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HOME > PAST ISSUE > May-June 2003 > Article Detail

SCIENCE OBSERVER

Why Did the Crow Cross the Road?

Michael Szpir


The crow does not hide its prey, but calls for others to share it;
so wealth will be with those of a like disposition.--Tiruvalluvar, 5th century A.D.

In my morning drive to work I occasionally spot a crow picking at something on the road. As my car approaches the bird, it doesn't fly off in a panic or take to the trees; instead it calmly hops to the shoulder of the road or to the other side of the double yellow line. It's a measured response; one gets the impression the crow knows that automobiles tend to stay within these boundaries. That kind of cool intelligence is also evident in a crow's interactions with other members of its species.

Crows that are relatedClick to Enlarge Image

Behavioral biologist Renee Robinette Ha and her colleagues at the University of Washington have been studying the subtleties of how crows steal food from one another (Bird Behavior 15:65, 2003). Ha had been watching the birds at the beach along Puget Sound as they fed on fish, clams and other small animals in the intertidal zone. She noticed that if a crow had found a particularly large meal that couldn't be eaten in a single gulp, another crow would often come by and try to steal the food away. Food theft is fairly common in the avian world, so the crows' thievery wasn't unexpected. What really intrigued Ha was that the birds employed two different tactics to take the food.

In some instances the thieving bird would take an aggressive approachtypically involving some chasing or physical contact, such as pecking. In other exchanges, however, the thief would use a more passive method: merely sidling up to the other bird and pinching the food without any commotion at all. How did these tactics fit into the group foraging practices of the crows?

Ha and her colleagues decided to investigate whether social relations such as dominance or kinship played any role. They briefly restrained 55 crows, drew blood to take DNA samples, and marked each with identifying leg bands. Over the course of 30 months, they observed nearly 3,000 instances of group foraging and 390 attempts to steal food.

When they analyzed all of the data, Ha and her colleagues found some complex social interactions. For one thing, the crows seemed to have preferred social companions: Certain birds regularly foraged with each other, even if they weren't genetically related. Moreover, the relative social positions of the birds had little to do with the stealing tactics they employed—the dominance of males over females, or of adults over juveniles, wasn't a factor. What did become clear was that the crows discriminated between their relatives and others when it came to filching their food.

Although the birds would steal from kith and kin alike, closely related birds tended to be more cordial to each other. Relatives were inclined to use the passive method of stealing food, whereas aggressive thefts usually occurred when the two birds were more distantly related. The kindness shown to a relative attempting to steal food suggests to Ha and her colleagues that the discerning crows are gaining indirect benefits in evolutionary fitness. As Ha explains, "[W]e put up with more from our relatives than we might from our nonrelatives, because they share more genes in common with us than strangers."

The results also have implications for behavioral ecologists, who have suggested that animals forage in groups so that some can be on the lookout for predators. But Ha and her colleagues question whether the "vigilant" birds might actually be scanning the horizon to see whether another bird has found food worth stealing. Crows may indeed be inclined to share their food, but they're also pretty quick to take it from one another if they get the chance. The next time you see a crow on the road, you might think twice before you decide you know what's on its mind.—Michael Szpir


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