The American Crow- The fraternity everyone wants kicked off campus

Author: Christopher Rizk

It was a Monday evening as I drove towards a city park in Alabama; as I approached the park I thought my radio signal was getting weaker because there was an annoying and increasing background noise.  The instant I turned off the car I realized that the problem was hardly the radio but rather several hundred American crows, Corvus brachyrhynchos, making cawing sounds that blended together into an unbelievably loud rumble.  The crows had turned what used to be a relaxing park into a place guaranteed to induce migraine headaches upon arrival.  As I stood there, I realized these crows were just like that one obnoxious college fraternity that everyone wants gone but just can’t get rid of.

After my failed attempt at relaxation in the park, I went home and read about the American crow only to find out that they were in fact very similar to college fraternity boys in many aspects.  Along with the noise pollution and disturbance that crows and frat boys create, they are both willing to sleep anywhere and eat anything.  The American crow is willing to eat just about anything from mice to grain and settle anywhere from forests to parking lots.  This allows the crow to adapt and become successful in a myriad of unique environments.

As I walked through observing the crows I noted the sheer number and boldness of the crows.  As I would approach them, instead of fly away, the crows would just watch me as if trying to ask, “can we help you?”  It wasn’t until I was almost within arms reach that they would move away.  Richard Knight showed how crows living in urban areas are much less threatened by human presence then crows living in a near by rural area.  When threatened, the urban crows were more aggressive towards humans then the crows in the rural area.  The urban crows would also come much nearer to humans then rural crows (details in the chart below).

Just like the frat boys, the crows are social, stick together, and help each other out.  Typically crows will live together, forming colonies containing hundreds or thousands of birds.  As the crows form families, their young will stay around for a few years to help take care of the next generation of children, allowing the parents to get food.  Crows will also work together when chasing away predators or getting dinner.  When foraging, some crows will distract an animal from its nest as the other crows seize the goods from the nest.  In his paper, Marc Hauser shows how American crows recognize sounds made by predators and react in a similar and synchronized manner.  For example hearing the red-shouldered hawk led all the crows to fly away, whereas hearing the great horned owl lead the crows to “mob” the source of the sound and warn others.

Its no secret that teamwork goes a long way in helping these crows survive.  However, the fact that they are freakishly crafty and intelligent really helps them thrive in certain areas.  According to Carolee Caffrey and Louis Lefebvre, American crows have the ability to make and use tools for a multitude of tasks.  Most commonly, crows have been spotted making and utilizing probes made from wood in order to capture prey, reach a previously inaccessible area, or bore holes.  These crows are also remarkably clever in utilizing their surroundings.  American crows have been known to drop objects, including walnuts, from various heights in order to break them open. In a BBC special, David Attenborough showed just how ingenious these crows can be.  In order to safely consume the walnuts, the crows would perch on a power-line directly above a road crosswalk, drop the walnut in the middle of the crosswalk, wait for a car to run over the walnut, wait for traffic to stop, then walk along the cross walk with humans and consume the walnut until traffic resumed!!!

At this point the American crow seems basically unstoppable, they eat anything, live anywhere, are very intelligent, and work together.  However, the American crow took a significant hit to its population upon the introduction of the West Nile virus into the United States.  According to Robert McLean, the American crow is almost guaranteed to die upon infection with the West Nile virus.  The virus usually kills the infected crow in about a week.  Although the exact mechanism isn’t known, infected crows were able to infect healthy crows through contact (most likely through sharing food, or grooming).  Luckily crows aren’t able to transmit the virus to humans.  In his paper, Nicholas Panella discusses how crow deaths due to West Nile virus (confirmed via brain tissue) are actually a quite reliable biological indicator of West Nile virus levels in the area.  Michel Bunning was able to show that a significant number of crows could be saved via vaccination.  He tested four different methods of vaccination and found that administering an intramuscular DNA vaccine with adjuvant was able to save about 60% of the crows that were exposed to West Nile virus 10 weeks after vaccination (details in the chart below).  This is pretty impressive considering none of the unvaccinated crows survived.

Despite West Nile Virus’ attempt at wiping out the American crow, they are still going strong.  Some may consider the American crow a nuisance; as for myself, I can only say that I am greatly impressed with their resilience, intelligence, and resourcefulness.  Thus, it doesn’t look like this fraternity is leaving campus anytime soon, so buy a scarecrow or pair of earplugs.  If you want to read more about the American crow, the information I used in this blog can be found in the articles I have cited below.

Attenborough, D. 2007. Wild crows inhabiting the city use it to their advantage. BBCWorldwide.

This is a BBC video showing the crows ability to utilize their resources.

Bunning, L., Fox, E., Bowen, A. 2007. DNA Vaccination of the American Crow (Corvus brachyrhynchos) Provides Partial Protection Against Lethal Challenge with West Nile Virus. Avian Diseases. 51(2):573-577.

This paper discusses several methods for vaccinating American crows against West Nile virus.

Caccamise, D., Reed, L., Romanowski, J., Stouffer, P. 1997. Roosting Behavior and Group Territoriality in American Crows. Auk. 114(4):628-637.

This paper discusses group behavior with respect to foraging and nesting.

Caffrey, C. 2000. Tool Modification and Use by an American Crow. The Wilson Bulletin. 112(2):283-284.

This paper outlines observations of a crow making and using a wooden tool.

Caffrey, C. 2001. Goal-Directed Use of Objects by American Crows. The Wilson Bulletin. 113(1):114-115.

This paper outlines observations of crows dropping various objects and using tools.

Chamberlain-Auger, J., Auger, P., Strauss. E. 1990. Breeding Biology of American Crows. The Wilson Bulletin. 102(4):615-622.

This paper contains a lot of details about American crow breeding.

Hauser, M., Caffrey, C. 1994. Anti-predator response to raptor calls in wild crows, Corvus brachyrhynchos hesperis. Animal Behavior. 48:1469-1471.

This paper shows how the American crow can recognize and respond to predator’s sounds in a similar and synchronized manner.

Knight, R., Grout, D., Temple, S. 1987. Nest-Defense Behavior of the American Crow in Urban and Rural Areas. The Condor. 89(1):175-177.

This paper shows how the American crow adapts its behavior depending on the environment (rural vs. urban in this case).

Lefebvre, L., Nicolakakis, N., Boire, D. 2002. Tools and Brains in Birds. Behaviour. 139(7):939-973.

This paper has a lot of information about tool utilization in birds (with respect to the brain).

McLean, R., Ubico, S., Docherty, D., Hansen, W., Sileo, L., McNamara, T. 2001. West Nile Virus Transmission and Ecology in Birds. Annals of the New York Academy of Sciences.  951:54-57.

This paper discusses how West Nile virus may have been spread in the American crow population.

Panella, N., Kerst, A., Lanciotti, R., Bryant, P., Wolf, B., Komar, N. 2001. Comparative West Nile Virus Detection in Organs of Naturally Infected American Crows (Corvus brachyrhynchos). Emerging Infectious Diseases. 7(4):754-755.

This paper discusses how West Nile virus affects various crow organs.

Powell, R. 1972. Operant Conditioning in the Common Crow (Corvus brachyrhynchos). The Auk. 89(4):738-742.

This paper discusses the American crow’s impressive learning ability.

Powell, R., Kelly, W. 1975. A Method for the Objective Study of Tool-Using Behavior. Journal of the Experimental Analysis of Behavior. 24(2):249-253.

This paper discusses a combination of the American crow’s learning ability and use of tools.

Photo of an American crow, Corvus brachyrhynchos. Taken from Wikimedia Commons- http://commons.wikimedia.org/wiki/File:Corvus-brachyrhynchos-001.jpg

Photo of an American crow, Corvus brachyrhynchos, taking off. Photo taken by Dr. Gordon Robertson- http://en.wikipedia.org/wiki/File:Crow_taking_off.jpg

The above chart was taken from the 1987 Richard Knight paper on Nest-defense behavior. This chart highlights the behavioral differences between crows living in unique environments; specifically the way they can adapt to very different environments. As shown, urban crows did not fly or call when intruders (humans) approached their nests, whereas all rural crows did. Some urban crows even dove at the intruder, and were willing to come much closer to the intruders then rural crows.

The above chart was taken from the 2007 Michel Bunning paper on vaccinating crows against West Nile virus. As expected the placebo, adjuvant (alone), and oral vaccine had no effect. However, the intramuscular DNA vaccines with and without the adjuvant were able to protect 60% and 44% of the crows, respectively. The killed vaccine did a very poor job of protecting the crows (11%) because this is a form of passive (short term) immunity versus long-term immunity (conferred by the live strain).

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