Category Archives: Crow life history

What happens to baby crows at the end of summer?

Watching each year’s new crop of kiddos is one of my favorite parts of the summer field season.  I love observing the awkward, dumpy juveniles making their first forays into the wild.  I get a special kick out of watching them interact with other animals, especially the pigeons that they often find themselves competing for handouts with.  In one instance, I watched as a young crow decided to practice a bit of tail pulling on an unlucky pigeon.  After grabbing it by the tail feathers, the crow started swinging the pigeon around its head; the pigeon remaining aloft with its useless flapping while the crow just went round and round, seemingly very pleased with its animate lasso.  Antics like these are much of what I suspect drives many people’s interests in crows.  They’re just so charismatic!  So it’s no wonder I encounter so many people who take an interest in their resident family of birds.  For those of you that do, you may find yourself wondering what happens to each year’s batch of young fledgies.  Although there’s still much that remains mysterious about the transition from crow kid to crow adult, we know, or at least have begun to understand, some of the options that face these young birds.

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Without being able to fly just yet, this fledgling appears to be vying for an alternate means of transportation!

Let’s get the painful part out of the way first.  Although estimates vary, it’s safe to say many, if not most, of young crows die within their first year.  We know this by putting light (3% of a bird’s body weight) radio transmitters on a reasonable sample size of birds and then following them over the course of the next several years.  Previous work in the PNW showed that about half died.1  This is consistent with observations of banded birds out of the McGowen lab at Cornell.2  So that, unfortunately, takes care of a lot of the young birds that delight us during June and July.  But what of the others?

For the first few months after they leave the nest, young crows stick close to the family unit.  They are fiercely defended by their parents, and as a result of this safety net have the time and energy to engage in the play behaviors like those I watched.  Allowing for this period of relatively safe exploration and play may be part of the reason long lived social animals such as corvids, primates, cetaceans (dolphins and whales) and elephants, are known for being so smart.  Though, I’ll qualify that by saying exactly why that correlation exists is very poorly understood despite what your high school bio teacher may have told you about the function of play.

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A baby crow begs while mom and dad attempt to enjoy some parental bonding time.

After this security period, young birds will begin to spend more of their days with larger flocks that may or may not include members of their family.  At this time of year I’m often seeing large groups of local juvies hanging out and generally causing mayhem.  Come the fall and winter, however, each individual is faced with a more distinct option:  They can either take off to “float” before finding a mate and establishing a territory of their own, or remain on their home turf and act as a “helper” for next year’s brood.  The latter option is known as cooperative breeding and I’ll save a more full discussion of it for another post.  If they float, they may do so close to home and regularly visit the territory, or far from home returning only occasionally or not at all. Suffice it to say that crows who take this option are making a fascinating “choice” to delay their own reproduction in favor of helping their parents, something that is the subject of much study.   Of the crows tracked as apart of the UW study, about 1/5 remained with parents and there was a very strong sex bias in favor of males for this option.  Of those that disperse they’ll likely settle down within 2-3 years and find a territory similar to the one they were raised in.  So keep your eyes out as the fall progresses, and watch how the dynamic of your resident family plays out.

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Literature cited

  1. Marzluff, J. M., et al. 2001. Pp 332-363
    in Avian ecology and conservation in an urbanizing world (J. M. Marzluff, R. Bowman, and R. Donelly, eds.). Kluwer Academic Press, Norwell, MA.
  2. McGowan, K.J. (2001). P 365-381 in Avian ecology and conservation in an
    urbanizing world (J. M. Marzluff, R. Bowman, and R. Donelly, eds.).
    Kluwer Academic Press, Norwell, MA.

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Filed under Crow behavior, Crow life history

Identifying crow dieases: Avian pox

Recently, I received an email from someone in distress over an obviously sick crow in their yard. They sent some photos and were wondering about the cause of the crusty areas around the bill and eyes. Although I am by no means an expert in avian diseases, there’s one that’s easy to identify and by far the most common I see in the field so I feel it’s appropriate to provide a brief description of it here.

Bird with a likely avian pox infection as identified by the whitish/pink lesions around the exposed skin.  Photo c/o D. Wright

Bird with a likely avian pox infection as identified by the whitish/pink lesions around the exposed skin. Photo c/o D. Wright

Avian pox is a common disease affecting birds across many different orders including songbirds, raptors and game birds. It’s caused by the avipoxvirus of which there are at least 3 different strains. There’s multiple modes of transmission but it’s most often spread by mosquitoes which is why it’s observed more commonly in the spring and summer months. It can also be spread via direct transmission or indirectly through inhalation of dander, feather debris or sharing contaminated food or water sources (a good reminder to regularly clean feeders and bird baths).

There are two types of avian pox: wet and dry. Wet pox affects the mouth, throat, trachea and lungs and is most likely to be fatal because it can eventually cause suffocation. Dry pox infections are easy to identify because they result in visible lesions and scarring on the non feathered areas of the bird. In the early stages small yellow, white or pink blisters form which become large raised areas that eventually burst. This results in the formation of crusty scabs that, on crows, will appear brown or black. As long as the the bird’s ability to feed isn’t influenced by the location of the scabbing the lesions will generally heal in 2-4 weeks. The general health of the bird before the infection, and the presence of any secondary infections that result from the open lesions will influence the bird’s likelihood of survival.

The pox covered feet of a crow fatally weakened by this disease.

The pox covered feet of a crow fatally weakened by this disease. c/o Sarah Ramirez

If you notice pox infected birds at your feeder(s) the best way to protect your visitors is to stop feeding them. This may seem cruel or counterproductive since sick birds need access to consist food sources to fight the infection, but providing food bonanzas concentrate birds and increases the risk of transmission to healthy individuals. Depending on your situation you may be able to provide food to the individual without worry of creating contact between multiple birds.

Please leave a message in the comments if you have high quality photos of your infected birds and would be willing to let me use them in this post.  Thank you!

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Filed under Crow disease, Crow life history

Crow curiosities: what causes white feathers?

ZB in flight.  Photo c/o Sarah Ramirez

ZB in flight. Photo c/o Sarah Ramirez

At a recent field site in Bellevue, one of my regular visitors was the most fantastically leucistic crow I had ever seen.  Naturally, I dubbed him or her “ZB” for Zebra Bird.  One or two white feathers is pretty common but this was something far more spectacular.  Something that, to the naive observer, may look like a whole different species of bird.  In response, I thought it might be helpful to talk about how and why crows have white feathers or other kinds of color aberrations.  First off, let’s put some definitions on the table since there are a few terms that often get mixed up, or have different definitions depending on your source.  The following definitions are based on those provided by Guay et al. 2012.

Albinism results from a complete lack of melanin in both the feathers and all the soft body tissues.  This causes red eyes and pink legs, making it very easy to spot.  Albinism is often associated with poor vision and hyper-activity which quickly removes it from the general population and why, when it is spotted, it’s usually only in young or captive birds.

A complete leucistic crow.  What makes it leucistic and not albino?  The colored irises.

Albino crow spotted in Franklin Park, Seattle*

Leucism is a complete lack of melanin in all or part of the plumage, but not necessarily the soft tissues.  It is sometimes referred to as ‘partial albanism’ but if you’re familiar with the definition of albanism (which hopefully you are now!) you know the term ‘partial albinism’ is oxymoronic.  Leusistic birds can have one or multiple white feathers, as is the case with my friend in Bellevue, or be completely white but with regularly colored eyes.  Their feet and bills may or may not appear pink like that of an albino bird’s.

Schizochrosim is a lack of a particular pigment.  So a bird lacking the phaeomelanin (brown) pigment, for example, would appear grey.

Melanism is exceptionally high deposits of melanin that make the animal appear darker overall.

Carotenism is a change in the amount, distribution or composition of caroteniod (red, yellow, orange) pigments.

Dilution is, as the name suggests, a muting of colors across all or part of a bird’s plumage.

How do these color abnormalities arise?  There are a couple of different pathways including genetics, diet and injury/disease.

Genetics

Albinism is genetic, specifically, it’s linked to a recessive autosomal gene.  If you’re reading this and thinking “autosomal recesisve…what?” remember that humans have 23 pairs of chromosomes. Twenty two of them are autosomal and the last pair are sex chromosomes (you’re either XX or XY, sound familiar?).  Recessive means you need two copies of the gene to express the trait.  What this mutation does is cause an absence of the enzyme tyrosinase, which is used by the body to create some of the colored pigments. Because albinism is heritable, it can be bred into an artificial population by a skilled breeder, which is why you may see things like white tigers and lions in the entertainment business.  Despite their dramatic color variation from their peers, they are not distinct species-an idea I occasionally see being perpetuated on social media.  Genetics also plays a role in leucism, though it’s often only part of a more complicated mechanism.

A leucistic dark-eyed junco spotted at my feeder.

A leucistic dark-eyed junco spotted at my feeder.

Diet

Diets low in protein may also contribute to leucism, as the amino acid lysine has been correlated with increased white feathers.  This is supported by the observation that urban birds (who presumably have a diet lower in meat and protein) typically have more color aberrations than their rural or forested peers.  Carotenism, on the other hand, is very strongly influenced by diet, since animals cannot produce this color on their own.  A very familiar example of this is seeing the white young of flamingos who, in this early stage of life, have not yet had enough time to begin producing mass quantities of their pink pigments.

Age/injury

Lastly, age and injury may also contribute to feathers which fail to correctly pigment though this is poorly understood.  Somatic genetic mutation (i.e mutations that occur after conception) are associated with increased age, and indeed, older crows are more often seen with white feathers.  Avian Pox is known to play a role in carotenism though not much is understood about this.

ZB's eye catching leucism in fligh

ZB’s eye catching leucism in flight.  Photo c/o Sarah Ramirez

*Updates* a previous version of this post contained a typo stating that humans have 24 pairs of chromosomes.
A previous version stated incorrectly that a complete leucitic juvenile could be identified by its correctly colored iris.  Since blue pigments occur irrelevant of melanin, this is not the case.

Lit cited

Guay, P.J., Potvin, D.A., and Robinson, R.W. 2012. Abberations in plumage coloration in birds. Australian Field Ornithology 29 23-30.

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Filed under Crow curiosities, Crow life history

Reaching the limits of crow intelligence

When I was in college it became a joke among my friends and I that they would greet or bid me farewell with the following phrase “I believe in crow intelligence.”  Even as an undergrad, my passion for crow behavior and cognition was evident to my friends and family and I relished the emerging data demonstrating that this relative underdog was far exceeding our expectations of what an animal, especially a bird, could do.  While I still carry this phrase as a mantra in my research, it’s something I’ve also grown cautious to keep in check.  I’ll come back to this point in a minute, but for now let me rather crudely transition to some exciting new research.

The fantastic Alex Taylor and his group at Auckland University have once again dazzled us with another one of their eloquent studies on the New Caledonian crows.  This time they were looking at yet another aspect of crow’s learning intelligence: the ability to observe cause and effect and exercise a new behavioral pattern i.e causal intervention.  Essentially the researchers presented both the crows and two year old children with cylinder that, when hit with a block, would reward them with food.  The subjects were first exposed to the set up by baiting the block with food, thereby  demonstrating that, when moved in an effort to reach the bait food, the block would drop and release even more food via hitting the cylinder.  Babies quickly learned how to use an unbaited block provided in a new location to access the food hidden by the cylinder, but the crows failed to make the cause and effect connection.

The researchers were apt to point out that while this failure provides a fascinating insight into the evolution of causal reasoning, it does not negate the ways in which these animals remain exceptional in this respect as well.  Indeed, crows outperform children in some aspects of causal reasoning as demonstrated by the Aesop’s Fable experiments they conducted looking at object discrimination.

For me, it also provides one other important reminder: that crows are not feathered humans.  Reflecting on my earlier anecdote about my iconic catch phrase, something I’ve had to come to terms with as a graduate student is recognizing my own bias regarding these animals.  Occasionally, I find myself truly disappointed by results like the aforementioned one.  Perhaps it’s an all-American love for the underdog, or a hope that if only people understood how smart these animals are they would show them more respect.  Whatever the reason, an important area of growth for me has been acknowledging  my desire to continuing showing that these animals are exceptional and being aware of when or how that might be affecting my interpretation of my results.  This is indeed what it means to be a scientist.  Even when I have a my civilian hat on, accepting that crows are not simply feathered humans is, I think, an important part of truly embracing the natural world for what it is: a rich source of both diversity and overlap all of which deserve our admiration and preservation.

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Do crows reduce other songbirds?

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 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.

The short answer is: not usually.  Now, let’s be clear, crows will absolutely kill and eat eggs, nestlings and even adult birds if they can get their hands on one.  I once saw a crow take down an adult house sparrow in an attack so quick and dexterous I only realized what had happened after the crow had already started eating its meal.  It’s important to keep in mind, however, that crows are one of many, many animals that are eating the young and adults of other bird species.  Raccoons, squirrels, foxes, hawks, owls, bullfrogs, rats, mice, and of course cats will all gladly eat birds, especially eggs and nestlings.  The vulnerability of young birds is in fact why the breeding strategy of many birds is to have multiple clutches over the course of the breeding season.  Crows themselves are subject to these same predators and very few of their young will make it to adulthood.

Why do we think that crows aren’t responsible for the any observed decrease in feeder birds?  Predator removal studies.  These studies are straightforward and essentially create two populations, a control population that has been unmodified and a second where the predator in question has been actively removed.  Prey abundance or productivity is monitored and compared at the end of the trial.

Recently, Madden et al. published a comprehensive literature review of 42 studies across 9 countries that looked at the impacts of corvid removal on a variety of avian groups including gamebirds, passerines, waders and other ground nesting birds.  They found that in 81% of cases corvid removal made no impact on prey abundance or productivity.  They also found that impacts of corvids on prey species was similar, and no one group was particularly more sensitive than any others.  Of the corvids studied, magpies consistently had the smallest impact on prey productivity, but no difference was found if the study was looking at prey abundance.  So if corvids are such conspicuous avian predators, why doesn’t their removal seem to matter in most cases?

This is explained by idea of compensatory mortality, which is essentially that removing one predator just means that the other predators will account for its absence by eating the prey it otherwise would have.  Kevin McGowan provided a great description for this idea on his site I like to use the analogy of handicapped parking spaces at the mall You drive up to the mall, looking for a parking space in a crowded lot. You can’t find a parking space, but there are four near the entrance that are reserved for handicapped permits only. You complain and think that if only those handicapped restrictions weren’t there, you could park in those spots (common sense). In truth, of course, if those spaces were not reserved they would have been taken long ago, just like all the other spaces in the lot.”  Indeed, Madden et al.  found this to be true.  When they looked at studies that only conducted corvid removal, they found that only 16% of cases saw a difference in prey productivity.  Whereas if all predators were removed the researchers reported that 60% of studies found a significant difference in prey productivity.

What this means for those of us trying to improve the bad reputations of crows and other corvids is that the data is on our side, crows are not usually the problem predators they’re often made out to be (though in a small number of instances they are, and it’s important to acknowledge when that’s the case).  In fact, in 6% of cases the researchers found that corvid presence actually benefited other birds.  So what I suspect is happening when residents ask me why they see fewer birds and if crows are to blame is that crows often follow urban development and it’s possible that what these residents are experiencing is a change in species diversity as habitats are disrupted and modified to make way for new human settlements.  Though it’s also possible they simply don’t know where to look.  It wouldn’t be the first time I’ve listened to folks complain about crows driving away their birds in the presence of yellow-rumped warblers, white crowned sparrows, juncos and chickadees.  Indeed, suburbia is often a great place to enjoy both crows and other smaller songbirds.

John Marzluff's newest book which describes the awesome power of suburbia to become a heaven for a huge diversity of birds.  Illustrations by my friend and colleague, Jack Delapp.

John Marzluff’s newest book which describes the awesome power of suburbia to become a heaven for a huge diversity of birds. Illustrations by my friend and colleague, Jack DeLap.

John Marzluff’s new book Welcome to Subirdia, highlights that vast species diversity that can come with suburban development, showing that these types of habitat modifications aren’t doomed to be low diversity.  With a bit of thought on our part, we can create habitats that attract a variety of birds.  Namely, by limiting lawn space, increasing snags, native plants and bushes and keeping our cats indoors, we can expect to see a great variety of birds visiting our feeders, crows included!

Madden, C.F., Beatriz A., Amar, A. (2015) A review of the impacts of corvids on bird productivity and abundance. Ibis: 157, 1-16.

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

The many sounds of crow, including the jaws theme

I am constantly fascinated by the vocal repertoire of crows.  They produce over 20 calls, but I would venture a guess that for most people (I being one of them) the vast majority of those sounds are too subtle to distinguish between.  As I hear it, there are four majors “classes” of call that are easy to differentiate, even for a completely novice birder.  Those are: The classic ‘caw’, the harsh ‘scold’, the female courting ‘rattle/knock’ call, and the juvenile begging sound.  To make up the rest of the over 20 sounds, they use a combo of caws, clicks, coos, grunts and rattles.  While more has been decoded than I offer here, there is still much that remains a mystery.  Take for example their scold call.

Me getting yelled at by a Raven in Yellowstone that wanted a treat NOW!

Me, getting yelled at by a raven in Yellowstone that wanted a treat NOW!

Stimulus days featuring either our dead crow or our dead crow+the hawk consistently draw the largest number of birds.  In fact based on last year’s data, the dead crow+hawk stimulus draws the greatest numbers of birds by a large margin.  That’s not a huge surprise since hawks are a primary predator and responding very strongly to an obvious kill would provide a safety mechanism to the ‘neighborhood’.  Needless to say, with few exception once the first bird comes across this scene it alarm calls (scolds) and either very quickly or over a few minutes will draw a crowd of 10-30 birds.  It can certainly be more or less but somewhere in that range is most typical in my experience.  Now, none of this is particularly interesting.  What’s interesting is what happens the next day.  As a reminder, all our stimulus trials feature one of the known dangers paired with a person.  In the case of the dead crow+hawk this person is standing 2m away from the stuffed birds.  In half my trials I send this person out the next day and in the other half they get sent out a week later.  This is to test for any “guilty by association” inferences the birds are making about these people simply because they were near a known threat.  To my elation I’ve found that many birds do indeed respond to these people when they see them in the future.  While that’s certainly cool in and of itself, what makes it interesting from the perspective of their calls is that although their alarm call in this instance does not sound different to me, they almost never draw the crowds the initial stimulus day does.  It’s almost as if the hawk or dead crow is a neighborhood problem but a dangerous person is only the territory holder’s problem.  Is the caller advertising this distinction and intentionally not drawing in the other birds or are the others making that choice themselves?  I have no idea.  But there’s no arguing that there’s a level of complexity there worth investigating.

As for the story on my Jaws loving bird.  This afternoon I was returning to a field site I had, just the previous day, tested the stimulus I described above on.  While I was walking through the woods on my way there I noticed a shadow occasionally passing above me.  Finally I saw my stalker: A crow who I can only assume had spotted me and was hoping I’d offer some food before we actually arrived at the now tainted feeding site.  Once we arrived I threw my food out and, unsurprisingly, was not greeted by eager feeding.  Instead the bird merely perched in the tree and stared wishfully at the food it was too nervous to eat.   After a few minutes I started to hear an inexplicable noise.  A very low duh-nuh.  Duh-nuh.  The field site is located on the edge of a residential street and at first I assumed it was some kids, maybe just playing or perhaps intentionally trying to mess with me.  It was clearly the intro to the Jaws theme song.  But the more I listened, I realized there were no kids around and the sound was coming from the wrong direction to be made my a person.  With some careful watching I realized that, although I could not perceive it to be moving its bill, my perching crow bowed its head in perfect unison with the sound.  It became clear it was the sound’s author.  In all my time watching these birds I have never heard anything like it.  Have you?  You can listen to it here, but be warned it’s pretty quiet so turn your sound up.

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