Monthly Archives: February 2016

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?

DSC_2960

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.

DSC_2705

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.

21 Comments

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

Crows with broken beaks

It hurts to look at.  The physical pain incurred at the time of the injury, the likely chronic pain on the path to recovery, the dubious chance of survival, it all makes me reach for my mouth in horror when I see this bird.  To me, the idea of living on in spite of such a grotesque injury seems impossible.   Yet here this bird is, surviving, reminding me of what life is capable of.

DSC_1625

So now that I had my moment of sadness and awe, let’s get to what everyone wonders when they see a bird like this: Will a crow’s beak grow back if it’s broken and if not, can it survive?

Cracks or complete fractures like this can result from a number of things, though the list could be longer since these accidents are so rarely observed firsthand.  Perhaps it was traumatic run in with a window, or perhaps the upper or lower bill got trapped against a fulcrum point and an opposing surface.

As far as the prognosis is concerned, I asked birds experts, wildlife rehabilitators and veterinarians and the answer was always an equivocal ‘maybe’.  Maybe it would heal back to something resembling normal and maybe it would remain stunted. Maybe it would survive and maybe it wouldn’t.  To fully understand the reasoning behind this ambiguity you need to understand how a bird’s beak is actually formed.

Like mammals, birds have two jaws bones that form the upper and lower mandibles.  These bones are surrounded by the nerves and blood vessels that support the beak’s functionality and growth. Protecting these layers is the outer lightweight layer of keratin called the rhamphotheca.  Like our fingernails, this layer is always growing and being replaced.

Depending on where the fracture occurs, the rhamphotheca can grow back enough to abolish the injury.  Unfortunately though, there’s not much room to work with before you hit bone, and the bone cannot be regrown.  In these cases the rhampotheca may heal over the exposed bone, but it may not grow back to full size since the template for its shape (the bone) has been stunted.  Even if it does grow back, it may not do so correctly, leading to twisty shapes.

crowskull21

A crow skull.  You can see that the jaws bones are almost the full length of a crow’s beak.  The black outer layer, the rhampotheca, adds only a little (think mm) extra length so there’s not much that can be removed from the tip of the bill without hitting bone.

So what’s the prognosis?  Well again, that depends.  Even if the bill does not grow back correctly, or at all, some crows can learn to compensate.  Fortunately, being a generalist helps their chances considerably.  Although some foods may now be out of reach, many crows lean how to scoop, poke, and jab their way to a full stomach everyday. The same cannot be said for many species of bird whose beaks are the cornerstone for consuming a specialized diet.

So while it’s fair to be heartbroken at such an injury, it’s not cause for hopelessness. Many crows will learn to compensate, and go on to remind us of the beautiful stubbornness of life.

8 Comments

Filed under Corvid health, Crow behavior, Crow disease, Uncategorized