Tag Archives: crow diet

Dumpster diving is giving crows higher cholesterol—but does it matter?

Whether it’s from actively watching crows, or simply just existing in a city, we’ve all seen it: the overflowing garbage bin with fat-stained wrappers littered at its base, and the crows snapping up each bit of leftover junk like spilled money.  Cheetos, cheeseburgers, fries, nuggets, chips, or pizza, they will devour basically anything fatty and salty with absolute glee.  This behavior is so canonically crow that it’s stapled into our contemporary imagery of these birds.  Take this 12ft statue called “Crow with Fries” by artist Peter Reiquam.

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Photo c/o salishsea

I’d wager that most people don’t think about this behavior beyond simply finding it amusing or annoying, but I suspect that if you describe yourself as crow lover, naturalist, or bird watcher, you’ve been struck with the same thought as me: “This stuff is called junk food for a reason—it’s bad for you.  What’s it doing to these birds?”

Given that anthropogenic foods can account for as much as 65% of an urban crow’s diet, it seems essential to understand what a diet derived from regularly feasting at McDonalds might do to an animal with 0.7% the body mass of a typical human.1  Unfortunately, we could do little more than shrug and speculate as to its effects.  The data for a more informed understanding just didn’t exist.  That is, until today.

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A new study published in The Condor by Dr. Andrea Townsend et al. examines the relationship between urbanization, junk food, and the body conditions of crows.2  To conduct this study, her team blood sampled 140 wild crow nestlings along an urban to rural gradient.  They found that plasma blood cholesterol levels increased in correlation with the amount of impervious surface, which is a typical way we measure urbanization.  This finding suggests that crows in the city have more access to high cholesterol foods and they make haste in gobbling it up.

Correlation is not causation, however, so to confirm this, they ran an additional supplementation study where they provided 10 rural crow parents with 3 McDonalds cheeseburgers 5-6 days a week, and then looked at how their nestling’s blood cholesterol levels compared with unsupplemented nestlings from the same area. They found that eating cheeseburgers most days of the week had a demonstrable effect on the subject’s cholesterol levels.  While this finding may not be raising any eyebrows, the actual logistics of carrying out a study that required buying hundreds of cheeseburgers each week, and sometimes in one order, certainly did.  In one of their more memorable attempts, Hannah Staab called to place an order for 125 pickle-less cheeseburgers, a request to which McD’s staff replied, “Sure, we’ll get right on that.”  When she arrived several hours later to pick them up, however, they hadn’t made any, having been convinced that the call was a prank.  The peculiarities of urban fieldwork never falter.

prank

So far, these findings tell us little more than what most people could have probably intuited, but they were crucial to laying the foundation for the real clogged heart of this study: Whether any of this is actually a bad thing.  In the final piece, they examined the body condition, and 2-3 year survival of the 140 nestlings sampled along the urban to rural gradient.  They found that cholesterol levels had no detectable effect on survival and were actually correlated with higher indices of body condition (meaning mass adjusted for size), a feature that is sometimes tied to higher reproductive success and survival. In other words, there might actually be a scenario where regularly pigging out on McDonalds doesn’t kill you and is maybe kinda helpful?

Needless to say, this caught everyone off guard, including Dr. Townsend, who told me, “I was surprised that we didn’t detect any negative health effects. I was thinking—based on the human literature—that high-cholesterol birds would have lower survival rates, but we didn’t see any effect of cholesterol on survival.”

So what gives? Is the universe really just this unfair?  While we can’t rule out that the answer is simply, “Yes,” the authors speculated that it’s possible a longer study would bear out health consequences that take more than a few years to accrue. There’s also something to be said for the fact that body condition has complex and not always agreed upon relationship with fitness and survival.3 While some studies show pudgy birds have more resources to produce more offspring and keep on ticking, others find inconsistent support. Alternatively, crows may just not live long enough to see their lifestyle catch up to them.  Future long-term studies will be necessary to fully understand whether crows have truly found a loophole in the junk-food problem. For now however, I’m happy to wish my favorite dumpster divers well, though I’ll hold off placing my own orders.

Literature cited

  1. Marzluff JM, McGowen KJ, Roarke D. and Knight RL. 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.
  2. Townsend AK, Staab HA, and Barker CM. 2019. Urbanization and elevated cholesterol in American Crows. The Condor page 1-20
  3. Milenkaya O, Catlin DH, Legge S, and Walters JR. 2015. Body Condition Indices Predict Reproductive Success but Not Survival in a Sedentary, Tropical Bird. Plos One https://doi.org/10.1371/journal.pone.0136582

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Filed under Corvid health, Crow behavior, crow diet, Crows and humans, New Research

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

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

Crows, quotes, and cooking.

In an effort to keep up with the onslaught of crow news, one of the first things I did as a newly minted graduate student was setup a daily Google news alert to the key term “crow”.  Although most of it is news about the Australian football team the Adeliade Crows, or futile efforts on behalf of desperate business districts to rid themselves of problem roosts, every once and a while some big new study will have also made the daily report.  While this is more of what I was expecting when I  signed up for the alert, it’s not what I’ve found to be the most important part.  I’ve realized that new  studies will always find their way to my desk-that’s the benefit of being a graduate student in a particular field.  No, what’s been the most savory part of this endeavor are the small, obscure observations from local and international sources.  It’s there that I’m reminded of the details that make these animals so special to me, and inspired this harried journey to try and study them in academia.

Take this recent article out of an Indian newspaper, The Hindu.  While the notes about protective crow parents are relatively banal, it’s her story about her crow visitors that struck me, particularly the last two lines of the piece.  “They do not like anything white, plain and stale. If it finds the food delicious, it calls out for others in the community.”  I found myself saying these lines over and over again both because I love their poetic feeling, and because I find myself in them as well.

When I started graduate school I knew that there were certain pieces of my life it wasn’t worth giving up, even if accommodating them would take considerable effort.  One of these things was cooking.  My husband and I love to cook, it’s an expression of passion both as individuals and as partners and something we make happen every night, even if it means eating at 9:30.  Even during my field season when he was away on the road, and I was working 14 hour days I would come home and prepare a meal from scratch.  Food is something I feel privileged to use as a form of love and expression, and nothing beats a comforting meal after a long day of discovery or failure (or more often the case, both).  And sharing that passion is a primary way my partner and I connect with our friends and family, we love cooking for others and it’s something I give up during the summer field season in exchange for all day crow watching.

Photo by Nicole Nicky

Photo by Nicole Nicky

With summer fast approaching, I’m confident this quote will float around my head as I watch crows pick at their peanuts in the dwindling hours of daylight and I’ll be reminded of exactly why I’m out there: to better understand an animal who’s avian biology is so different from ours, but whose behavior is strikingly similar. For don’t we all crave the joy of pleasurable food in the company of others?

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Filed under Crow behavior