Category Archives: Breeding

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.

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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

The birds and the bees

~This post describes sex in explicit terms ~

It’s springtime in Seattle which means the air is marked with the scent of warm, wet earth, the cherry trees are in full bloom, and our conversations of the goings on in the natural world are couched in the phrase “the birds and the bees”.  While its origin story is not completely clear, it’s very likely that this euphemism has been around for hundreds of years, dating back as early as 1644 in the Evelyn Diaries.  Despite the phrases’ ubiquity for discussing human sexuality, ironically very few people know much about the sex lives of the phrase’s subjects.  This is attributable largely to the fact that how these animals procreate is quite different from the mammalian systems we are familiar with.  So in the spirit of springtime let’s talk about the birds and the bees, the real story of the birds and the bees.

Birds
The most consequential departure by birds from mammalian systems is that most birds lack any kind of external sex organ. In contrast, most other kinds of animals have sex organs that are either maintained outside the body permanently, or can emerge temporarily as needed. Among birds, there are exceptions to this but fair warning you’ll probably never look at a duck in the same lighthearted way again if you pursue this line of knowledge. Which is to say of course that you absolutely should.

As far as crows and other songbirds are concerned though, sex must be conducted without penetration by a sex organ. So instead, these birds copulate by way of the “cloacal kiss”.  The cloaca is a bird’s single external vent and it’s used for both waste elimination and reproductive activities. During the breeding season, both the male and female’s vents swell.  To copulate, the male mounts the female and pushes her tail aside with his.  Once they make cloacal contact, the sperm that’s been stored in his cloaca is transferred.  It’s an awkward looking balancing act that lasts only a few seconds.  After the initial copulatory event the pair may mate again several times within minutes or over the course of weeks.

Whereas people need to consider timing, birds can successfully mate even in the absence of ovulation.  That’s because females have specialized sperm storage tubes inside their oviducts.  A female’s vagina is very picky, however, and only a very small number of the male’s sperm will reach this holding chamber.  Depending on the species, once there a female can store the living sperm for weeks or months. The exact mechanism of how the sperm are subsequently released is not entirely known, but possibilities include that it’s the mechanical pressure of the passing ovum (unfertilized eggs) that releases the sperm, or that it’s directed by hormonal changes during ovulation.1

One additional consequence of sperm storage is that it allows for different eggs within the same clutch to be sired by unique males.  In crows for example, about 80% of a nest has been sired by the female’s permanent partner.  It is unclear how many of these extra pair copulations are either solicited by or forced on the female.

Bees
While mid-air copulations have never been confirmed in birds, honey bees do in fact mate on the wing during what’s called the mating flight.  A newly minted, week old queen will take to their air followed by thousands of male drones.  She will only mate with about 10-20 of these individuals though.  To mate, the male inserts his endophallus (bee penis) into the queen’s sting chamber.  Once he has deposited his sperm, the male detaches.  Unfortunately for the male, however, he doesn’t do so with all the parts he arrived with.  His endophallus will rip off and remain inside the female, most often killing him.  As if that wasn’t brutal enough, the next male is required to remove the previous suitor’s leftovers before being granted the ability to seal his own fate.

By the end of the mating flight, the queen will be carrying hundreds of millions of sperm, most of which will be used immediately to begin fertilizing her eggs.  Like birds, the queen has a specialized storage chamber called a spermathecal where she keeps the unused sperm.  The stored sperm from her mating flight will last up to 4 years, after which the queen’s reign will come to an end, and the cycle begins anew.

Literature cited

  1. SASANAMI, T., MATSUZAKI, M., MIZUSHIMA, S., & HIYAMA, G. (2013). Sperm Storage in the Female Reproductive Tract in Birds. The Journal of Reproduction and Development, 59(4), 334–338. http://doi.org/10.1262/jrd.2013-038

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Filed under Breeding, Uncategorized

RAVENous for crow eggs 

Given their similarities, it might surprise folks to see crows occasionally harassing and chasing ravens. After all, birds of a feather right? Not in this case.  Rather than being in cahoots, the relationship between crows and ravens is most often competitive, though it can also be predatory.

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A raven barrel rolls to scold an attacking crow.

Such is the case in a recent video shared with me by a reader, Ty Lieberman.  To the dismay of him and his colleagues, a crow nest they had been observing outside their Los Angeles office window was partially dismantled, and at least one egg taken by what they believed was a pair of crows.   Concerned for the survival of the nest, Ty reached out for my interpretation.  Based on his initial description, I wondered if maybe he had witnessed egg transport, something I knew had been observed in black-billed magpies and pinon jays.1  Previous accounts of these species included descriptions of eggs being taken, and then returned to the nest, as well as eggs being deposited into the nests of neighbors, both of which are utterly fascinating behaviors and probably warrant their own post.

To date, however, there are no accounts of crows engaging in this behavior, though there is one documented observation of a nestling being deposited into a nest from which it did not originate.2  Again, utterly fascinating, but not helpful here.

Later, a more detailed account from Ty made mention of the size of the intruding birds, which quickly led me to the story’s true explanation.  Shortly after my ‘ah ha’ moment, to the dismay of he and his colleagues the nest raiders returned, and this time were caught on video by one of Ty’s colleagues (who you can follow on twitter, @namnam).  Rather than being crows, these literal homewreckers were common ravens.

Instead of being something out of the ordinary, Ty had witnessed a typical breeding season interaction between crows and ravens.  It’s no wonder then, that crows can be so hostilie when ravens enter their territory. 

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Crows (top) mobbing a raven (bottom) in Kent, WA

Eggs of all kinds are one of the most power-packed meals in the animal kingdom, so it’s no surprise ravens would take advantage of crow nests when they find them.  Around this same time back in 2015, a black bear made a similarly memorable meal out of a raven nest, reminding us that for corvids of all kinds, it’s a constant fight between being predator or prey.

Literature cited

  1.  Trost CH and CL Webb. 1986. Egg moving by two species of corvid. Animal Behaviour 34: 294-295.
  2. Schaefer JM and Dinsmore JJ.  1992.  Movement of a nestling between American crow nests.  The Wilson Bulletin 104: 185-187

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Filed under Birding, Breeding, Crow behavior, Raven behavior

American crow nesting ecology 101

*This article was updated and renamed from Everything you want to know about crow nests on April 14th, 2020.

Spring marks one of my favorite times of year. Cherry blossoms abound, the rain smells sweet and the birds get busy putting their carpentry skills to good use. In fact for me, there’s nothing more iconically spring than watching the silhouettes of crows with bill loads of timber or soft material dot the skies as they shuttle back and forth to their nest trees. 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. Watching your local nest is both a great way to learn more about your neighborhood crow family, and avoid unpleasant conflicts with protective crow parents.  Whether you’re years into this tradition or just getting started, there’s always more to learn and enjoy!

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Nest site selection, construction, and reuse

Nest construction is initiated anytime from early February to late April, depending on the region.  In Washington, nest construction generally kicks of by the second week of March. Crows will nest in an astounding array of places, depending on where they live and what’s available.  In Seattle, I see them nest anywhere from the eaves of skyscrapers, to the crooks of well concealed tree limbs, to within reach in saplings that are struggling to support their weight. In areas where appropriate trees are unavailable they may even nest right on the ground!¹ How crows make their precise nest site selection is unknown, but most commonly in the PNW, nests are placed close to the trunk in a fork or on a horizontal branch in the top third of a conifer.

Both the male and female participate in building the nest. In areas where auxiliary helpers are present, helpers may also contribute to gathering nesting materials and may add these materials themselves, or leave for the female to work in.

If trees are abundant, the nest exterior is constructed mainly from twigs pulled from live trees.  In areas where such materials are in short supply, nests may be composed of as much as 50% grass and other plant stems.² After the bulk of construction is complete, they’ll line the cup of the nest with soft materials like grass, bark, moss, flowers, mud, cow dung, roots, paper, fabric, fur, etc. Fur may be found or collected from live animals, as this large-billed crow (Corvus macrorhynchos) demonstrated on a panda at the Beijing Zoo in 2015.  Contrary to the news anchor’s fears, this would not have been painful for the panda. 

It takes 1-2.5 weeks to finish a nest, though second attempts can take as little as 5 days in areas where helpers are present.  A new nest is usually about 1.5 ft across and 8-10 in deep. The life of a typical nest is only about 10 weeks (1-2.5 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 for years. After the young fledge, the crows will not return to the nest. Generally speaking, crows will only use a nest once, though the occasional observation of a pair repairing and reusing an old nest have been reported across the country. More often it appears that if they are going to reuse a nest site, they will 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.

Eggs and nestlings

Egg laying may begin immediately, or up to a week after the nest is complete.  Crows, like nearly all birds, have a single ovary and oviduct and can only lay one egg a day.  In some cases they may even skip a day or two between laying. Crows generally lay a clutch of 4-5 eggs, but nests with up to 9 eggs have been observed, though it’s possible this was the result of a second female laying in the same nest.³  Females will usually start incubating the nest once the third egg is laid. To aid with incubation, females develop a patch of featherless skin on their underside called a brood patch.  Brood patches are common in birds, and generally only occur on females but observations of male crows with brood patches have been reported. Only female crows incubate, though eggs may be briefly “incubated” by helpers.

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Common raven eggs left | American crow eggs right

For a couple of days before the full clutch is laid, the female will sit next to the nest and give begging calls.  This behavior continues even after she starts incubating, which in Seattle is most often the explanation for begging calls emanating from nests during April. Both her partner and helpers will bring her food, usually 2-4 times an hour. She may hop off the nest to help chase away threats, feed, preen, or stretch, but generally doesn’t leave for more than a few minutes.

The eggs will start hatching after about 15-20 days of incubation.  Since the female starts incubating before the full clutch is laid, crows exhibit asynchronous hatching, where not all the young hatch on the same day. In other species of birds like mallards, even though the eggs were laid days apart, the young all hatch within a few hours of each other because the female waited to start incubating until only after all the eggs were laid.

Like other songbirds, crow chicks are altricial and nidicolous, meaning they hatch blind and helpless, and will remain in the nest for many days after hatching.  Ducks, chickens, quails, etc. all produce precocial, nidifugous young which hatch sighted, covered in downy feathers, and ready to follow their parent(s) away from the nest within a few hours.

baby crow

Once the eggs start to hatch the female will continue to brood the nestlings continuously for the next couple of weeks.  Once the nestlings are more developed and covered in feathers, she will brood less and less often and transition mostly to food provisioning.  Nestlings appear to be fed primarily invertebrates, but their diets vary depending on local resources.  For the first couple of weeks after hatching, nestlings are fed about every 30min by parents and helpers, if they are present. After about 4 weeks the young will fledge (leave the nest permanently). Prior to fledging you may see the nestlings sitting on the rim of the nest and flapping around awkwardly.  Not all fledglings are flighted at the time of leaving the nest, so take care not to assume young have simply fallen out. After the chicks fledge, they remain in the care of their parents for the duration of summer, and will continue to be fed for about 4 months.

Differences among corvids

Crow, jay and raven nests are similar in shape and choice of materials and mostly differ in overall size. The main standout are magpies,  which build incredible domed-shaped nests the size of a large beach ball.  The nests require so much material that 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.

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A Jungle crow nest in urban Japan. Photo: Götz

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

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. Physical contact between birds and people during these altercations are rare, but can happen and might hurt. In areas where crows are less persecuted (like cities) they 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 a good strategy is to invest in an umbrella you don’t care about.  It’s a simple and inexpensive solution that protects both yourself and the legacy of recognizing that outdoor spaces are shared space between ourselves and wildlife.

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Crows build their worlds on our backs.  We might as well lean in and appreciate the joys of watching nesting birds!

Have more questions? Let me know in the comments!

 

  1. Gross, A. O. (1946b). “Eastern Crow.” In Life histories of North American jays, crows, and titmice., edited by A. C. Bent, 226-259. U.S. Natl. Mus. Bull. 191
  2. Good EE. (1952). The life history of the American Crow Corvus brachyrhynchos Brehm. Phd Thesis, Ohio State Univ., Columbus.
  3. Peck, G. K., and R. D. James (1987). Breeding Birds of Ontario: Nidiology and Distribution. Volume 2: Passerines. Miscellaneous Publications of the Royal Ontario Museum, Toronto, ON, Canada.

Most of the general information was sourced from:

  1. 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. https://doi-org.offcampus.lib.washington.edu/10.2173/bow.amecro.01

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