implications.

An AKD-afflicted American crow in Seattle, WA.

Among the initial suspects were environmental contaminants such as heavy metals, organic pesticides, and toxic environmental pollutants like PCBs, PCDDs, and PCDFs. Blood work done on afflicted Northwestern crows, however, showed no significant difference in the 30 blood elements tested compared to unaffected adults or juveniles³. Fortunately, new research may finally be shedding light on what’s going on.

Disease can be an easy thing to rule out if you know what you’re looking for, but new to science pathogens can evade traditional diagnostic techniques. To account for this, a team of USGS and university scientists conducted a sequencing study comparing pooled RNA of healthy and AKD positive chickadees, crows and nuthatches in attempt to identify a candidate pathogen². Their work appears to have paid off, revealing evidence for a new picornavirus (a family of viruses previously known to science) they are calling poecivirus. Whereas 100% of AKD-affected birds (23 subjects) tested positive only 22% of the 9 control individuals did.

black-capped_chickadee_in_homer_alaska_c_martin_renner-768x432

Alaskan black-capped chickadee with severe AKD. Photo c/o Martin Renner

Given these small sample sizes, it’s too early to throw our hands up in complete relief of having identified the cause of the AKD outbreak, especially since there’s still much to be done in understanding the potential relationship of this new virus to the environment. Nevertheless, these findings offer some insight and hope that scientists are on the right track. With more dedicated work we may soon have a much better understanding of this novel pathogen, its link to AKD, and management options moving forward.

Literature cited

1. Van Hemert C, & Handel CM. 2010. Beak deformities in Northwestern crows: Evidence of a multispecies epizootic. The Auk 127: 746-751. doi: http://dx.doi.org/10.1525/auk.2010.10132

2. Zylberberg M, Van Hemert C, Dumbacher JP, Handel CM, Tihan T, and DeRisi JL. 2016. Novel picornhttps://wordpress.com/post/corvidresearch.wordpress.com/3363avirus associated with Avian Keratin Disorder in Alaskan birds. mBio 7 doi: 10.7589/2015-10-287

3. Van Hemert C, Handel C. 2016. Elements in whole blood of Northwestern crows (Corvus caurinus) in Alaska USA: No evidence for an association with beak deformities. Journal of Wildlife Diseases 52:713-718 doi: http://dx.doi.org/10.7589/2015-10-287

13 Comments

Filed under Birding, Corvid health, Crow curiosities, Crow disease, Crow life history, Ecosystem, Uncategorized

Tagged as AKD, Avian keratin disorder, beak, bill, deformed bill, deformity, disease, health, sick crow

June 9, 2016 · 2:57 am

5 reasons to leave baby crows alone

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

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

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

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

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

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

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

Tagged as Baby crow, rescue

March 29, 2016 · 4:50 pm

Everything you want to know about crow nests

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

Nest construction and site selection

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

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

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

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

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

Not sure how they did it, but this pair managed to build a nest on the underside of the Alaska Way Viaduct in downtown Seattle

This is one of my favorite nesting locations I’ve discovered so far. Rather than being deterred by the bird spikes, they’ve essentially used it as rebar!

Differences among corvids

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

Creativity_crows_nest_made_of_clothes_hangers

A Jungle crow nest in urban Japan. Photo: Götz

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

Reuse

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

Avoiding conflicts

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

***

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

Can you spot the nest?

Have more questions? Let me know in the comments!

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

Tagged as Breeding, crow behavior, nest, nesting, nests

March 17, 2016 · 1:48 am

Why the crow smiles

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

A New Caledonian crow’s bill appears in sharp contrast to a typical American crow. Photo: “Science Meets Adventure”

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

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

Literature cited

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

9 Comments

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

Tagged as crow behavior, Crow facts, Crow intelligence, crow research, New Caledonian crow, raven intelligence, Ravens

March 14, 2016 · 3:34 pm

Australian magpies are not corvids

Ah the Australian magpie. With its glossy tuxedo plumage, heavy bill, and charismatic reputation it’s no wonder it’s a favorite among corvid lovers. Why then, do scientists keep insisting that it’s not, in fact, a corvid? This insistence of ours can feel arbitrary, even perhaps insulting, to a bird that superficially looks and acts like the corvids we know.

A still frame from the infamously cute video of an Aussie magpie and a puppy play-wrestling together.

To address this question, corvid expert and my colleague, Jennifer Campbell-Smith, recently penned a terrific piece to lay the confusion to rest. I recommend everyone take the time to read it in full.

If you do not have time, the short version of the story is that physiologically, Australian magpies, like the other butcherbirds they are classified with, lack the nasal bristles indicative of corvids. Genetically, DNA work done in the late 80’s also showed that, while they share a common ancestor, are are phylogentically distinct from other corvids. There has been some back and forth since then on the details, but there’s no scientific evidence that we should be lumping them in with corvids.

The nasal feathers are those thin, wire like feathers covering the base of the bill on the crow to the right, but conspicuously absent on the Aussie magpie to the left. Australian magpie photo: Guy Poisson

Why this bums so many corvids lovers out is a curious mystery to me. Personally, I find the convergent evolution with respect to both appearance and behavior much more interesting than if we simply made a taxonomic mistake. As for whether corvid lovers should continue to find joy and fascination in observing these birds well, I’ll direct you to this video and let you be the judge.

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Filed under Being a scientist, Birding, Crow curiosities, Crow life history, Taxonomy