Category Archives: Crow life history

February 17, 2016 · 6:24 pm

Crow curiosities: What do crows eat?

Spoiler alert: They’re not, as so many people believe, true scavengers.  Meaning, they’re not mostly eating carrion.  I know what you’re thinking: MIND BLOWN.  Also you might be thinking PBS lied to you, and you’d technically be correct.  So why is this myth so pervasive that even PBS fell victim to its ubiquity?

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An American crow picks at the torn up belly of a rat in a Bellevue neighborhood.  After a few minutes, it had its fill and moved on to other feeding opportunities, leaving most of the rat untouched.  

Well, a huge part of the problem is that like so many words in science, their use in general discourse has parted from their scientific meaning.  Typically we use this word to describe say, grad students at the end of the party stuffing their pockets with the leftovers but, biologically speaking, scavengers are organisms who are specialized to consume, or obtain most of their food, from the decaying tissue of animals or herbaceous matter.  Now don’t get me wrong, the title of ‘scavenger’ can get a bit blurry as Bernd Heinrich argues in his book, Life Everlasting.  Ravens for instance, switch primarily to scavenging during lean winter months.  For most American crows, however, the identity of ‘scavenger’ simply will not do.

Which is really too bad, since the title of scavenger is bestowed with honor given how they make our living on planet earth possible.  I’m not being hyperbolic when I say thanking the undertakers of our ecosystem should be part of everyone’s pre-meal ritual, but perhaps that argument should be saved for another post.

As for crows, carrion makes up only a very small part of their diet.  In Seattle, roadkill accounts for <5% of crow food, and in wildland areas carrion accounts for even less1.  Crow beaks aren’t even strong enough to break through the skin of a grey squirrel, though they will usually give it a try.

So what are they eating?  Mostly human refuse (no surprise) and invertebrates.  In fact human garbage (meat, grain products and veggies) account for about 65% of their diet in urban areas, whereas in wildland areas it’s roughly split between garbage and inverts (35% and 35% respectively)1.

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Crows spend much of their time patrolling lawns looking for invertebrates

 

These data correct another common misconception about crows: they’re not mostly eating the eggs and nestlings of other birds.  In fact, crows only account for 1 of 20 observed nest predators in WA and have been found to have a nonsignificant, negative relationship between abundance and rate of predation in experiments using artificial ground nests, shrub nests, and canopy nests1.

So there you have it, American crows are neither true scavengers nor meaningful nest predators. They’re primarily omnivores with an emphasis on human refuse and invertebrates.  So the next time you see one patrolling your grassy lawn remember; they’re busy trying to bring home the bacon.  Er, bugs.  Well, probably bugs, but preferably bacon provided you were crazy enough to throw some out.

Literature cited

  1.  Marzluff, J.M., McGowen, K.J., Roarke, D. and Knight, R.L.  2001.  Causes and consequences of expanding American crow populations.  in Avian ecology and conservation in an urbanizing world (J.M. Marzluff, R. Bowmanm and R Donelly, eds).  Kluwer academic Press, norwell, Ma.

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Filed under Crow behavior, Crow curiosities, crow diet, Crow life history

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December 4, 2015 · 4:58 pm

Crow curiosities: Do crows collect shiny objects?

The notion that corvids, especially magpies, have a special affinity for shiny object has been around for more than a century.  In fact to refer to someone as a magpie is to describe them as someone who ‘compulsively collects or hoards small objects’.   This idea is so old hat that it can feel a bit frivolous to even wonder if it’s true.  The trouble with this bit of corvid whimsy, however, is that when we do investigate it, and scientists have, we find there’s no empirical evidence to support it.

'Crow Collects' by Cori Lee Marvin.

‘Crow Collects’ by Cori Lee Marvin.

For instance, one study presented both captive and free-living magpies piles of blue or shiny silver screws, rings, and pieces of tin foil near piles of food to which they had been previously habituated.1  They found that, rather than thieving and subsequently caching the gleaming objects, the birds were actually more nervous to take food than they had been previously.  In the 64 conducted tests, only two instances of contact between a bird and an object were recorded.

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Experimental set-up for magpie study.

Cornell crow expert Kevin McGowen, elaborates on this general conclusion, suggesting that perhaps the origin of this folklore is pet crows who are attracted to the objects of obvious value to their owner like coins, keys or jewelry.  Speaking personally as someone who has spent countless hours observing hundreds of individual crows in the field, I can also attest to the fact I have never witnessed anything resembling this behavior.  So there you have it, corvids do not, according to the best empirical evidence, show an attraction to, or are otherwise known to collect shiny objects.

And yet…

And yet I still hear anecdotes about this behavior that peak my curiosity.  For instance, once or twice a year I’ll see a headline about crows thieving shiny stones at the expense of bereft family members.  In Jewish culture, it’s tradition to leave a small stone atop a gravestone, as a way to honor the deceased and indicate that they’ve been visited.  For whatever reason, particularly across Ireland, these stones occasionally go for joy rides in the mouths of crows.  In Omagh, Patsy Kerlin who mounts headstones in his town’s graveyard recently told a local reporter that “It seems to be only the black shiny ones they take and a lot of them go missing.”  Even in my own neck of the woods at the University of Washington one of the gardeners at the Urban Horticulture Center regaled John Marzluff and I with his story of how the crows regularly steal the shiny metal placards that identify the center’s plants.

In science, we often like to say “the plural of anecdote is not data”.  This is unequivocally true.  But just because they’re not data doesn’t mean they’re meaningless either.  I’m inclined to believe there’s more to these stories than random chance and I think they are worth exploring.  Perhaps these stories emerge out of confirmation bias, meaning people tend to report theft with respect to shiny things more often because they’re looking to confirm a suspicion they already had.  If so, it would be yet another fascinating example of the extent to which corvids have infiltrated our culture.  Or perhaps this is the work of curious juveniles as has been suggested by my crow colleague Dr. Jennifer Cambell-Smith.  If so, teasing out any evidence of discrimination or bias juveniles are using when selecting objects to explore could give us insight into how they learn about the world, or how our garbage is modifying that behavior.  Or perhaps crows do like to carry off with glossy objects, but for textural, rather than visual reasons.  At least some corvid species swallow small stones to aid in digestion and these stones are most often partially smoothed.2  These ‘grit stones’, however, are considerably smaller (on average only 2.9 mm) than I imagine grave stones are, so perhaps this behavior is evidence of poor grit stone selection among naive birds.

Or maybe it’s none of the above, we simply cannot say.  Which, for me, is exactly why I find these anecdotes so interesting.  While we can rule out that this behavior isn’t a manifestation of corvids’ love for bling, we can’t exactly explain this behavior either.   It’s yet another item on the shelf along with thieving golf balls and wiper blades where we can’t do much more than offer an educated guess.  So while I’m quick to clarify that crows are not attracted to shiny objects, I’m not dismissive of these anecdotes either.  My friend and colleague David Craig likes to say that every bird has a story, and citizen science is part of sharing that story.  In my book, the story of corvids and their light fingered behavior seems an ideal project for the crow minded bird nerd.

  1.  Shepard, T. V, Lea, S. E. G., and Hempel de Ibarra, N.  2014.  Thieving magpie’?  No evidence for attraction to shiny objects.  Animal Cognition 18: 393-397.
  2. Gionfriddo, J.P., and Best, L. B. 1996.  Grit-use patterns in North American birds: The influence of diet, body size and gender.  The Wilson Bulletin 108: 685-696

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Filed under Corvid mythology, Crow behavior, crow conflicts, Crow curiosities, Crow life history, Crows and humans, In the news

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September 3, 2015 · 1:30 am

Baby crow detective work

By now, most of us have come across these images of  “baby crows” so often it induces more of a yawn than a fit of aggravation.
Image-1If, somehow, these images are new to you feel free to check out my post fully debunking them, as I will not dedicate any further time to them here.   But there’s a new photo circulating social media, and it makes for a much more compelling crow doppelganger;

baby crow
You’ve got three, black, altricial baby birds in a nest and really, they’re not terribly un-crow like.   It doesn’t make you a complete crow rookie to make this mistake, but there are some key things wrong here.  And this is the moment where, as a scientist, these photos elevate from being simply another source of annoying misinformation (which, they are) to the kind detective work that childhood doctor visits fostered a deep love for.  Because, not unlike my favorite activity in the Highlights magazines I anxiously parsed through in those waiting rooms, there are 4 things that are different between these two photos and it’s up to you to find them.  So take a minute and see what jumps out at you….

Crows_v_no crows
Figured it out (or given up)?  The first thing to know is that all bird species are very specific in terms of both nest materials and nest construction.  Sure, some birds can happily use some ribbon in place of straw (like orioles) or build nests in old shoes just as easily as in gutters (like bewick’s wrens) but the basic style is always the same.  Robins will always use mud as a binder and bushtit nests will always look like cozy sleeping bags made of moss.  Knowing that, the material used in the nest on the right should jump out as a red flag.  Of course you’ll find crow nests with a bit of string, fabric or grass (especially for lining) but the bulk of the nest is always made of pinky-wide sticks.   Really, you need look no further at this point to know immediately that the photo on the right is an impostor but let’s keep going because it’s fun.

Next let’s look at the babies themselves, which is where the three remaining differences are.  Two of them are color-coded, did you catch them?  Ah yes, gape and eye color.  See that brightly colored area on the corner of the bird’s developing beak?  That’s called the gape, and the bright color that flashes when they open their mouths is a powerful signal that tells parents to “insert food here”.  Crows have bright pink gapes, whereas these other birds have yellow gapes.  Our other color coded giveaway is the eyes.  Granted the lighting is not great, but it’s clear that the crows on the left have light blue eyes whereas these other birds have dark eyes.  In some species of crow the babies are born with brown eyes that turn blue as they age, but such is not the case with our American crows and you can expect that nestlings will always have blue eyes.  The last clue, which takes more expert level knowledge to notice, is the bill shape.  The birds on the right have a slightly more embellished curve to the bill than a typical crow chick.

The final mystery, of course, is what the birds on the right actually are.  Unfortunately, I failed to track down the original poster, but as best I can tell they’re black drongo chicks.  Black drongos are members of the drongo family (Dicruridae) and are native to Southern and Eastern Asia.  Here’s another photo I found that looks consistent with the previous one.  If any drongo experts read this blog though and want to correct me, I’d love to hear from you!

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August 14, 2015 · 3:48 am

Corvid of the Month: How abundant food may be killing the Mariana crow

For most of us, it’s hard to imagine crows being anything but ubiquitous.  Here in Seattle, American crows can nest so densely, I once found myself within 50 m of three different active nests.  Such is the case for many other parts of the world too, where house crows, jungle crows, or hooded crows are an almost inescapable part of the landscape.  Given these species’ success, it might be tempting to assume that all crows welcome human presence and habitat modification.  Rules don’t exist without exceptions however, (especially in nature!) as our Corvid of the Month, the Mariana crow, tragically illustrates.

A female Aga and fledgling do some exploring. Photo: Matt Henschen

A female Aga and fledgling do some exploring. Photo: Matt Henschen

The Mariana crow, or Aga, is endemic to Guam and Rota and is the only corvid native to Micronesia1.  In appearance, they bear a striking resemblance to the American crow, only they’re 40% smaller (cue adoring sound effects).  Across their range they’re considered critically endangered and as of today, all of Guam’s birds have been extirpated by the invasive brown tree snake, and only about 46 breeding pairs remain on Rota.  If that wasn’t alarming enough, their numbers continue to dwindle and researchers at the University of Washington project they could be extinct within the next 75 years2.  Unlike Guam, however, there are no brown tree snakes on Rota.  So what is causing the drastic decline of this island crow?   As my colleague and Mariana crow researcher, Sarah Faegre, is beginning to tease out, the answer may lie in the delicate nature of island food webs, and the unanticipated butterfly effect that started with a few errant snails.

Like our American crows, Mariana crows are generalists and eat a wide variety of foods from insects, to geckos, to fruits and seeds.  But adult Mariana crows have one other food source they’ve come to specialize on: the humble hermit crab. Despite the presence of hermit crabs near other species of corvus, the Mariana crow’s frequent predation on them is unique, especially when you look at how they extract them.  Unlike most coastal or inland living crows that drop tough objects like clams or nuts onto hard surfaces to open them, the Mariana crow actually uses its bill to peck and break the shell at the seams to extract the vulnerable crab, a process that takes place entirely on the ground and is only shared by two other known bird species in the world (one of which is now extinct).  So what does this have to do with wanderlusting snails?  As it turns out, everything.

What's crackin' crabby? Photo: C. Brevimanus

What’s crackin’ crabby? Photo: Sarah Faegre

Rota is home to several species of native land and sea snail, though hermit crabs only utilize the larger shell of the sea snail.  Critically, these shells are extra hard and apparently impenetrable to even the most determined crow.  In the late 1930’s, however, humans introduced the Giant African Land snail which quickly invaded the island.  Two major differences between the native and invasive snails are 1) that the invasive snails have thinner shells, and 2) people were anxious to get rid of them.  So, naturally, we introduced  yet another invasive species (a predatory flatworm) and…it actually worked.  By the 1970’s the island was brimming with large, thin, empty shells, ready and waiting to be filled with hermit crabs.  Gradually, the crows learned that these shells were possible to peck open and now hermit crabs are an important staple for Rota’s crows.

Photo: Matt Henschen

Photo: Phil Hannon

On its surface, this seems like the making of an ecological disaster turned into a conservation blessing.  After all, we successfully controlled an invasive species while simultaneously creating a new food source for a threatened bird.  But in our tangled web of introduced species and ecological fallout we must considering the one remaining player: cats.  Although further study is needed, Sarah’s work3 suggests that all that extra time adult crows now spend on the ground cracking open hermit crabs may be making them more susceptible to predation by cats.

Couple the effect of cats with habitat destruction and persecution by people and the results project a bleak outlook for crow recovery.  But conservationists and researchers like Sarah are working tirelessly to better understand the threats facing this bird and how to solve them.  In fact Sarah and her husband, Phil Hannon, recently started a non-profit called Luta Bird Conservation to help raise awareness and conservation funds to better protect this unique crow.  At the top of their priorities is funding initiatives that would bring the science of crow conservation to the classrooms of local people, helping to raise both pride and awareness for the plight of this endemic species.

So the next time you look at a crow and experience a slight feeling of fatigue at such a ubiquitous bird remember; not all corvid species welcome the consequences of people and some have suffered greatly from them.  Aldo Leopold once said “to keep every cog and wheel is the first precaution of intelligent tinkering.” The lesson from Rota, and so many others, is that the same can be said of not adding any either.

If you wish to contribute directly to Mariana crow conservation, I encourage you to send Luta Bird Conservation Inc. a check at:

Luta Bird Conservation Inc. c/o Aron Faegre
520 SW Yamhill Street, Roofgarden 1
Portland, OR 97204
Sunny, Luta's educational Mariana crow captivates the students in a local school

Sunny the captive Aga on an ambassadorial trip to a local classroom with Luta Bird Conservation Inc.

Literature cited:

  1. http://www.fws.gov/pacificislands/fauna/marianacrow.html
  2. http://www.washington.edu/news/2010/12/20/without-intervention-mariana-crow-to-become-extinct-in-75-years-2/
  3. Faegre, S. (2014) Age-related differences in diet and foraging behavior of the critically endangered Mariana Crow (Corvus kubaryi) (Masters thesis; University of Washington).  https://digital.lib.washington.edu/researchworks/handle/1773/27571?show=full

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June 3, 2015 · 2:42 pm

Jays do those sassy peanut moves for good reason

After I lay down my handful of un-shelled peanuts it’s only a matter of minutes before I can hear the “thwap!” as the steller’s jay hits the cable wire that runs above my balcony’s railing.  It balances there for a moment before descending onto the pile I’ve offered it.  Quickly, and with obvious purpose, it springs down the railing and picks up a nut.  Its mohawk feathers bounce as it snaps and cocks its head around in various direction.  After only a few seconds, I hear the sound of rejection; the distinct hollow tap as the nut is returned to the railing.   The jay repeats the same process with two more nuts before abruptly flying off with one that, to my eyes, appeared identical to the first two.

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Certainly most manner of corvids engage in some kind of choosy behavior though I don’t think any of them go about it with as much frenetic spunk as jays do.  So the questions arise: Why are they being so picky?  What do they know about the rejected nuts that my eyes can’t see?

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According to a new study1, Mexican jays are actually ‘weighing’ nuts during this process.  By using specialized slow motion cameras, researchers showed that those snappy head movements are actually a way for the birds to get a tactile feel for the the nut’s weight and listen to the sound the peanuts makes as it rattles in its shell.  By providing nuts that were visually similar but different in mass, the researchers were able to show that jays could consistently select nuts with the most nutmeat density.  A further test showed that large shells that were altered to contain only one nut were typically selected first, only be be rejected, while single nut shells were accepted.  This suggests that jays either have a sense for how much nut should weigh (and thus reject nuts that contain less than they should) or that the correlation between hollow sounds and nut density lead to the ability to choose denser nuts.

So the next time your visiting jay delights you with its sassy head snaps remember; it may simply be amusing to you but for jays, it’s an impressive product of evolution that helps keep them alive.

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Literature cited:

1) Piotr G. Jablonski, Sang-im Lee, Elzbieta Fuszara, Maciej Fuszara, Choongwon Jeong, Won Young Lee. Proximate mechanisms of detecting nut properties in a wild population of Mexican Jays (Aphelocoma ultramarina). Journal of Ornithology, 2015

 

 

 

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May 28, 2015 · 8:30 am

Help, I’ve found a baby crow!

You’re walking down the sidewalk when suddenly you notice a waddling little mass of black feathers trying awkwardly to hide under a bush or car.  When you begin to approach the young crow, it simply stares up at you or perhaps continues on its poorly planned waddle down the sidewalk or worse, into traffic. Clearly this baby crow cannot fly and has a habit of making bad decisions.  Your instinct (and perhaps the opportunity to interact with a baby animal) are tempting you to intervene and “save” this young crow who seems ill prepared to be out of the nest.  What should you do?

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The bright blue eyes and pink bill tell you right away that this is a baby crow. Within about a week the bill will turn black like and adults’ but the eyes and mouth corners with remain blue and pink respectively.

The answer, almost always, is to ignore your instincts and good intentions.  I have many friends and colleagues who are either licensed wildlife rehabbers or who spent summers volunteering with their local rehab facilities, that can attest to the dozens of animals that get brought in during the spring and summer by well intentioned folks who assumed that finding a baby meant it needed help.  In many of these cases, these animals did not need help and now they’ve been separated from their parents or mother and stand a much lower chance of surviving once they’re released.  So how do you avoid making the same error?

First, understand that corvid babies (and many other open-cup nesting songbirds) very often leave the nest before they are completely flighted.  For crows, this early departure can be on the order of 7-10 days before they can fly.  Although this strategy is risky and leads to lower fledgling survival rates than for species that wait until the babies are fully flighted, the alternative is quite literally an “all your eggs in one basket” situation where the longer the kids stay in the nest the more chance they stand of being discovered by a predator who will then wipe out the whole brood.1  By pushing kids out sooner, the less developed ones may get caught by a predator or killed by some other means, but the stronger individuals stand a better chance of escaping. It’s not a great compromise, but its continued selection suggests it’s the one that works best.

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A crow’s nest is an example of an “open-cup” style nest, where the top is exposed, making them easier to spot and access by predators.  Other styles include cavity nests, which are made in holes, and pendant nests which are sock shaped nests.

During this vunerable time, the young are still in the care of their parents who will continue to feed and defend them until they reach independance. So finding a flightless baby crow or jay is totally normal between late May and July and does not imply that it has been abandoned or fallen out of the nest.

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This is the stage of development that most babies will be at when they leave the nest.  If your fledgling looks like this then it is okay.

Simply knowing this piece of corvid biology will lay to rest the concerns for most situations you may find yourself in this summer.  For babies that are naked, bleeding, have drooping wings, or are within reach of a dog or cat, etc., the following flow chart is excellent and will help navigate the situation (I’ve thrown in the mammal one too just for good measure).

Help, I’ve found a baby bird
Help, I’ve found a baby mammal

The main thing to remember is; as long as the baby is mostly feathered and being attended by its parents, it’s just where it needs to be.  Only if it’s trapped in a storm drain, naked, injured, cornered by a cat, or after several hours has not been visited by its parents is it appropriate to intervene.  Even in most of those cases, simply creating a makeshift nest out of a basket and securing it to a tree, or placing the baby in a bush and leaving the area, is much better than taking it away to a facility.  So use your best judgment this summer and remember; if you feel your situation is unique and has not been addressed by the flow charts provided, give a rehab facility a call and let them help you decide if an animal needs to be removed before you make the mistake of taking babies away from their parents.

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This baby has left the nest earlier than most of its peers. See how much more of its legs are exposed and how much shorter the wing feathers are in contrast to the other babies in this post? It will be quite a long time before it can fly. Still, this little one will be cared for and fed by its parents making it a good candidate for a makeshift nest secured to a high branch. If it jumps out right away then wish it luck and leave it be.

Literature cited

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April 11, 2015 · 3:49 am

Why climate change may be deadly for crows

In general, crows love the ways that humans transform the environment.  We plow down forests and replace them with suburbia, creating a perfect mix of the occasional nesting tree and plenty of grassy yards perfect for looking for bugs.  On top of that, we litter and waste food, providing lots of additional foraging opportunities outside of our yards.  And even when we replace the forests with urban epicenters, we build towers perfect for nesting, leave out even more garbage, and make the environment less hospitable to predators.  But this climate change thing, well now we’ve gone too far. Even for crows.

Scientists know that climate change is going to be bad for lots of species, with some projections estimating that by 2050, 15-37% of species in the areas sampled will be committed to extinction if current emission trends continue1. Not all creatures will be worse off as the climate warms, however, some will do quite well.  Who are these lucky creatures you ask?  No, not beautiful sword-billed hummingbirds, or rare pygmy marmosets but…mosquitoes.

GO is a proud (albeit perhaps oblivious) participant in the #iamclimatechange campaign.

GO is a proud (albeit perhaps oblivious) participant in the #iamclimatechange campaign.

Warmer, and in some places wetter, climates are projected to not only increase mosquito numbers but, importantly, increase the incidence rate of West Nile virus2.  Crows are particularly susceptible to WNV over other non-corvids, making this especially bad news for them3.  One study showed that some regional crow populations across the US declined 45% since WNV was first introduced in 19994. As warmer temperatures march north, scientists predict we’ll see WNV not only increasing in areas where is exists now, but spreading to northern latitudes where it may have a particularly acute effects2 (I’m talking to you, Canada!).

By extension, this is also bad news for us, since humans can also get sick from being bitten by infected mosquitoes (there is no evidence we can be infected from handling sick or dead crows, however5).  Not to mention all the other diseases mosquitoes carry that kill people and birds.  So whether you care about climate change because you care about crows, humans, or any of the other organisms that are/will be negatively affected, now is the time to do something about it.  Individuals can have profound effects by making changes in their own lives, and by putting social pressure on others to do the same.  There’s plenty of reason to be hopeful if we act!  To learn more about the #iamclimatechange campaign that GO was nice enough to participate in please check out the tumblr or facebook page.

Citations

1. Thomas, C.D., Cameron,A., Green, R.E., Bakkenes, M., Beaumont, L.J., Collingham, Y.C., Erasmus, F.N., Ferreira de Siqueira, M., Grainger, A., Hannah, L., Hufhes, L., Huntley, B., van Jaarsveld, A., Midgley, G.F., Miles, L., Ortega-Huertam, M.A., Townsend Peterson, A., Philips, O.L., and Williams, S.E. (2003) Extinction risk from climate change. Nature 427: 145-148

2. Tam, B. Y., and Tsuji, L.J.S. (2014) West Nile virus in American crows (corvus brachyrhynchos) in Canada: projecting the influences of climate change.  GeoJournal DOI 10.1007/s10708-014-9609-z

3. Yaremych, S. A., Warner, R.E., Mankin, P.C., Brawn J.D., Raim, A., & Novak, R. (2004) West Nile virus and high death rates in American crows.  Emerging Infectious Diseases, 10, 709.

4. LaDeau, S.L., Kilpatrick, A.M., and Marra, P.P. (2007) West Nile virus emergence and the large-scale declines of North American bird populations. Nature Letters. 447

5. http://www.cdc.gov/westnile/faq/deadBirds.html

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March 5, 2015 · 12:00 am

All in the (crow) family

With the breeding season fast approaching, I thought it would be fun to write a post dedicated to some of the finer details of growing up in a crow family.  From territory selection to the function of helper birds I hope this post illuminates some of the less familiar aspects of what goes on between crow parents and kids.

Mate/Territory selection
Crows reach sexual maturity between 2-4 years, with females generally maturing faster.  Once they’ve bonded with a mate it’s time to secure a territory.  After they’ve settled on a territory they’ll hold on to it for years, if not the rest of their lives, providing a great opportunity for some in depth crow watching!  With few exceptions, crows generally nest in areas similar to where they were raised1.  Meaning, rural crows settle down in rural areas, suburban crows in suburban areas and so on and so forth.  Dispersal distance ranges (generally) from 0-60 km, and some birds will settle down right next door to their natal territory.

Monogamy
As far as mating goes, we refer to crows as being socially monogamous but genetically promiscuous, as is the case with most birds.  This means mated pairs will typically stay together for life, but extra pair copulations are not unusual, at least in some populations. For example, in one study in New York, breeding males sired 82% of their offspring, while the rest of the clutch resulting from extra pair matings.2 A separate study showed that among 25 families, 36% experienced extra pair attempts, and of 252 offspring, 19% were not sired but their mother’s permanent partner.³ Whether females are soliciting these extra pair matings is unclear, but most data suggest that females do not have complete control over their extrapair partners, if any.2  Loss of paternity increases dramatically if the male has been non-fatally injured.  Although his partner won’t leave him altogether, he is generally at risk for smaller brood sizes and paternity loss jumps to about 48%, likely due to more difficultly mating and guarding his mate, as well as lower sperm counts as a result of stress3.  In these cases, females may be more willing to accept fertilization from extrapair males,  though this is poorly supported.

The mechanics of mating
loversFor those that want the biological details on crow sex, male crows, like the majority of birds, do not have an external penis.  So mating is generally preceded by solicitation from the female, followed by the male mounting and rubbing his cloaca against hers, which transfers the sperm.  This is known as the cloacal kiss (which, bleh, saying reminds me of Tina Fey in 30 Rock talking about the word ‘lovers’).  The whole process takes a couple of seconds.

Nesting

A typical looking crow nest with a water bottle for scale.

A typical looking crow nest with a water bottle for scale.

Crows start to nest in mid to late march with both males and females participating in nest building.  Nests are constructed from sticks and lined with grass, fur, feathers or other soft materials.  The building process is pretty conspicuous if you know what you’re looking for (and very fun to observe) but be warned; I have little doubt that crows will build fake out nests if they believe they are being watched by someone they don’t know or trust.   Females lay a clutch of usually 3-4 blue and brown speckled eggs.  Although males will occasionally (rarely) sit on the nest while the female is away they can’t really incubate because they lack the necessary brood patch to transfer heat.  Chicks hatch after about 19 days of incubation followed by another 30-45 days in the nest before fledging.   Once they fledge they spend 1.5-2 months dependent on their parents before they’re ready to strike it out on their own.

Helpers
I dedicated another post to talking about the behavior and survival rates of hatch-year offspring which you can find here.  Let me use this space to pick up where that post left off addressing helpers.  Crows, like a number of other birds including nuthatches and kingfishers, engage in cooperative breeding, though they are not obligate cooperative breeders meaning they can successfully fledge young without helpers.  Cooperative breeding is defined as when more than two individuals contribute to the care of young in a single brood.  For crows this means that, in addition to the mated pair, there can be up to 10 additional birds helping to raise this year’s brood.1  Generally, these are young males that are related to the male breeder.2 The motive behind cooperative breeding is somewhat mysterious since there are costs to both breeders and helpers.  Costs to parents including diversion of food provisioning towards helpers and, for males, threat of paternity loss to helpers.  Costs to helpers are more straight forward; they’re delaying their own breeding efforts to rear offspring which only share some of their genetic identity.  So why do crows bother?

Let me preface this by saying that quantifying the helpfulness of helpers is surprisingly difficult. A central roadblock can be; how do you tease out the effects of helpers, vs the underlying quality of parents that have lots of helpers? One study that controlled for differences among parents, found that helpers were’t hardly helpful at all, begging the question if “helper” is really the most apt word choice here.5  Still they are fact of life for many crow pairs, so let’s attempt to understand what’s going on here, based on the available evidence.

Why parents keep them around:

One insight comes from studies done on carrion crows (a species native to Western Europe and Asia). In these birds, differences in helper helpfulness are so obvious that some individuals are scientifically referred to as “lazy” group members.  Amazingly, however, if one of the parents is injured it’s these lazy group members that pick up the most slack to compensate for the reduction in effort by another group member.6 So they’re kinda like your State Farm policy.  Most of the time they feel like an annoying burden that will never come in handy, until that one time they do.  Alternatively, in other birds, it’s been shown that although helpers don’t make much of a difference as far as each individual breeding attempt goes, they do increase the breeding female’s overall life expectancy and therefore pay off in the long run.7

Why helpers help:

Now the connection between HBO's Game of Thrones and corvids takes on yet another meaning...

Now the connection between HBO’s Game of Thrones and corvids takes on yet another meaning…

For the helpers themselves, there are more potential benefits.  Whether it’s by choice or force,  helper males can sire as much as 7% of offspring, so breeding is in fact not out of the question.4  Although it’s a small number, some of that percent is even from mother/son relationships, though this isn’t consistent among corvids and some species do a better job avoiding incest than others. Even if they don’t breed with the female, they’re still helping to raise siblings that share some of their genetic material so it’s not a complete loss from the standpoint of their DNA (something known as inclusive fitness).  They also stand to inherit the territory, should something befall the territorial pair.  Finally, though this has been especially difficult to quantify, it’s possible that sticking around and observing a breeding attempt offers valuable insights that helper birds will go one to use when they raise their own brood.

How did such a behavior evolve?
That really is the million dollar question.  It may have to do with delayed dispersal, which is basically a fancy way of saying there wasn’t always places for these young juveniles to go so they just stuck around at home (ah ha! See, millennial are just doing what comes naturally).  It’s notable that the majority of birds that partake in cooperative breeding do so in kin based groups, which seems to suggest that perhaps this is a product of the fact that kin tend to settle down close to each other and as time went on a more loose, flexible system of helping became the more formal cooperative system we see today.

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!

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!

Literature cited

1McGowen, K. 2001.  Avian ecology and conservation in an urbanizing world.  Kluwer Academic Press, Norwell, MA.  p 365-381

2Townsend, A.K., Clark, A.B., McGowen, K.J., and Lovette, I.J. 2009.  Reproductive partitioning and the assumptions of reproductive skew models in the cooperatively breeidng American crow,  Animal Behavior 77(2)

³Townsend, A.K. (2009). Extrapair copulations predict extrapair fertilizations in the American crow. The Condor 111: 387-392

4Townsend, A.K., Clark, A.B., and McGowen, K.J. 2011.  Injury and paternity loss in cooperatively breeding American crows.  J. Field Ornithology 82(4): 415-421

5Caffery, C. (2000). Correlates of reproductive success in cooperatively breeding crows: If helpers help, it’s not by much. The Condor 102: 333-341

6Baglione, V., Canestrari, D., Chiarati, E., Vera, R., and Marcos, J.M. 2010.  Lazy group members are substitute helpers in carrion crows. The R. Soc. Proc. B: 282(1804)

7Wright, J., amd Russell, A.F., How helpers help: Disentangling ecological confounds from the benefits of cooperative breeding.  British Ecological Society 77: 427-429

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January 31, 2015 · 6:13 pm

How to troll a corvid lover

In general, I find that crow people are lovely, and easy to get along with folk.  There are, however, two widely shared images that have proven themselves an effective trigger to turn an otherwise gentle corvid lover into a foaming at the mouth fact checker.  I say that as someone who has found themselves on the foaming end of that equation over some social media perpetuated misinformation on many occasions.  In the wealth of silly nonsense posts  a person could manufacture on corvids, why these are the two images that have become so ubiquitous on social media is a mystery to me.  Maybe it’s because people genuinely enjoy the narratives they offer, or have no reason to be suspicious of the images because they’re just beginning to learn about crow biology.   Or maybe some people know exactly what they’re doing and are just trolling to, ahem, ruffle a few feathers.

Either way, consider the following two images debunked.  So rest easy fellow fact checkers, until of course you see them posted again.  Probably tomorrow.

Myth: The infamous “baby crow”

These are not baby crows.

These are not baby crows.

I blame this one on BuzzFeed.  In what was trying to be a cool post on crows, but really turned out to be just riddled with terrible photo choices, they start off with one of these pictures as evidence for how cute baby crows are (despite photos of very obviously different looking actual baby crow further down the in the same post!).  Since BuzzFeed ranks one of the more trafficked sites on the internet, searching “baby crow” brings up the photo attached to the story.

Fact: Those cute fluffy babies are actually a variety of baby rails, including a corncrake.  Notice how feathery and soft they look, like a baby duck or chicken?  That’s because there are basically two strategies for how young are born.   They can be altricial, which means you’re born naked and blind (i.e helpless) or precocial, which means you’re born fully feathered or furred and are ready to go from day one.

Actual baby crow

Actual baby crow

Fun fact, most birds that are ground-nesters, so game birds, waterbirds, etc., are born feathered and adorable.  Whereas most birds born in arboreal nests are little naked jelly beans.  Crows, fall into the latter category as anyone who studies crows, rehabs crows, or curiously peeked into a crow nest can attest to.

 Myth: White ravens are cannibalized and are social pariahs

poor raven

I have no hypothesis as to where this came from or how it generated such momentum on social media, other than that it’s dramatic and offers a kind of ‘misery loves company’ on a crumby day.

Fact:  Leucism, which is what this is, is pretty common in crows and ravens (check out this post if you want to learn more about the causes).  Not to this degree, of course, but even a complete leucistic like this are not unheard of.  Although life is certainly a bit harder for them (they’re a little more conspicuous, leucism is often nested in other health problems, etc.) the suggestion that they’re eaten by their mothers is nonsense.  They are however, generally subordinate to regularly colored ravens which is maybe the kernel of truth that this originates from.

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January 25, 2015 · 7:04 pm

Do crows reduce other songbirds?

Updated with data from a new review of studies looking at impacts of corvid removal on other birds.

A comment I occasionally hear, especially while conducting my research in neighborhoods is, “Ugh, I hate the crows.  All of a sudden we have tons of crows and they’ve scared off all our songbirds!”  This comment always pains me, but I understand that for most people it arises from a genuine concern for songbird abundance and conservation.  First off, as a reminder crows are songbirds themselves; ravens are our biggest songbird.  Semantics aside, I understand that there are many, many bird lovers who just can’t get on the crow bandwagon and when they talk about wanting songbirds at their feeders they mean chickadees, juncos, grosbeaks, etc.  They feel that since the “arrival” of the crows their observations of these other birds have diminished.  So is there anything to this?  Do crows indeed drive down populations of small, “desirable” backyard birds?

I came across this grizzly scene while conducting research in Bellevue. An adult robin calling frantically while a crow munched on one of its young. Later that same week I would watch of pair of adult crows chase hopelessly after a cooper's hawk that had taken one of their offspring. I came across this grizzly scene while conducting research in…

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