Sunday, February 5, 2012

The American Crow

The American Crow, Corvus brachyrhynchos.
In one of Aesop's Fables, a thirsty crow finds a tall pitcher with water in the bottom, beyond its reach. The crow drops pebbles into the pitcher to raise the water level, and drinks.  The moral of the tale is that necessity is the mother of invention. Another message is that even the ancients realized that crows are smart.

One of my favorite scientific debates is about a smart thing that crows do. Some scientists think the birds drop nuts onto roads from the air so that cars will crush them open; other scientists insist that crows are dropping nuts on hard roads surfaces to break them as they always do, but that cars are coincidental and not part of the nut-cracking plan.

Regardless of how the car debate is resolved, there is no question that crows drop nuts from the air to break them open. And it's more complicated than you might expect. Nuts may not crack on the first drop, but they weaken with each drop and break open eventually. Nuts break differently when dropped on different surfaces. Another crow might dash in and steal the broken nut.

A scientific study compared the way crows dropped hard California walnuts and softer English walnuts. (You can read the original paper by by clicking here.) The scientists observed and documented that crows drop harder nuts from greater heights, drop the same nut from decreasing heights on subsequent drops, and drop both kinds of nuts closer to hard roads. If other crows are watching, they drop from lower altitudes, on guard for possible thieves. How smart is that!

Stiff bristly feathers on the beak. 
The American crow is a large glossy black bird up to about 20 inches long. Its bill is heavy, shiny, and black; the top of the bill near the face is covered with stiff short bristly feathers. In adults, even the inside of the mouth is black. It calls a familiar caw-caw-caw. It breeds from central Canada south through most of the United States.

Crows spend winters in most of the parts of their breeding range inside the United States. During winter when less of their natural food is available they are more likely to visit landfills and dumpsters. They masterfully exploit urban food sources. They don't just eat trash -- they open lunch bags and unwrap the contents. I have watched them around outdoor lunch tables and trash cans at the Bronx Zoo. They pecked open unused ketchup packets, up-ended unfinished milk containers to drink the remains, and stuck their heads inside discarded potato chip bags to finish off the crumbs.

One study found that crows spend more time watching for danger while foraging around denser human populations than in rural areas; they share vigilance by forming groups and may depend on that more in cities, where line-of-sight can be short. I can attest that they are hard to photograph; they seem only casually aware of me, but I never seem to be able to get close. As I advance, using maximum stealth, they find interesting things to do just a few steps further away.

Click to enlarge. 
There is a nursery rhyme called Counting Crows, in which the number of crows you see predicts a future event. There seem to be endless version. Here is one --
One for sadness, two for mirth;
Three for marriage, four for birth;
Five for laughing, six for crying;
Seven for sickness, eight for dying;
Nine for silver, ten for gold;
Eleven for a secret that will never by told. 

1 comment:

  1. Love crows, and find them endlessly fascinating. Watched them run off a hawk in Riverside Park last week with a minimum of effort and a well co-ordinated strategy. Very impressive birds, indeed. I recently read "Crow Planet," and highly recommend it. Thanks for the post and for the nursery rhyme - I didn't realize it was about crows - guess I never heard the title!