I’ve never really understood the fear of large black birds that many people report. I’ve been told by those more well-read on the subject of human psychology that we are instinctively afraid of such creatures, archetypes of doom. Something must be wrong with my instincts then, because I find the American crow (Corvus Brachyrhynchos) in particular to be absolutely fascinating and in no way frightening.
Perhaps this is in part because I learned about this species’ extreme intelligence before I ever heard about Alfred Hitchcock’s The Birds (1963) or any mutterings by Edgar Allen Poe. American crows have been observed to display ingenious tool use – a sign of great intelligence within the animal kingdom – for numerous reasons, including foraging, nest defense, and even play (Caffrey 2001). American crows have been observed to not only use sticks for poking into the holely homes of potential arthropod snacks, but to modify the sticks for better fit.
Caffrey (2000) directly observed an American crow doing just that in an attempt to grab a spider lunch from a hole in a wooden fence post. The crow, finding its own beak was too big to fit into the hole, broke off a small piece of the fence, shaped the piece by pecking at it with its beak, and then used this now narrow piece to prod for the spider. The fact that the crow used an external tool to forage is an obvious sign of intelligence and ingenuity. However, a less obvious but possibly even more significant proof of intelligence in this story is that the crow realized that the piece it broke off was slightly too big to fit the hole before attempting the probe; the crow had remembered the general size of the hole, knew the piece of wood was larger, and had applied this knowledge by shaping the piece for successful use.
American crows living in close proximity to human development have found a way to use our tools as well, although for different functions and in different fashion. Grobecker and Pietsch (1978) first reported the use of automobiles by crows as a means for more efficient foraging. (No, they don’t drive to the store.) Crows were found to drop palm fruit from a power line onto the street below for cars to run over and crack open. One might question the validity of this observation. Are we sure that the crows are purposefully using the cars as nutcrackers, or are they just very clumsy while sitting on power lines? It almost certainly is the former, as confirmed by the observation that the crows would not go down to inspect the fruit until a car had passed, sometimes waiting up to 7 minutes if the fruit had not cracked on impact with the pavement and had to wait for a car to deal the fatal blow (Grobecker and Pietsch 1978). American crows in other areas of the country have also been found to exhibit this behavior with walnuts and pecans (Caffrey 2001).
American crows have understandably found it quite easy to transition to an urban environment. Knight (1987) found that crows in urban areas are far less defensive of their nests than crows in rural areas due to differences in human interaction; crows in rural areas are often persecuted and hunted by humans, while urban crows are not. Knight found that rural crows were more cautious in defending their nests, preferring to stay farther away (out of gun shot range) and call as humans approached. The urban crows, on the other hand, only started to call when the humans actually began to climb the tree in which the nest was located, and proceeded to actively defend the nest by diving at the intruder. Knight (1987) concluded that urban crows have “habituated to human beings on the ground.”
In a later study by Knight (1991), a comparison was made between American crows, bald eagles (Haliaetus leucocephalus), and common ravens (Corvus corax) that were anglers off the coast of Washington state. Knight found that both the bald eagles and the common ravens showed a stronger preference for perching – as opposed to foraging on the ground – while anglers were present, which doesn’t seem very surprising. What is interesting, however, is that the American crows were actually more likely to be on the ground with anglers present than without. Knight (1991) found that both bald eagles and ravens shifted their diurnal eating patterns on the days that anglers were present, yet crows continued to forage throughout the day, “thereby depleting the food at the expense of eagles and ravens.” Perhaps the crows had adapted more quickly to the anglers, just as they had to the less-persecuting urban humans, and found a unique window of opportunity for less foraging competition during the angler’s visits.
I recently found myself lucky enough to observe a group (aka “murder”) of American crows first hand near my house. A small group of four crows were foraging in the grass just off the main road, seemingly oblivious to the cars rushing by. I think instead that they had adapted to the sight and sounds of the large metal beasts and knew that it was a very rare occasion that these strange creatures would venture off their hard grey path. The crows had little to fear here. I unfortunately did not observe any tool use that day, but nonetheless found the behavior of the birds to reveal their intelligence. The birds walked slowly at times, looking in the grass with beady eyes that belied their no doubt complex cortexes. They appeared to examine the ground carefully, rather than just pecking incessantly like some of their less intelligent feathered relatives.
While I find its label as a portent of doom to be dubious at best, there should be no doubt as to the intelligence of the American crow. Recent studies have all but confirmed many ornithologists’ beliefs that the crow represents “the most advanced stage of avian evolution” (Powell 1976). Perhaps we should consider changing the name for a group of crows from a “murder” to a “mensa.” I for one find it far more appropriate.
For anyone interested in learning more about the American crow, I recommend watching PBS Nature’s 50 minute episode on recent research into the intelligence of crows, entitled A Murder of Crows. The full episode can be found here.
Caffrey, C. 2001. Goal-directed use of objects by American crows. The Wilson Bulletin 113(1): 114-115. doi: 10.1676/0043-5643(2001)113[0114:GDUOOB]2.0.CO;2
This paper is an observational look at a group of American crows in Encino, CA that drop nuts onto streets for passing cars to crack, as well as the use of pine cones as tools for nest-defense in a breeding pair in Stillwater, OK. Also noted in Encino, CA is the use of flower petals (tools) for play.
Caffrey, C. 2000. Tool modification and use by an American crow. The Wilson Bulletin 112(2): 283-284. doi: 10.1676/0043-5643(2000)112[0283:TMAUBA]2.0.CO;2
This paper is an observational look at an American crow in Stillwater, OK displaying tool use and modification to reach inside a hole on a fence post containing a spider.
Grobecker, D. B. and Pietsch, T. W. 1978. Crows use automobiles as nutcrackers. The Auk 95(4): 760-761.
This paper is an observational look at a group of crows in Long Beach, CA that drop palm fruit onto the street below and only go to retrieve pieces of the fruit after cars have passed, assumingly breaking apart the fruit.
Knight, R. L., Grout, D. J., and Temple, S. A. 1987. Nest-defense behavior of the American crow in urban and rural areas. The Condor 89(1): 175-177.
This study compares the nest defense behaviors of American crows living in rural areas, where they are highly persecuted, with those crows living in urban areas, where they are not persecuted. The study finds that urban crows are far less defensive as a result.
Knight, R. L., Anderson, D. P., and Marr, N. V. 1991. Responses of an avian scavenging guild to anglers. Biological Conservation 56: 195-205. doi: 10.1016/0006-3207(91)90017-4
This study compares the scavenging behavior of a group of bald eagles, common ravens, and American crows off the coast of Washington with and without the presence of anglers. The study finds that American crows are less affected by the presence of anglers and may even be more willing to forage on the ground in their presence, giving them a foraging advantage over the other studied species.
Powell, R. W. and Kelley, W. 1976. Responding under positive and negative response contingencies in pigeons and crows. Journal of the Experimental Analysis of Behavior 25(2): 219-225. doi: 10.1901/jeab.1976.25-219.
This study uses both American crows and pigeons in a psychological experiment examining positive and negative response contingencies. The crows were find to be highly adaptable to negative contingencies and quick to learn.
American Crow (Corvus Brachyrhynchos) showing its patriotism,
taken by Dori in Milwaukee, Wisconsin (public domain).
American Crow (Corvus Brachyrhynchos) with a nutty snack
(photo from Britannica Online).
The graphs above from Powell (1976) show the results for the four American crows (Corvus Brachyrhynchos) used in their response-key experiment to determine the effect of negative response contingencies and negative automaintenance. The crows showed remarkable success in obtaining reinforcers and adaptive learning.
The graph above from Knight (1991) compares the behavior of three bird species in the presence of anglers on the coast of Washington. While both bald eagles (Haliaetus leucocephalus) and common ravens (Corvus corax) showed a stronger preference for perching while anglers were present, American crows (Corvus Brachyrhynchos) were actually more likely to be on the ground with anglers present than without – an example of adaptability to human interaction.